fix: ctor of time and null

fix: manually cast file size to std::streamsize

.github/workflows/main.yml

@@ -11,6 +11,9 @@ jobs:
         standard: ['11', '14', '17', '20']
         precompile: ['ON', 'OFF']
     steps:
+      - name: Get number of CPU cores
+        uses: SimenB/github-actions-cpu-cores@v2
+        id: cpu-cores
       - name: Checkout
         uses: actions/checkout@v4
         with:
@@ -25,7 +28,7 @@ jobs:
             cmake -B build/ -DCMAKE_CXX_COMPILER=${{ matrix.compiler }} -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build build/
+            cmake --build build/ -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --output-on-failure --test-dir build/
@@ -56,7 +59,7 @@ jobs:
             cmake -B build/ -DCMAKE_CXX_COMPILER=clang++-${{ matrix.compiler }} -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build build/
+            cmake --build build/ -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --output-on-failure --test-dir build/
@@ -86,7 +89,7 @@ jobs:
             cmake -B build/ -DCMAKE_CXX_COMPILER=${{ matrix.compiler }} -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build build/
+            cmake --build build/ -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --output-on-failure --test-dir build/
@@ -118,7 +121,7 @@ jobs:
             cmake -B build/ -DCMAKE_CXX_COMPILER=clang++-${{ matrix.compiler }} -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build build/
+            cmake --build build/ -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --output-on-failure --test-dir build/
@@ -139,7 +142,7 @@ jobs:
             cmake -B build/ -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build build/
+            cmake --build build/ -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --output-on-failure --test-dir build/
@@ -160,7 +163,7 @@ jobs:
             cmake -B build/ -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_BUILD_TESTS=ON -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build build/
+            cmake --build build/ -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --output-on-failure --test-dir build/
@@ -184,7 +187,7 @@ jobs:
             cmake -B build/ -G "NMake Makefiles" -DTOML11_BUILD_TESTS=ON -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DTOML11_PRECOMPILE=${{ matrix.precompile }}
       - name: Build
         run: |
-            cmake --build ./build --config "${{ matrix.config }}"
+            cmake --build ./build --config "${{ matrix.config }}" -j${{ steps.cpu-cores.outputs.count }}
       - name: Test
         run: |
             ctest --build-config "${{ matrix.config }}" --test-dir build/ --output-on-failure

README.md

@@ -109,7 +109,7 @@ include(FetchContent)
 FetchContent_Declare(
   toml11
   GIT_REPOSITORY https://github.com/ToruNiina/toml11.git
-  GIT_TAG        v4.0.3
+  GIT_TAG        v4.1.0
 )
 FetchContent_MakeAvailable(toml11)
 
@@ -117,6 +117,19 @@ add_executable(main main.cpp)
 target_link_libraries(main PRIVATE toml11::toml11)
 ```
 
+### CMake Package Manager (CPM)
+
+After [adding cpm to your project](https://github.com/cpm-cmake/CPM.cmake?tab=readme-ov-file#adding-cpm), you can use toml11 by doing:
+
+```cmake
+include(cmake/CPM.cmake)
+
+CPMAddPackage("gh:ToruNiina/toml11@4.1.0")
+
+add_executable(main main.cpp)
+target_link_libraries(main PUBLIC toml11::toml11)
+```
+
 ### Install Using CMake
 
 You can install toml11 using CMake with the following steps:
@@ -627,7 +640,10 @@ I appreciate the help of the contributors who introduced the great feature to th
   - Fix issues with CMake package configuration when used with vcpkg
 - Egor Pugin (@egorpugin)
   - Fix incorrect operator<<() argument type that gives build error
-
+- Andreas Keller (@andreaskeller96)
+  - Fix not checking for \r\n when parsing line comments
+- 萧迩珀 (@CDK6182CHR)
+  - Support template into_toml members
 
 ## Licensing terms
 

README_ja.md

@@ -114,6 +114,19 @@ add_executable(main main.cpp)
 target_link_libraries(main PRIVATE toml11::toml11)
 ```
 
+### CMake Package Manager (CPM)
+
+[CMake package manager](https://github.com/cpm-cmake/CPM.cmake)を導入すると、以下のようにして使用することができます。
+
+```cmake
+include(cmake/CPM.cmake)
+
+CPMAddPackage("gh:ToruNiina/toml11@4.1.0")
+
+add_executable(main main.cpp)
+target_link_libraries(main PUBLIC toml11::toml11)
+```
+
 ### Install using CMake
 
 以下の手順で、CMakeを使ってインストールすることができます。
@@ -629,6 +642,10 @@ toml11 v3からは複数の破壊的変更が追加されています。
   - Fix issues with CMake package configuration when used with vcpkg
 - Egor Pugin (@egorpugin)
   - Fix incorrect operator<<() argument type that gives build error
+- Andreas Keller (@andreaskeller96)
+  - Fix not checking for \r\n when parsing line comments
+- 萧迩珀 (@CDK6182CHR)
+  - Support template into_toml members
 
 ## Licensing terms
 

docs/content.en/docs/changelog/_index.md

@@ -6,12 +6,60 @@ weight = 4
 
 # Change Log
 
+# v4.1.0
+
+## Added
+
+- support `std::get<std::u8string>`
+- support `toml::value(std::u8string)`
+- support `string_type = std::u8string`
+- support `operator<<(std::ostream&, toml::value)`
+- add `bool uppercase` to `toml::integer_format`
+- support `template into_toml()` (by 萧迩珀)
+
+## Fixed
+
+- Fix not checking for `\r\n` when parsing line comments (by Andreas Keller)
+
+## Changed
+
+- speedup CI tests by multicore build
+
+# v4.0.3
+
+## Fixed
+
+- remove default template argument from forward declaration in `toml_fwd.hpp`
+- enable to call `toml::make_error_info` with multiple `toml::value`s
+- enable to copy/move `toml::result` having `std::reference_wrapper`
+- fix document generation error with the latest version of hugo
+- fix several tiny errors that causes warning
+- fix CMake compatibility warning
+
+## Changed
+
+add `-Werror` / `/WX` to build script
+set MSVC warning level to `/W4`
+add explanation of features to README
+
+# v4.0.2
+
+## Fixed
+
+- check return value of fread in `parse(FILE*)`
+- version macro defined in `toml11/version.hpp`
+- update docs about compilation
+- update docs about file open mode
+
+## Changed
+- use version macros defined in `toml11/version.hpp` as the project version in `CMakeLists.txt`
+
 # v4.0.1
 
 ## Fixed
 
 - Resolved naming conflict of `sematic_version::{major, minor}` with macros defined in `<sys/sysmacro.h>`
-- Fixed the definition of `operator<<` in `discard_comments`
+- Fixed the definition of `operator<<` in `discard_comments` (by Egor Pugin)
 - Fixed the issue where the first blank line was not output in `format_location`
 - Fixed the issue where error messages pointing to `source_location` referring to lines containing only newline characters were displayed in two lines
 - Corrected links in the README

docs/content.en/docs/reference/format.md

@@ -60,10 +60,11 @@ Specifies the radix of an `integer`.
 ```cpp
 struct integer_format_info
 {
-    integer_format fmt = integer_format::dec;
-    std::size_t width  = 0;  // minimal width (may exceed)
-    std::size_t spacer = 0;  // position of `_` (if 0, no spacer)
-    std::string suffix = ""; // _suffix (library extension)
+    integer_format fmt    = integer_format::dec;
+    bool        uppercase = true; // use uppercase letters
+    std::size_t width     = 0;  // minimal width (may exceed)
+    std::size_t spacer    = 0;  // position of `_` (if 0, no spacer)
+    std::string suffix    = ""; // _suffix (library extension)
 };
 
 bool operator==(const integer_format_info&, const integer_format_info&) noexcept;
@@ -76,6 +77,10 @@ bool operator!=(const integer_format_info&, const integer_format_info&) noexcept
 
 Specifies the radix.
 
+### `bool uppercase`
+
+Uses uppercase letters when formatted as a hexadecimal integer.
+
 ### `std::size_t width`
 
 Specifies the minimum width. The formatted value may exceed this width.

docs/content.ja/docs/changelog/_index.md

@@ -6,12 +6,61 @@ weight = 4
 
 # Change Log
 
+# v4.1.0
+
+## Added
+
+- `std::get<std::u8string>`をサポート
+- `toml::value(std::u8string)`をサポート
+- `string_type = std::u8string`をサポート
+- `operator<<(std::ostream&, toml::value)`をサポート
+- `toml::integer_format`に、16進数表示で大文字を使うことを指定する`bool uppercase`を追加
+- `template`を使って実装された `into_toml()` をサポート (by 萧迩珀)
+
+## Fixed
+
+- Windowsで`\r\n`が改行に使われた際、出力時に`\r`が重複する問題を修正 (by Andreas Keller)
+
+## Changed
+
+- CIで複数コアを使うことでビルドを高速化
+
+# v4.0.3
+
+## Fixed
+
+- `toml_fwd.hpp`を使用した際にデフォルト引数が重複する問題を修正
+- `toml::value`を複数渡した際に`toml::make_error_info`が使えない問題を修正
+- `std::reference_wrapper`を持つ`toml::result`をコピー・ムーブする際の問題を修正
+- hugoの最新版でドキュメントがビルドできない問題を修正
+- コンパイラによる軽微な警告を多数修正
+- CMakeの互換性に関する警告を抑制
+
+## Changed
+
+- CIビルドで`-Werror`, `/WX`を追加
+- CIビルドでMSVCのwarningレベルを`/W3`から`/W4`に変更
+- READMEでより多くの機能を紹介するよう更新
+
+# v4.0.2
+
+## Fixed
+
+- `parse(FILE*)` 内の `fread` の結果をチェック
+- `toml11/version.hpp`のマクロを修正
+- コンパイルオプションに関するドキュメントの更新
+- ファイルオープンモードに関するドキュメントの更新
+
+## Changed
+
+- `CMakeLists.txt`内のバージョン番号を`version.hpp`から取得するように変更
+
 # v4.0.1
 
 ## Fixed
 
 - `sematic_version::{major, minor}` と `<sys/sysmacro.h>` 内で定義されるマクロの衝突を解消
-- `discard_comments` の `operator<<` の定義を修正
+- `discard_comments` の `operator<<` の定義を修正 (by Egor Pugin)
 - `format_location`を使用した際に最初の空行が出ない問題を解決
 - 改行文字のみを含む行を指す`source_location`でエラーメッセージを生成した際に、同じ行が二回表示される問題を解決
 - `README.md`内のリンクを修正

docs/content.ja/docs/reference/format.md

@@ -60,10 +60,11 @@ std::string to_string(const integer_format);
 ```cpp
 struct integer_format_info
 {
-    integer_format fmt = integer_format::dec;
-    std::size_t width  = 0;  // minimal width (may exceed)
-    std::size_t spacer = 0;  // position of `_` (if 0, no spacer)
-    std::string suffix = ""; // _suffix (library extension)
+    integer_format fmt    = integer_format::dec;
+    bool        uppercase = true; // use uppercase letters
+    std::size_t width     = 0;       // minimal width (may exceed)
+    std::size_t spacer    = 0;       // position of `_` (if 0, no spacer)
+    std::string suffix    = "";      // _suffix (library extension)
 };
 
 bool operator==(const integer_format_info&, const integer_format_info&) noexcept;
@@ -76,6 +77,10 @@ bool operator!=(const integer_format_info&, const integer_format_info&) noexcept
 
 基数を指定します。
 
+### `bool uppercase`
+
+16進数表記で大文字を使用します。
+
 ### `std::size_t width`
 
 最小の幅を指定します。値によってはこの幅を超えることがあります。

examples/CMakeLists.txt

@@ -2,4 +2,5 @@ add_subdirectory(boost_container)
 add_subdirectory(boost_multiprecision)
 add_subdirectory(parse_file)
 add_subdirectory(reflect)
+add_subdirectory(u8string)
 add_subdirectory(unicode)

examples/u8string/.gitignore

@@ -0,0 +1 @@
+u8string

examples/u8string/CMakeLists.txt

@@ -0,0 +1,5 @@
+add_executable(u8string u8string.cpp)
+target_link_libraries(u8string PRIVATE toml11::toml11)
+target_compile_features(u8string PRIVATE cxx_std_20)
+set_target_properties(u8string PROPERTIES
+    RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")

examples/u8string/spec_example.toml

@@ -0,0 +1,23 @@
+# This is a TOML document
+
+title = "TOML Example"
+
+[owner]
+name = "Tom Preston-Werner"
+dob = 1979-05-27T07:32:00-08:00
+
+[database]
+enabled = true
+ports = [ 8000, 8001, 8002 ]
+data = [ ["delta", "phi"], [3.14] ]
+temp_targets = { cpu = 79.5, case = 72.0 }
+
+[servers]
+
+[servers.alpha]
+ip = "10.0.0.1"
+role = "frontend"
+
+[servers.beta]
+ip = "10.0.0.2"
+role = "backend"

examples/u8string/u8string.cpp

@@ -0,0 +1,120 @@
+#include <toml.hpp>
+
+#include <iostream>
+#include <map>
+
+#include <cassert>
+
+struct u8config
+{
+    using comment_type  = toml::preserve_comments;
+
+    using boolean_type  = bool;
+    using integer_type  = std::int64_t;
+    using floating_type = double;
+    using string_type   = std::u8string; // XXX
+
+    template<typename T>
+    using array_type = std::vector<T>;
+    template<typename K, typename T>
+    using table_type = std::unordered_map<K, T>;
+
+    static toml::result<integer_type, toml::error_info>
+    parse_int(const std::string& str, const toml::source_location src, const std::uint8_t base)
+    {
+        return toml::read_int<integer_type>(str, src, base);
+    }
+    static toml::result<floating_type, toml::error_info>
+    parse_float(const std::string& str, const toml::source_location src, const bool is_hex)
+    {
+        return toml::read_float<floating_type>(str, src, is_hex);
+    }
+};
+
+int main()
+{
+    const auto root = toml::parse<u8config>("spec_example.toml");
+
+    // using member functions
+    {
+        assert(root.at(u8"title").as_string() == u8"TOML Example");
+
+        assert(root.at(u8"owner").at(u8"name").as_string() == u8"Tom Preston-Werner");
+
+        const auto dob = root.at(u8"owner").at(u8"dob" ).as_offset_datetime();
+        assert(dob.date  .year   ==  1979);
+        assert(dob.date  .month  ==  static_cast<int>(toml::month_t::May));
+        assert(dob.date  .day    ==  27);
+        assert(dob.time  .hour   ==  7);
+        assert(dob.time  .minute ==  32);
+        assert(dob.time  .second ==  0);
+        assert(dob.offset.hour   == -8);
+        assert(dob.offset.minute ==  0);
+
+        assert(root.at(u8"database").at(u8"enabled").as_boolean());
+        assert(root.at(u8"database").at(u8"ports").at(0).as_integer() == 8000);
+        assert(root.at(u8"database").at(u8"ports").at(1).as_integer() == 8001);
+        assert(root.at(u8"database").at(u8"ports").at(2).as_integer() == 8002);
+        assert(root.at(u8"database").at(u8"data").at(0).at(0).as_string()   == u8"delta");
+        assert(root.at(u8"database").at(u8"data").at(0).at(1).as_string()   == u8"phi");
+        assert(root.at(u8"database").at(u8"data").at(1).at(0).as_floating() == 3.14);
+        assert(root.at(u8"database").at(u8"temp_targets").at(u8"cpu" ).as_floating() == 79.5);
+        assert(root.at(u8"database").at(u8"temp_targets").at(u8"case").as_floating() == 72.0);
+
+        assert(root.at(u8"servers").at(u8"alpha").at(u8"ip"  ).as_string() == u8"10.0.0.1");
+        assert(root.at(u8"servers").at(u8"alpha").at(u8"role").as_string() == u8"frontend");
+        assert(root.at(u8"servers").at(u8"beta" ).at(u8"ip"  ).as_string() == u8"10.0.0.2");
+        assert(root.at(u8"servers").at(u8"beta" ).at(u8"role").as_string() == u8"backend");
+    }
+
+    // using toml::find
+    {
+        // you can get as std::string from u8string, using toml::get/find
+        assert(toml::find<std::string>(root, u8"title") == "TOML Example");
+
+        assert(toml::find<std::string>(root, u8"owner", u8"name") == "Tom Preston-Werner");
+
+        const auto dob = toml::find<toml::offset_datetime>(root, u8"owner", u8"dob");
+        assert(dob.date  .year   ==  1979);
+        assert(dob.date  .month  ==  static_cast<int>(toml::month_t::May));
+        assert(dob.date  .day    ==  27);
+        assert(dob.time  .hour   ==  7);
+        assert(dob.time  .minute ==  32);
+        assert(dob.time  .second ==  0);
+        assert(dob.offset.hour   == -8);
+        assert(dob.offset.minute ==  0);
+
+        assert(toml::find<bool>(root, u8"database", u8"enabled"));
+
+        const auto ports = toml::find<std::vector<int>>(root, u8"database", u8"ports");
+        assert(ports.at(0) == 8000);
+        assert(ports.at(1) == 8001);
+        assert(ports.at(2) == 8002);
+
+        const auto data = toml::find<std::pair<std::vector<std::string>, std::vector<double>>>(root, u8"database", u8"data");
+        assert(data.first.at(0)  == "delta");
+        assert(data.first.at(1)  == "phi");
+        assert(data.second.at(0) == 3.14);
+
+        const auto temp_targets = toml::find<std::map<std::string, double>>(root, u8"database", u8"temp_targets");
+        assert(temp_targets.at("cpu" ) == 79.5);
+        assert(temp_targets.at("case") == 72.0);
+
+        const auto servers = toml::find<std::map<std::string, std::map<std::string, std::string>>>(root, u8"servers");
+        assert(servers.at("alpha").at("ip"  ) == "10.0.0.1");
+        assert(servers.at("alpha").at("role") == "frontend");
+        assert(servers.at("beta" ).at("ip"  ) == "10.0.0.2");
+        assert(servers.at("beta" ).at("role") == "backend" );
+    }
+
+    const std::u8string out = toml::format(root);
+
+    std::string printable;
+    std::transform(out.begin(), out.end(), std::back_inserter(printable),
+            [](const char8_t c) {return static_cast<char>(c);});
+    std::cout << printable << std::endl;
+
+    std::cout << "ok." << std::endl;
+
+    return 0;
+}

include/toml11/fwd/format_fwd.hpp

@@ -57,9 +57,10 @@ std::string to_string(const integer_format);
 struct integer_format_info
 {
     integer_format fmt = integer_format::dec;
-    std::size_t width  = 0;  // minimal width (may exceed)
-    std::size_t spacer = 0;  // position of `_` (if 0, no spacer)
-    std::string suffix = ""; // _suffix (library extension)
+    bool        uppercase = true; // hex with uppercase
+    std::size_t width     = 0;    // minimal width (may exceed)
+    std::size_t spacer    = 0;    // position of `_` (if 0, no spacer)
+    std::string suffix    = "";   // _suffix (library extension)
 };
 
 bool operator==(const integer_format_info&, const integer_format_info&) noexcept;
@@ -213,6 +214,29 @@ struct table_format_info
 bool operator==(const table_format_info&, const table_format_info&) noexcept;
 bool operator!=(const table_format_info&, const table_format_info&) noexcept;
 
+// ----------------------------------------------------------------------------
+// key
+
+enum class key_format : std::uint8_t
+{
+    bare           = 0,
+    quoted         = 1, // ""
+    quoted_literal = 2  // ''
+};
+
+std::ostream& operator<<(std::ostream& os, const key_format f);
+std::string to_string(const key_format);
+
+struct key_format_info
+{
+    key_format fmt = key_format::bare;
+    std::int32_t spaces_before_equal = 1;
+    std::int32_t spaces_after_equal  = 1;
+};
+
+bool operator==(const key_format_info&, const key_format_info&) noexcept;
+bool operator!=(const key_format_info&, const key_format_info&) noexcept;
+
 // ----------------------------------------------------------------------------
 // wrapper
 

include/toml11/get.hpp

@@ -109,16 +109,29 @@ get(const basic_value<TC>& v)
     return static_cast<T>(v.as_floating());
 }
 
+// ============================================================================
+// std::string with different char/trait/allocator
+
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_not_toml_type<T, basic_value<TC>>,
+    detail::is_1byte_std_basic_string<T>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    return detail::string_conv<cxx::remove_cvref_t<T>>(v.as_string());
+}
+
 // ============================================================================
 // std::string_view
 
 #if defined(TOML11_HAS_STRING_VIEW)
 
 template<typename T, typename TC>
-cxx::enable_if_t<std::is_same<T, std::string_view>::value, std::string_view>
+cxx::enable_if_t<detail::is_string_view_of<T, typename basic_value<TC>::string_type>::value, T>
 get(const basic_value<TC>& v)
 {
-    return std::string_view(v.as_string());
+    return T(v.as_string());
 }
 
 #endif // string_view
@@ -175,6 +188,10 @@ cxx::enable_if_t<cxx::conjunction<
     detail::is_container<T>,                            // T is a container
     detail::has_push_back_method<T>,                    // .push_back() works
     detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::array
+    cxx::negation<detail::is_std_basic_string<T>>,      // but not std::basic_string<CharT>
+#if defined(TOML11_HAS_STRING_VIEW)
+    cxx::negation<detail::is_std_basic_string_view<T>>,      // but not std::basic_string_view<CharT>
+#endif
     cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
     cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
     cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
@@ -201,16 +218,33 @@ template<typename T, typename TC>
 cxx::enable_if_t<detail::is_std_tuple<T>::value, T>
 get(const basic_value<TC>&);
 
-// map-like
+// std::map<key, value> (key is convertible from toml::value::key_type)
 template<typename T, typename TC>
 cxx::enable_if_t<cxx::conjunction<
     detail::is_map<T>,                                  // T is map
     detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::table
+    std::is_convertible<typename basic_value<TC>::key_type,
+                        typename T::key_type>,          // keys are convertible
     cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
     cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
     cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
     >::value, T>
-get(const basic_value<TC>&);
+get(const basic_value<TC>& v);
+
+// std::map<key, value> (key is not convertible from toml::value::key_type, but
+// is a std::basic_string)
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_map<T>,                                  // T is map
+    detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::table
+    cxx::negation<std::is_convertible<typename basic_value<TC>::key_type,
+        typename T::key_type>>,                         // keys are NOT convertible
+    detail::is_1byte_std_basic_string<typename T::key_type>, // is std::basic_string
+    cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v);
 
 // toml::from<T>::from_toml(v)
 template<typename T, typename TC>
@@ -245,6 +279,10 @@ cxx::enable_if_t<cxx::conjunction<
     detail::is_container<T>,                            // T is a container
     detail::has_push_back_method<T>,                    // .push_back() works
     detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::array
+    cxx::negation<detail::is_std_basic_string<T>>,      // but not std::basic_string<CharT>
+#if defined(TOML11_HAS_STRING_VIEW)
+    cxx::negation<detail::is_std_basic_string_view<T>>, // but not std::basic_string_view<CharT>
+#endif
     cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
     cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
     cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
@@ -363,10 +401,13 @@ get(const basic_value<TC>& v)
 // ============================================================================
 // map-like types; most likely STL map, like std::map or std::unordered_map.
 
+// key is convertible from toml::value::key_type
 template<typename T, typename TC>
 cxx::enable_if_t<cxx::conjunction<
     detail::is_map<T>,                                  // T is map
     detail::is_not_toml_type<T, basic_value<TC>>,       // but not toml::table
+    std::is_convertible<typename basic_value<TC>::key_type,
+                        typename T::key_type>,          // keys are convertible
     cxx::negation<detail::has_from_toml_method<T, TC>>, // no T.from_toml()
     cxx::negation<detail::has_specialized_from<T>>,     // no toml::from<T>
     cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
@@ -388,6 +429,31 @@ get(const basic_value<TC>& v)
     return m;
 }
 
+// key is NOT convertible from toml::value::key_type but std::basic_string
+template<typename T, typename TC>
+cxx::enable_if_t<cxx::conjunction<
+    detail::is_map<T>,                                       // T is map
+    detail::is_not_toml_type<T, basic_value<TC>>,            // but not toml::table
+    cxx::negation<std::is_convertible<typename basic_value<TC>::key_type,
+        typename T::key_type>>,                              // keys are NOT convertible
+    detail::is_1byte_std_basic_string<typename T::key_type>, // is std::basic_string
+    cxx::negation<detail::has_from_toml_method<T, TC>>,      // no T.from_toml()
+    cxx::negation<detail::has_specialized_from<T>>,          // no toml::from<T>
+    cxx::negation<std::is_constructible<T, const basic_value<TC>&>>
+    >::value, T>
+get(const basic_value<TC>& v)
+{
+    using key_type    = typename T::key_type;
+    using mapped_type = typename T::mapped_type;
+
+    T m;
+    for(const auto& kv : v.as_table())
+    {
+        m.emplace(detail::string_conv<key_type>(kv.first), get<mapped_type>(kv.second));
+    }
+    return m;
+}
+
 // ============================================================================
 // user-defined, but convertible types.
 

include/toml11/impl/format_impl.hpp

@@ -66,10 +66,11 @@ TOML11_INLINE std::string to_string(const integer_format c)
 
 TOML11_INLINE bool operator==(const integer_format_info& lhs, const integer_format_info& rhs) noexcept
 {
-    return lhs.fmt    == rhs.fmt    &&
-           lhs.width  == rhs.width  &&
-           lhs.spacer == rhs.spacer &&
-           lhs.suffix == rhs.suffix ;
+    return lhs.fmt       == rhs.fmt    &&
+           lhs.uppercase == rhs.uppercase &&
+           lhs.width     == rhs.width  &&
+           lhs.spacer    == rhs.spacer &&
+           lhs.suffix    == rhs.suffix ;
 }
 TOML11_INLINE bool operator!=(const integer_format_info& lhs, const integer_format_info& rhs) noexcept
 {
@@ -292,5 +293,41 @@ TOML11_INLINE bool operator!=(const table_format_info& lhs, const table_format_i
     return !(lhs == rhs);
 }
 
+// ----------------------------------------------------------------------------
+// key
+
+TOML11_INLINE std::ostream& operator<<(std::ostream& os, const key_format f)
+{
+    switch(f)
+    {
+        case key_format::bare           : {os << "bare"          ; break;}
+        case key_format::quoted         : {os << "quoted"        ; break;}
+        case key_format::quoted_literal : {os << "quoted_literal"; break;}
+        default:
+        {
+            os << "unknown key_format: " << static_cast<std::uint8_t>(f);
+            break;
+        }
+    }
+    return os;
+}
+TOML11_INLINE std::string to_string(const key_format c)
+{
+    std::ostringstream oss;
+    oss << c;
+    return oss.str();
+}
+
+TOML11_INLINE bool operator==(const key_format_info& lhs, const key_format_info& rhs) noexcept
+{
+    return lhs.fmt                 == rhs.fmt                 &&
+           lhs.spaces_before_equal == rhs.spaces_before_equal &&
+           lhs.spaces_after_equal  == rhs.spaces_after_equal  ;
+}
+TOML11_INLINE bool operator!=(const key_format_info& lhs, const key_format_info& rhs) noexcept
+{
+    return !(lhs == rhs);
+}
+
 } // namespace toml
 #endif // TOML11_FORMAT_IMPL_HPP

include/toml11/parser.hpp

@@ -172,7 +172,7 @@ parse_boolean(location& loc, const context<TC>& ctx)
     // no format info for boolean
     boolean_format_info fmt;
 
-    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(val, std::move(fmt), {}, {}, std::move(reg)));
 }
 
 /* ============================================================================
@@ -221,7 +221,7 @@ parse_bin_integer(location& loc, const context<TC>& ctx)
     const auto val = TC::parse_int(str, source_location(region(loc)), 2);
     if(val.is_ok())
     {
-        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, {}, std::move(reg)));
     }
     else
     {
@@ -273,7 +273,7 @@ parse_oct_integer(location& loc, const context<TC>& ctx)
     const auto val = TC::parse_int(str, source_location(region(loc)), 8);
     if(val.is_ok())
     {
-        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, {}, std::move(reg)));
     }
     else
     {
@@ -292,7 +292,7 @@ parse_hex_integer(location& loc, const context<TC>& ctx)
     if( ! reg.is_ok())
     {
         return err(make_syntax_error("toml::parse_hex_integer: "
-            "invalid integer: hex_int must be like: 0xC0FFEE, 0xDEAD_BEEF",
+            "invalid integer: hex_int must be like: 0xC0FFEE, 0xdead_beef",
             syntax::hex_int(spec), loc));
     }
 
@@ -320,10 +320,18 @@ parse_hex_integer(location& loc, const context<TC>& ctx)
     // 0x0000_0000 becomes empty.
     if(str.empty()) { str = "0"; }
 
+    // prefix zero and _ is removed. check if it uses upper/lower case.
+    // if both upper and lower case letters are found, set upper=true.
+    const auto lower_not_found = std::find_if(str.begin(), str.end(),
+        [](const char c) { return std::islower(static_cast<int>(c)) != 0; }) == str.end();
+    const auto upper_found = std::find_if(str.begin(), str.end(),
+        [](const char c) { return std::isupper(static_cast<int>(c)) != 0; }) != str.end();
+    fmt.uppercase = lower_not_found || upper_found;
+
     const auto val = TC::parse_int(str, source_location(region(loc)), 16);
     if(val.is_ok())
     {
-        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+        return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, {}, std::move(reg)));
     }
     else
     {
@@ -394,7 +402,7 @@ parse_dec_integer(location& loc, const context<TC>& ctx)
         fmt.suffix = sfx;
     }
 
-    return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(val.as_ok(), std::move(fmt), {}, {}, std::move(reg)));
 }
 
 template<typename TC>
@@ -659,7 +667,7 @@ parse_floating(location& loc, const context<TC>& ctx)
         fmt.suffix = sfx;
     }
 
-    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(val, std::move(fmt), {}, {}, std::move(reg)));
 }
 
 /* ============================================================================
@@ -777,7 +785,7 @@ parse_local_date(location& loc, const context<TC>& ctx)
     auto fmt = std::move(std::get<1>(val_fmt_reg.unwrap()));
     auto reg = std::move(std::get<2>(val_fmt_reg.unwrap()));
 
-    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, {}, std::move(reg)));
 }
 
 // all the offset_datetime, local_datetime, local_time parses date part.
@@ -946,7 +954,7 @@ parse_local_time(location& loc, const context<TC>& ctx)
     auto fmt = std::move(std::get<1>(val_fmt_reg.unwrap()));
     auto reg = std::move(std::get<2>(val_fmt_reg.unwrap()));
 
-    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, {}, std::move(reg)));
 }
 
 template<typename TC>
@@ -1005,7 +1013,7 @@ parse_local_datetime(location& loc, const context<TC>& ctx)
     local_datetime val(std::get<0>(date_fmt_reg.unwrap()),
                        std::get<0>(time_fmt_reg.unwrap()));
 
-    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(val, std::move(fmt), {}, {}, std::move(reg)));
 }
 
 template<typename TC>
@@ -1132,7 +1140,7 @@ parse_offset_datetime(location& loc, const context<TC>& ctx)
                                        std::get<0>(time_fmt_reg.unwrap())),
                                        offset);
 
-    return ok(basic_value<TC>(val, std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(val, std::move(fmt), {}, {}, std::move(reg)));
 }
 
 /* ============================================================================
@@ -1393,7 +1401,7 @@ parse_ml_basic_string(location& loc, const context<TC>& ctx)
     }
 
     return ok(basic_value<TC>(
-            std::move(val), std::move(fmt), {}, std::move(reg)
+            std::move(val), std::move(fmt), {}, {}, std::move(reg)
         ));
 }
 
@@ -1473,7 +1481,7 @@ parse_basic_string(location& loc, const context<TC>& ctx)
     auto val = std::move(val_res.unwrap().first );
     auto reg = std::move(val_res.unwrap().second);
 
-    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, std::move(reg)));
+    return ok(basic_value<TC>(std::move(val), std::move(fmt), {}, {}, std::move(reg)));
 }
 
 template<typename TC>
@@ -1520,7 +1528,7 @@ parse_ml_literal_string(location& loc, const context<TC>& ctx)
     string_type val(str.begin(), str.end());
 
     return ok(basic_value<TC>(
-            std::move(val), std::move(fmt), {}, std::move(reg)
+            std::move(val), std::move(fmt), {}, {}, std::move(reg)
         ));
 }
 
@@ -1573,7 +1581,7 @@ parse_literal_string(location& loc, const context<TC>& ctx)
     auto reg = std::move(val_res.unwrap().second);
 
     return ok(basic_value<TC>(
-            std::move(val), std::move(fmt), {}, std::move(reg)
+            std::move(val), std::move(fmt), {}, {}, std::move(reg)
         ));
 }
 
@@ -1693,21 +1701,7 @@ parse_simple_key(location& loc, const context<TC>& ctx)
 
     if(const auto bare = syntax::unquoted_key(spec).scan(loc))
     {
-        const auto reg = bare.as_string();
-        // here we cannot use `if constexpr` because it is C++11.
-        if(std::is_same<key_type, std::string>::value)
-        {
-            return ok(reg);
-        }
-        else
-        {
-            key_type k;
-            for(const auto c : reg)
-            {
-                k += typename key_type::value_type(c);
-            }
-            return ok(k);
-        }
+        return ok(string_conv<key_type>(bare.as_string()));
     }
     else
     {
@@ -1799,6 +1793,9 @@ parse_key_value_pair(location& loc, context<TC>& ctx)
         // loc = first;
         return err(v_res.unwrap_err());
     }
+
+    // set key reg/fmt
+
     return ok(std::make_pair(std::move(key_res.unwrap()), std::move(v_res.unwrap())));
 }
 
@@ -1855,7 +1852,14 @@ skip_multiline_spacer(location& loc, context<TC>& ctx, const bool newline_found
         {
             spacer.newline_found = true;
             auto comment = comm.as_string();
-            if( ! comment.empty() && comment.back() == '\n') {comment.pop_back();}
+            if( ! comment.empty() && comment.back() == '\n')
+            {
+                comment.pop_back();
+                if (!comment.empty() && comment.back() == '\r')
+                {
+                    comment.pop_back();
+                }
+            }
 
             spacer.comments.push_back(std::move(comment));
             spacer.indent_type = indent_char::none;
@@ -2048,7 +2052,7 @@ parse_array(location& loc, context<TC>& ctx)
     }
 
     return ok(basic_value<TC>(
-            std::move(val), std::move(fmt), {}, region(first, loc)
+            std::move(val), std::move(fmt), {}, {}, region(first, loc)
         ));
 }
 
@@ -2205,7 +2209,7 @@ insert_value(const inserting_value_kind kind,
                     fmt.fmt = table_format::implicit;
                 }
                 current_table.emplace(key, value_type(
-                    table_type{}, fmt, std::vector<std::string>{}, key_reg));
+                    table_type{}, fmt, std::vector<std::string>{}, {}, key_reg));
 
                 assert(current_table.at(key).is_table());
                 current_table_ptr = std::addressof(current_table.at(key).as_table());
@@ -2352,7 +2356,8 @@ insert_value(const inserting_value_kind kind,
 
                         current_table.emplace(key, value_type(
                                 array_type{ std::move(val) }, std::move(fmt),
-                                std::vector<std::string>{}, std::move(key_reg)
+                                std::vector<std::string>{}, key_format_info{},
+                                key_reg, key_reg
                             ));
 
                         assert( ! current_table.at(key).as_array().empty());
@@ -2603,7 +2608,7 @@ parse_inline_table(location& loc, context<TC>& ctx)
     }
 
     basic_value<TC> retval(
-        std::move(table), std::move(fmt), {}, region(first, loc));
+        std::move(table), std::move(fmt), {}, {}, region(first, loc));
 
     return ok(std::move(retval));
 }
@@ -3170,10 +3175,10 @@ parse_file(location& loc, context<TC>& ctx)
 
     if(loc.eof())
     {
-        return ok(value_type(table_type(), table_format_info{}, {}, region(loc)));
+        return ok(value_type(table_type(), table_format_info{}, {}, {}, region(loc)));
     }
 
-    value_type root(table_type(), table_format_info{}, {}, region(loc));
+    value_type root(table_type(), table_format_info{}, {}, {}, region(loc));
     root.as_table_fmt().fmt = table_format::multiline;
     root.as_table_fmt().indent_type = indent_char::none;
 
@@ -3278,7 +3283,7 @@ parse_file(location& loc, context<TC>& ctx)
             table_format_info fmt;
             fmt.fmt = table_format::multiline;
             fmt.indent_type = indent_char::none;
-            auto tab = value_type(table_type{}, std::move(fmt), std::move(com), reg);
+            auto tab = value_type(table_type{}, std::move(fmt), std::move(com), {}, reg);
 
             auto inserted = insert_value(inserting_value_kind::array_table,
                 std::addressof(root.as_table()),
@@ -3365,7 +3370,7 @@ parse_file(location& loc, context<TC>& ctx)
             table_format_info fmt;
             fmt.fmt = table_format::multiline;
             fmt.indent_type = indent_char::none;
-            auto tab = value_type(table_type{}, std::move(fmt), std::move(com), reg);
+            auto tab = value_type(table_type{}, std::move(fmt), std::move(com), {}, reg);
 
             auto inserted = insert_value(inserting_value_kind::std_table,
                 std::addressof(root.as_table()),
@@ -3440,7 +3445,7 @@ parse_impl(std::vector<location::char_type> cs, std::string fname, const spec& s
     {
         auto src = std::make_shared<std::vector<location::char_type>>(std::move(cs));
         location loc(std::move(src), std::move(fname));
-        return ok(value_type(table_type(), table_format_info{}, std::vector<std::string>{}, region(loc)));
+        return ok(value_type(table_type(), table_format_info{}, std::vector<std::string>{}, {}, region(loc)));
     }
 
     // to simplify parser, add newline at the end if there is no LF.
@@ -3525,7 +3530,7 @@ try_parse(std::istream& is, std::string fname = "unknown file", spec s = spec::d
     // read whole file as a sequence of char
     assert(fsize >= 0);
     std::vector<detail::location::char_type> letters(static_cast<std::size_t>(fsize), '\0');
-    is.read(reinterpret_cast<char*>(letters.data()), fsize);
+    is.read(reinterpret_cast<char*>(letters.data()), static_cast<std::streamsize>(fsize));
 
     return detail::parse_impl<TC>(std::move(letters), std::move(fname), std::move(s));
 }

include/toml11/serializer.hpp

@@ -59,13 +59,15 @@ class serializer
     using array_type           = typename value_type::array_type          ;
     using table_type           = typename value_type::table_type          ;
 
+    using char_type            = typename string_type::value_type;
+
   public:
 
     explicit serializer(const spec& sp)
         : spec_(sp), force_inline_(false), current_indent_(0)
     {}
 
-    std::string operator()(const std::vector<key_type>& ks, const value_type& v)
+    string_type operator()(const std::vector<key_type>& ks, const value_type& v)
     {
         for(const auto& k : ks)
         {
@@ -74,13 +76,13 @@ class serializer
         return (*this)(v);
     }
 
-    std::string operator()(const key_type& k, const value_type& v)
+    string_type operator()(const key_type& k, const value_type& v)
     {
         this->keys_.push_back(k);
         return (*this)(v);
     }
 
-    std::string operator()(const value_type& v)
+    string_type operator()(const value_type& v)
     {
         switch(v.type())
         {
@@ -98,14 +100,14 @@ class serializer
             }
             case value_t::table          :
             {
-                std::string retval;
+                string_type retval;
                 if(this->keys_.empty()) // it might be the root table. emit comments here.
                 {
                     retval += format_comments(v.comments(), v.as_table_fmt().indent_type);
                 }
                 if( ! retval.empty()) // we have comment.
                 {
-                    retval += '\n';
+                    retval += char_type('\n');
                 }
 
                 retval += (*this)(v.as_table(), v.as_table_fmt(), v.comments(), v.location());
@@ -115,7 +117,7 @@ class serializer
             {
                 if(this->spec_.ext_null_value)
                 {
-                    return "null";
+                    return string_conv<string_type>("null");
                 }
                 break;
             }
@@ -131,12 +133,19 @@ class serializer
 
   private:
 
-    std::string operator()(const boolean_type& b, const boolean_format_info&, const source_location&) // {{{
+    string_type operator()(const boolean_type& b, const boolean_format_info&, const source_location&) // {{{
     {
-        if(b) { return std::string("true"); } else { return std::string("false"); }
+        if(b)
+        {
+            return string_conv<string_type>("true");
+        }
+        else
+        {
+            return string_conv<string_type>("false");
+        }
     } // }}}
 
-    std::string operator()(const integer_type i, const integer_format_info& fmt, const source_location& loc) // {{{
+    string_type operator()(const integer_type i, const integer_format_info& fmt, const source_location& loc) // {{{
     {
         std::ostringstream oss;
         this->set_locale(oss);
@@ -180,7 +189,6 @@ class serializer
                 retval += '_';
                 retval += fmt.suffix;
             }
-            return retval;
         }
         else
         {
@@ -193,13 +201,27 @@ class serializer
             {
                 case integer_format::hex:
                 {
-                    oss << std::setw(static_cast<int>(fmt.width)) << std::setfill('0') << std::hex << i;
-                    return std::string("0x") + insert_spacer(oss.str());
+                    oss << std::noshowbase
+                        << std::setw(static_cast<int>(fmt.width))
+                        << std::setfill('0')
+                        << std::hex;
+                    if(fmt.uppercase)
+                    {
+                        oss << std::uppercase;
+                    }
+                    else
+                    {
+                        oss << std::nouppercase;
+                    }
+                    oss << i;
+                    retval = std::string("0x") + insert_spacer(oss.str());
+                    break;
                 }
                 case integer_format::oct:
                 {
                     oss << std::setw(static_cast<int>(fmt.width)) << std::setfill('0') << std::oct << i;
-                    return std::string("0o") + insert_spacer(oss.str());
+                    retval = std::string("0o") + insert_spacer(oss.str());
+                    break;
                 }
                 case integer_format::bin:
                 {
@@ -228,7 +250,8 @@ class serializer
                     {
                         oss << *iter;
                     }
-                    return std::string("0b") + oss.str();
+                    retval = std::string("0b") + oss.str();
+                    break;
                 }
                 default:
                 {
@@ -238,9 +261,10 @@ class serializer
                 }
             }
         }
+        return string_conv<string_type>(retval);
     } // }}}
 
-    std::string operator()(const floating_type f, const floating_format_info& fmt, const source_location&) // {{{
+    string_type operator()(const floating_type f, const floating_format_info& fmt, const source_location&) // {{{
     {
         using std::isnan;
         using std::isinf;
@@ -261,7 +285,7 @@ class serializer
                 oss << '_';
                 oss << fmt.suffix;
             }
-            return oss.str();
+            return string_conv<string_type>(oss.str());
         }
 
         if(isinf(f))
@@ -276,7 +300,7 @@ class serializer
                 oss << '_';
                 oss << fmt.suffix;
             }
-            return oss.str();
+            return string_conv<string_type>(oss.str());
         }
 
         switch(fmt.fmt)
@@ -301,7 +325,7 @@ class serializer
                     s += '_';
                     s += fmt.suffix;
                 }
-                return s;
+                return string_conv<string_type>(s);
             }
             case floating_format::fixed:
             {
@@ -314,7 +338,7 @@ class serializer
                 {
                     oss << '_' << fmt.suffix;
                 }
-                return oss.str();
+                return string_conv<string_type>(oss.str());
             }
             case floating_format::scientific:
             {
@@ -327,7 +351,7 @@ class serializer
                 {
                     oss << '_' << fmt.suffix;
                 }
-                return oss.str();
+                return string_conv<string_type>(oss.str());
             }
             case floating_format::hex:
             {
@@ -335,14 +359,14 @@ class serializer
                 {
                     oss << std::hexfloat << f;
                     // suffix is only for decimal numbers.
-                    return oss.str();
+                    return string_conv<string_type>(oss.str());
                 }
                 else // no hex allowed. output with max precision.
                 {
                     oss << std::setprecision(std::numeric_limits<floating_type>::max_digits10)
                         << std::scientific << f;
                     // suffix is only for decimal numbers.
-                    return oss.str();
+                    return string_conv<string_type>(oss.str());
                 }
             }
             default:
@@ -351,58 +375,58 @@ class serializer
                 {
                     oss << '_' << fmt.suffix;
                 }
-                return oss.str();
+                return string_conv<string_type>(oss.str());
             }
         }
     } // }}}
 
-    std::string operator()(string_type s, const string_format_info& fmt, const source_location& loc) // {{{
+    string_type operator()(string_type s, const string_format_info& fmt, const source_location& loc) // {{{
     {
-        std::string retval;
+        string_type retval;
         switch(fmt.fmt)
         {
             case string_format::basic:
             {
-                retval += '"';
+                retval += char_type('"');
                 retval += this->escape_basic_string(s);
-                retval += '"';
+                retval += char_type('"');
                 return retval;
             }
             case string_format::literal:
             {
-                if(std::find(s.begin(), s.end(), '\n') != s.end())
+                if(std::find(s.begin(), s.end(), char_type('\n')) != s.end())
                 {
                     throw serialization_error(format_error("toml::serializer: "
                         "(non-multiline) literal string cannot have a newline",
                         loc, "here"), loc);
                 }
-                retval += '\'';
+                retval += char_type('\'');
                 retval += s;
-                retval += '\'';
+                retval += char_type('\'');
                 return retval;
             }
             case string_format::multiline_basic:
             {
-                retval += "\"\"\"";
+                retval += string_conv<string_type>("\"\"\"");
                 if(fmt.start_with_newline)
                 {
-                    retval += '\n';
+                    retval += char_type('\n');
                 }
 
                 retval += this->escape_ml_basic_string(s);
 
-                retval += "\"\"\"";
+                retval += string_conv<string_type>("\"\"\"");
                 return retval;
             }
             case string_format::multiline_literal:
             {
-                retval += "'''";
+                retval += string_conv<string_type>("'''");
                 if(fmt.start_with_newline)
                 {
-                    retval += '\n';
+                    retval += char_type('\n');
                 }
                 retval += s;
-                retval += "'''";
+                retval += string_conv<string_type>("'''");
                 return retval;
             }
             default:
@@ -414,19 +438,19 @@ class serializer
         }
     } // }}}
 
-    std::string operator()(const local_date_type& d, const local_date_format_info&, const source_location&) // {{{
+    string_type operator()(const local_date_type& d, const local_date_format_info&, const source_location&) // {{{
     {
         std::ostringstream oss;
         oss << d;
-        return oss.str();
+        return string_conv<string_type>(oss.str());
     } // }}}
 
-    std::string operator()(const local_time_type& t, const local_time_format_info& fmt, const source_location&) // {{{
+    string_type operator()(const local_time_type& t, const local_time_format_info& fmt, const source_location&) // {{{
     {
         return this->format_local_time(t, fmt.has_seconds, fmt.subsecond_precision);
     } // }}}
 
-    std::string operator()(const local_datetime_type& dt, const local_datetime_format_info& fmt, const source_location&) // {{{
+    string_type operator()(const local_datetime_type& dt, const local_datetime_format_info& fmt, const source_location&) // {{{
     {
         std::ostringstream oss;
         oss << dt.date;
@@ -437,11 +461,11 @@ class serializer
             case datetime_delimiter_kind::space:   { oss << ' '; break; }
             default:                               { oss << 'T'; break; }
         }
-        oss << this->format_local_time(dt.time, fmt.has_seconds, fmt.subsecond_precision);
-        return oss.str();
+        return string_conv<string_type>(oss.str()) +
+            this->format_local_time(dt.time, fmt.has_seconds, fmt.subsecond_precision);
     } // }}}
 
-    std::string operator()(const offset_datetime_type& odt, const offset_datetime_format_info& fmt, const source_location&) // {{{
+    string_type operator()(const offset_datetime_type& odt, const offset_datetime_format_info& fmt, const source_location&) // {{{
     {
         std::ostringstream oss;
         oss << odt.date;
@@ -452,12 +476,12 @@ class serializer
             case datetime_delimiter_kind::space:   { oss << ' '; break; }
             default:                               { oss << 'T'; break; }
         }
-        oss << this->format_local_time(odt.time, fmt.has_seconds, fmt.subsecond_precision);
+        oss << string_conv<std::string>(this->format_local_time(odt.time, fmt.has_seconds, fmt.subsecond_precision));
         oss << odt.offset;
-        return oss.str();
+        return string_conv<string_type>(oss.str());
     } // }}}
 
-    std::string operator()(const array_type& a, const array_format_info& fmt, const comment_type& com, const source_location& loc) // {{{
+    string_type operator()(const array_type& a, const array_format_info& fmt, const comment_type& com, const source_location& loc) // {{{
     {
         array_format f = fmt.fmt;
         if(fmt.fmt == array_format::default_format)
@@ -546,7 +570,7 @@ class serializer
                 throw serialization_error("array of table must have its key. "
                         "use format(key, v)", loc);
             }
-            std::string retval;
+            string_type retval;
             for(const auto& e : a)
             {
                 assert(e.is_table());
@@ -556,9 +580,9 @@ class serializer
                 retval += this->format_indent(e.as_table_fmt().indent_type);
                 this->current_indent_ -= e.as_table_fmt().name_indent;
 
-                retval += "[[";
+                retval += string_conv<string_type>("[[");
                 retval += this->format_keys(this->keys_).value();
-                retval += "]]\n";
+                retval += string_conv<string_type>("]]\n");
 
                 retval += this->format_ml_table(e.as_table(), e.as_table_fmt());
             }
@@ -567,20 +591,20 @@ class serializer
         else if(f == array_format::oneline)
         {
             // ignore comments. we cannot emit comments
-            std::string retval;
-            retval += "[";
+            string_type retval;
+            retval += char_type('[');
             for(const auto& e : a)
             {
                 this->force_inline_ = true;
                 retval += (*this)(e);
-                retval += ", ";
+                retval += string_conv<string_type>(", ");
             }
             if( ! a.empty())
             {
                 retval.pop_back(); // ` `
                 retval.pop_back(); // `,`
             }
-            retval += "]";
+            retval += char_type(']');
             this->force_inline_ = false;
             return retval;
         }
@@ -588,32 +612,32 @@ class serializer
         {
             assert(f == array_format::multiline);
 
-            std::string retval;
-            retval += "[\n";
+            string_type retval;
+            retval += string_conv<string_type>("[\n");
 
             for(const auto& e : a)
             {
                 this->current_indent_ += fmt.body_indent;
-                retval += format_comments(e.comments(), fmt.indent_type);
-                retval += format_indent(fmt.indent_type);
+                retval += this->format_comments(e.comments(), fmt.indent_type);
+                retval += this->format_indent(fmt.indent_type);
                 this->current_indent_ -= fmt.body_indent;
 
                 this->force_inline_ = true;
                 retval += (*this)(e);
-                retval += ",\n";
+                retval += string_conv<string_type>(",\n");
             }
             this->force_inline_ = false;
 
             this->current_indent_ += fmt.closing_indent;
-            retval += format_indent(fmt.indent_type);
+            retval += this->format_indent(fmt.indent_type);
             this->current_indent_ -= fmt.closing_indent;
 
-            retval += "]";
+            retval += char_type(']');
             return retval;
         }
     } // }}}
 
-    std::string operator()(const table_type& t, const table_format_info& fmt, const comment_type& com, const source_location& loc) // {{{
+    string_type operator()(const table_type& t, const table_format_info& fmt, const comment_type& com, const source_location& loc) // {{{
     {
         if(this->force_inline_)
         {
@@ -630,17 +654,17 @@ class serializer
         {
             if(fmt.fmt == table_format::multiline)
             {
-                std::string retval;
+                string_type retval;
                 // comment is emitted inside format_ml_table
                 if(auto k = this->format_keys(this->keys_))
                 {
                     this->current_indent_ += fmt.name_indent;
-                    retval += format_comments(com, fmt.indent_type);
-                    retval += format_indent(fmt.indent_type);
+                    retval += this->format_comments(com, fmt.indent_type);
+                    retval += this->format_indent(fmt.indent_type);
                     this->current_indent_ -= fmt.name_indent;
-                    retval += '[';
+                    retval += char_type('[');
                     retval += k.value();
-                    retval += "]\n";
+                    retval += string_conv<string_type>("]\n");
                 }
                 // otherwise, its the root.
 
@@ -657,7 +681,7 @@ class serializer
             }
             else if(fmt.fmt == table_format::dotted)
             {
-                std::vector<std::string> keys;
+                std::vector<string_type> keys;
                 if(this->keys_.empty())
                 {
                     throw serialization_error(format_error("toml::serializer: "
@@ -674,7 +698,7 @@ class serializer
             {
                 assert(fmt.fmt == table_format::implicit);
 
-                std::string retval;
+                string_type retval;
                 for(const auto& kv : t)
                 {
                     const auto& k = kv.first;
@@ -722,46 +746,48 @@ class serializer
 
   private:
 
-    std::string escape_basic_string(const std::string& s) const // {{{
+    string_type escape_basic_string(const string_type& s) const // {{{
     {
-        std::string retval;
-        for(const char c : s)
+        string_type retval;
+        for(const char_type c : s)
         {
             switch(c)
             {
-                case '\\': {retval += "\\\\"; break;}
-                case '\"': {retval += "\\\""; break;}
-                case '\b': {retval += "\\b";  break;}
-                case '\t': {retval += "\\t";  break;}
-                case '\f': {retval += "\\f";  break;}
-                case '\n': {retval += "\\n";  break;}
-                case '\r': {retval += "\\r";  break;}
+                case char_type('\\'): {retval += string_conv<string_type>("\\\\"); break;}
+                case char_type('\"'): {retval += string_conv<string_type>("\\\""); break;}
+                case char_type('\b'): {retval += string_conv<string_type>("\\b" ); break;}
+                case char_type('\t'): {retval += string_conv<string_type>("\\t" ); break;}
+                case char_type('\f'): {retval += string_conv<string_type>("\\f" ); break;}
+                case char_type('\n'): {retval += string_conv<string_type>("\\n" ); break;}
+                case char_type('\r'): {retval += string_conv<string_type>("\\r" ); break;}
                 default  :
                 {
-                    if(c == 0x1B && spec_.v1_1_0_add_escape_sequence_e)
+                    if(c == char_type(0x1B) && spec_.v1_1_0_add_escape_sequence_e)
                     {
-                        retval += "\\e";
+                        retval += string_conv<string_type>("\\e");
                     }
-                    else if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
+                    else if((char_type(0x00) <= c && c <= char_type(0x08)) ||
+                            (char_type(0x0A) <= c && c <= char_type(0x1F)) ||
+                             c == char_type(0x7F))
                     {
                         if(spec_.v1_1_0_add_escape_sequence_x)
                         {
-                            retval += "\\x";
+                            retval += string_conv<string_type>("\\x");
                         }
                         else
                         {
-                            retval += "\\u00";
+                            retval += string_conv<string_type>("\\u00");
                         }
                         const auto c1 = c / 16;
                         const auto c2 = c % 16;
-                        retval += static_cast<char>('0' + c1);
+                        retval += static_cast<char_type>('0' + c1);
                         if(c2 < 10)
                         {
-                            retval += static_cast<char>('0' + c2);
+                            retval += static_cast<char_type>('0' + c2);
                         }
                         else // 10 <= c2
                         {
-                            retval += static_cast<char>('A' + (c2 - 10));
+                            retval += static_cast<char_type>('A' + (c2 - 10));
                         }
                     }
                     else
@@ -774,45 +800,47 @@ class serializer
         return retval;
     } // }}}
 
-    std::string escape_ml_basic_string(const std::string& s) // {{{
+    string_type escape_ml_basic_string(const string_type& s) // {{{
     {
-        std::string retval;
-        for(const char c : s)
+        string_type retval;
+        for(const char_type c : s)
         {
             switch(c)
             {
-                case '\\': {retval += "\\\\"; break;}
-                case '\b': {retval += "\\b";  break;}
-                case '\t': {retval += "\\t";  break;}
-                case '\f': {retval += "\\f";  break;}
-                case '\n': {retval += "\n";   break;}
-                case '\r': {retval += "\\r";  break;}
+                case char_type('\\'): {retval += string_conv<string_type>("\\\\"); break;}
+                case char_type('\b'): {retval += string_conv<string_type>("\\b" ); break;}
+                case char_type('\t'): {retval += string_conv<string_type>("\\t" ); break;}
+                case char_type('\f'): {retval += string_conv<string_type>("\\f" ); break;}
+                case char_type('\n'): {retval += string_conv<string_type>("\n"  ); break;}
+                case char_type('\r'): {retval += string_conv<string_type>("\\r" ); break;}
                 default  :
                 {
-                    if(c == 0x1B && spec_.v1_1_0_add_escape_sequence_e)
+                    if(c == char_type(0x1B) && spec_.v1_1_0_add_escape_sequence_e)
                     {
-                        retval += "\\e";
+                        retval += string_conv<string_type>("\\e");
                     }
-                    else if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
+                    else if((char_type(0x00) <= c && c <= char_type(0x08)) ||
+                            (char_type(0x0A) <= c && c <= char_type(0x1F)) ||
+                            c == char_type(0x7F))
                     {
                         if(spec_.v1_1_0_add_escape_sequence_x)
                         {
-                            retval += "\\x";
+                            retval += string_conv<string_type>("\\x");
                         }
                         else
                         {
-                            retval += "\\u00";
+                            retval += string_conv<string_type>("\\u00");
                         }
                         const auto c1 = c / 16;
                         const auto c2 = c % 16;
-                        retval += static_cast<char>('0' + c1);
+                        retval += static_cast<char_type>('0' + c1);
                         if(c2 < 10)
                         {
-                            retval += static_cast<char>('0' + c2);
+                            retval += static_cast<char_type>('0' + c2);
                         }
                         else // 10 <= c2
                         {
-                            retval += static_cast<char>('A' + (c2 - 10));
+                            retval += static_cast<char_type>('A' + (c2 - 10));
                         }
                     }
                     else
@@ -833,16 +861,16 @@ class serializer
         // str5 = """Here are three quotation marks: ""\"."""
         // str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
         // ```
-        auto found_3_quotes = retval.find("\"\"\"");
-        while(found_3_quotes != std::string::npos)
+        auto found_3_quotes = retval.find(string_conv<string_type>("\"\"\""));
+        while(found_3_quotes != string_type::npos)
         {
-            retval.replace(found_3_quotes, 3, "\"\"\\\"");
-            found_3_quotes = retval.find("\"\"\"");
+            retval.replace(found_3_quotes, 3, string_conv<string_type>("\"\"\\\""));
+            found_3_quotes = retval.find(string_conv<string_type>("\"\"\""));
         }
         return retval;
     } // }}}
 
-    std::string format_local_time(const local_time_type& t, const bool has_seconds, const std::size_t subsec_prec) // {{{
+    string_type format_local_time(const local_time_type& t, const bool has_seconds, const std::size_t subsec_prec) // {{{
     {
         std::ostringstream oss;
         oss << std::setfill('0') << std::setw(2) << static_cast<int>(t.hour);
@@ -862,10 +890,10 @@ class serializer
                 oss << '.' << subsec_str.substr(0, subsec_prec);
             }
         }
-        return oss.str();
+        return string_conv<string_type>(oss.str());
     } // }}}
 
-    std::string format_ml_table(const table_type& t, const table_format_info& fmt) // {{{
+    string_type format_ml_table(const table_type& t, const table_format_info& fmt) // {{{
     {
         const auto format_later = [](const value_type& v) -> bool {
 
@@ -881,7 +909,7 @@ class serializer
             return is_ml_table || is_ml_array_table;
         };
 
-        std::string retval;
+        string_type retval;
         this->current_indent_ += fmt.body_indent;
         for(const auto& kv : t)
         {
@@ -902,9 +930,9 @@ class serializer
             else
             {
                 retval += format_key(key);
-                retval += " = ";
+                retval += string_conv<string_type>(" = ");
                 retval += (*this)(val);
-                retval += "\n";
+                retval += char_type('\n');
             }
             this->keys_.pop_back();
         }
@@ -912,7 +940,7 @@ class serializer
 
         if( ! retval.empty())
         {
-            retval += "\n"; // for readability, add empty line between tables
+            retval += char_type('\n'); // for readability, add empty line between tables
         }
         for(const auto& kv : t)
         {
@@ -929,33 +957,33 @@ class serializer
         return retval;
     } // }}}
 
-    std::string format_inline_table(const table_type& t, const table_format_info&) // {{{
+    string_type format_inline_table(const table_type& t, const table_format_info&) // {{{
     {
         // comments are ignored because we cannot write without newline
-        std::string retval;
-        retval += '{';
+        string_type retval;
+        retval += char_type('{');
         for(const auto& kv : t)
         {
             this->force_inline_ = true;
             retval += this->format_key(kv.first);
-            retval += " = ";
+            retval += string_conv<string_type>(" = ");
             retval += (*this)(kv.second);
-            retval += ", ";
+            retval += string_conv<string_type>(", ");
         }
         if( ! t.empty())
         {
             retval.pop_back(); // ' '
             retval.pop_back(); // ','
         }
-        retval += '}';
+        retval += char_type('}');
         this->force_inline_ = false;
         return retval;
     } // }}}
 
-    std::string format_ml_inline_table(const table_type& t, const table_format_info& fmt) // {{{
+    string_type format_ml_inline_table(const table_type& t, const table_format_info& fmt) // {{{
     {
-        std::string retval;
-        retval += "{\n";
+        string_type retval;
+        retval += string_conv<string_type>("{\n");
         this->current_indent_ += fmt.body_indent;
         for(const auto& kv : t)
         {
@@ -963,12 +991,12 @@ class serializer
             retval += format_comments(kv.second.comments(), fmt.indent_type);
             retval += format_indent(fmt.indent_type);
             retval += kv.first;
-            retval += " = ";
+            retval += string_conv<string_type>(" = ");
 
             this->force_inline_ = true;
             retval += (*this)(kv.second);
 
-            retval += ",\n";
+            retval += string_conv<string_type>(",\n");
         }
         if( ! t.empty())
         {
@@ -982,12 +1010,12 @@ class serializer
         retval += format_indent(fmt.indent_type);
         this->current_indent_ -= fmt.closing_indent;
 
-        retval += '}';
+        retval += char_type('}');
         return retval;
     } // }}}
 
-    std::string format_dotted_table(const table_type& t, const table_format_info& fmt, // {{{
-            const source_location&, std::vector<std::string>& keys)
+    string_type format_dotted_table(const table_type& t, const table_format_info& fmt, // {{{
+            const source_location&, std::vector<string_type>& keys)
     {
         // lets say we have: `{"a": {"b": {"c": {"d": "foo", "e": "bar"} } }`
         // and `a` and `b` are `dotted`.
@@ -1003,7 +1031,7 @@ class serializer
         // a.b.c.e = "bar"
         // ```
 
-        std::string retval;
+        string_type retval;
 
         for(const auto& kv : t)
         {
@@ -1024,10 +1052,10 @@ class serializer
                 retval += format_comments(val.comments(), fmt.indent_type);
                 retval += format_indent(fmt.indent_type);
                 retval += format_keys(keys).value();
-                retval += " = ";
+                retval += string_conv<string_type>(" = ");
                 this->force_inline_ = true; // sub-table must be inlined
                 retval += (*this)(val);
-                retval += '\n';
+                retval += char_type('\n');
                 this->force_inline_ = false;
             }
             keys.pop_back();
@@ -1035,15 +1063,15 @@ class serializer
         return retval;
     } // }}}
 
-    std::string format_key(const key_type& key) // {{{
+    string_type format_key(const key_type& key) // {{{
     {
         if(key.empty())
         {
-            return std::string("\"\"");
+            return string_conv<string_type>("\"\"");
         }
 
         // check the key can be a bare (unquoted) key
-        auto loc = detail::make_temporary_location(key);
+        auto loc = detail::make_temporary_location(string_conv<std::string>(key));
         auto reg = detail::syntax::unquoted_key(this->spec_).scan(loc);
         if(reg.is_ok() && loc.eof())
         {
@@ -1051,41 +1079,43 @@ class serializer
         }
 
         //if it includes special characters, then format it in a "quoted" key.
-        std::string formatted("\"");
-        for(const char c : key)
+        string_type formatted = string_conv<string_type>("\"");
+        for(const char_type c : key)
         {
             switch(c)
             {
-                case '\\': {formatted += "\\\\"; break;}
-                case '\"': {formatted += "\\\""; break;}
-                case '\b': {formatted += "\\b";  break;}
-                case '\t': {formatted += "\\t";  break;}
-                case '\f': {formatted += "\\f";  break;}
-                case '\n': {formatted += "\\n";  break;}
-                case '\r': {formatted += "\\r";  break;}
+                case char_type('\\'): {formatted += string_conv<string_type>("\\\\"); break;}
+                case char_type('\"'): {formatted += string_conv<string_type>("\\\""); break;}
+                case char_type('\b'): {formatted += string_conv<string_type>("\\b" ); break;}
+                case char_type('\t'): {formatted += string_conv<string_type>("\\t" ); break;}
+                case char_type('\f'): {formatted += string_conv<string_type>("\\f" ); break;}
+                case char_type('\n'): {formatted += string_conv<string_type>("\\n" ); break;}
+                case char_type('\r'): {formatted += string_conv<string_type>("\\r" ); break;}
                 default  :
                 {
                     // ASCII ctrl char
-                    if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
+                    if( (char_type(0x00) <= c && c <= char_type(0x08)) ||
+                        (char_type(0x0A) <= c && c <= char_type(0x1F)) ||
+                        c == char_type(0x7F))
                     {
                         if(spec_.v1_1_0_add_escape_sequence_x)
                         {
-                            formatted += "\\x";
+                            formatted += string_conv<string_type>("\\x");
                         }
                         else
                         {
-                            formatted += "\\u00";
+                            formatted += string_conv<string_type>("\\u00");
                         }
                         const auto c1 = c / 16;
                         const auto c2 = c % 16;
-                        formatted += static_cast<char>('0' + c1);
+                        formatted += static_cast<char_type>('0' + c1);
                         if(c2 < 10)
                         {
-                            formatted += static_cast<char>('0' + c2);
+                            formatted += static_cast<char_type>('0' + c2);
                         }
                         else // 10 <= c2
                         {
-                            formatted += static_cast<char>('A' + (c2 - 10));
+                            formatted += static_cast<char_type>('A' + (c2 - 10));
                         }
                     }
                     else
@@ -1096,58 +1126,58 @@ class serializer
                 }
             }
         }
-        formatted += "\"";
+        formatted += string_conv<string_type>("\"");
         return formatted;
     } // }}}
-    cxx::optional<std::string> format_keys(const std::vector<key_type>& keys) // {{{
+    cxx::optional<string_type> format_keys(const std::vector<key_type>& keys) // {{{
     {
         if(keys.empty())
         {
             return cxx::make_nullopt();
         }
 
-        std::string formatted;
+        string_type formatted;
         for(const auto& ky : keys)
         {
             formatted += format_key(ky);
-            formatted += '.';
+            formatted += char_type('.');
         }
         formatted.pop_back(); // remove the last dot '.'
         return formatted;
     } // }}}
 
-    std::string format_comments(const discard_comments&, const indent_char) const // {{{
+    string_type format_comments(const discard_comments&, const indent_char) const // {{{
     {
-        return "";
+        return string_conv<string_type>("");
     } // }}}
-    std::string format_comments(const preserve_comments& comments, const indent_char indent_type) const // {{{
+    string_type format_comments(const preserve_comments& comments, const indent_char indent_type) const // {{{
     {
-        std::string retval;
+        string_type retval;
         for(const auto& c : comments)
         {
             if(c.empty()) {continue;}
             retval += format_indent(indent_type);
-            if(c.front() != '#') {retval += '#';}
-            retval += c;
-            if(c.back() != '\n') {retval += '\n';}
+            if(c.front() != '#') {retval += char_type('#');}
+            retval += string_conv<string_type>(c);
+            if(c.back() != '\n') {retval += char_type('\n');}
         }
         return retval;
     } // }}}
 
-    std::string format_indent(const indent_char indent_type) const // {{{
+    string_type format_indent(const indent_char indent_type) const // {{{
     {
         const auto indent = static_cast<std::size_t>((std::max)(0, this->current_indent_));
         if(indent_type == indent_char::space)
         {
-            return detail::make_string(indent, ' ');
+            return string_conv<string_type>(make_string(indent, ' '));
         }
         else if(indent_type == indent_char::tab)
         {
-            return detail::make_string(indent, '\t');
+            return string_conv<string_type>(make_string(indent, '\t'));
         }
         else
         {
-            return "";
+            return string_type{};
         }
     } // }}}
 
@@ -1166,29 +1196,38 @@ class serializer
 } // detail
 
 template<typename TC>
-std::string format(const basic_value<TC>& v,
-        const spec s = spec::default_version())
+typename basic_value<TC>::string_type
+format(const basic_value<TC>& v, const spec s = spec::default_version())
 {
     detail::serializer<TC> ser(s);
     return ser(v);
 }
 template<typename TC>
-std::string format(const typename basic_value<TC>::key_type& k,
-        const basic_value<TC>& v,
-        const spec s = spec::default_version())
+typename basic_value<TC>::string_type
+format(const typename basic_value<TC>::key_type& k,
+       const basic_value<TC>& v,
+       const spec s = spec::default_version())
 {
     detail::serializer<TC> ser(s);
     return ser(k, v);
 }
 template<typename TC>
-std::string format(const std::vector<typename basic_value<TC>::key_type>& ks,
-        const basic_value<TC>& v,
-        const spec s = spec::default_version())
+typename basic_value<TC>::string_type
+format(const std::vector<typename basic_value<TC>::key_type>& ks,
+       const basic_value<TC>& v,
+       const spec s = spec::default_version())
 {
     detail::serializer<TC> ser(s);
     return ser(ks, v);
 }
 
+template<typename TC>
+std::ostream& operator<<(std::ostream& os, const basic_value<TC>& v)
+{
+    os << format(v);
+    return os;
+}
+
 } // toml
 
 #if defined(TOML11_COMPILE_SOURCES)
@@ -1197,25 +1236,25 @@ namespace toml
 struct type_config;
 struct ordered_type_config;
 
-extern template std::string
+extern template typename basic_value<type_config>::string_type
 format<type_config>(const basic_value<type_config>&, const spec);
 
-extern template std::string
+extern template typename basic_value<type_config>::string_type
 format<type_config>(const typename basic_value<type_config>::key_type& k,
                     const basic_value<type_config>& v, const spec);
 
-extern template std::string
+extern template typename basic_value<type_config>::string_type
 format<type_config>(const std::vector<typename basic_value<type_config>::key_type>& ks,
                     const basic_value<type_config>& v, const spec s);
 
-extern template std::string
+extern template typename basic_value<type_config>::string_type
 format<ordered_type_config>(const basic_value<ordered_type_config>&, const spec);
 
-extern template std::string
+extern template typename basic_value<type_config>::string_type
 format<ordered_type_config>(const typename basic_value<ordered_type_config>::key_type& k,
                             const basic_value<ordered_type_config>& v, const spec);
 
-extern template std::string
+extern template typename basic_value<type_config>::string_type
 format<ordered_type_config>(const std::vector<typename basic_value<ordered_type_config>::key_type>& ks,
                             const basic_value<ordered_type_config>& v, const spec s);
 

include/toml11/traits.hpp

@@ -83,6 +83,14 @@ struct has_into_toml_method_impl
     static std::false_type check(...);
 };
 
+struct has_template_into_toml_method_impl
+{
+    template<typename T, typename TypeConfig>
+    static std::true_type  check(decltype(std::declval<T>().template into_toml<TypeConfig>())*);
+    template<typename T, typename TypeConfig>
+    static std::false_type check(...);
+};
+
 struct has_specialized_from_impl
 {
     template<typename T>
@@ -126,6 +134,9 @@ struct has_from_toml_method: decltype(has_from_toml_method_impl::check<T, TC>(nu
 template<typename T>
 struct has_into_toml_method: decltype(has_into_toml_method_impl::check<T>(nullptr)){};
 
+template<typename T, typename TypeConfig>
+struct has_template_into_toml_method: decltype(has_template_into_toml_method_impl::check<T, TypeConfig>(nullptr)){};
+
 template<typename T>
 struct has_specialized_from: decltype(has_specialized_from_impl::check<T>(nullptr)){};
 template<typename T>
@@ -162,6 +173,32 @@ struct is_std_forward_list_impl<std::forward_list<T>> : std::true_type{};
 template<typename T>
 using is_std_forward_list = is_std_forward_list_impl<cxx::remove_cvref_t<T>>;
 
+template<typename T> struct is_std_basic_string_impl : std::false_type{};
+template<typename C, typename T, typename A>
+struct is_std_basic_string_impl<std::basic_string<C, T, A>> : std::true_type{};
+template<typename T>
+using is_std_basic_string = is_std_basic_string_impl<cxx::remove_cvref_t<T>>;
+
+template<typename T> struct is_1byte_std_basic_string_impl : std::false_type{};
+template<typename C, typename T, typename A>
+struct is_1byte_std_basic_string_impl<std::basic_string<C, T, A>>
+    : std::integral_constant<bool, sizeof(C) == sizeof(char)> {};
+template<typename T>
+using is_1byte_std_basic_string = is_std_basic_string_impl<cxx::remove_cvref_t<T>>;
+
+#if defined(TOML11_HAS_STRING_VIEW)
+template<typename T> struct is_std_basic_string_view_impl : std::false_type{};
+template<typename C, typename T>
+struct is_std_basic_string_view_impl<std::basic_string_view<C, T>> : std::true_type{};
+template<typename T>
+using is_std_basic_string_view = is_std_basic_string_view_impl<cxx::remove_cvref_t<T>>;
+
+template<typename V, typename S>
+struct is_string_view_of : std::false_type {};
+template<typename C, typename T>
+struct is_string_view_of<std::basic_string_view<C, T>, std::basic_string<C, T>> : std::true_type {};
+#endif
+
 template<typename T> struct is_chrono_duration_impl: std::false_type{};
 template<typename Rep, typename Period>
 struct is_chrono_duration_impl<std::chrono::duration<Rep, Period>>: std::true_type{};

include/toml11/utility.hpp

@@ -99,6 +99,72 @@ inline std::string make_string(std::size_t len, char c)
     return std::string(len, c);
 }
 
+// ---------------------------------------------------------------------------
+
+template<typename Char,  typename Traits, typename Alloc,
+         typename Char2, typename Traits2, typename Alloc2>
+struct string_conv_impl
+{
+    static_assert(sizeof(Char)  == sizeof(char), "");
+    static_assert(sizeof(Char2) == sizeof(char), "");
+
+    static std::basic_string<Char, Traits, Alloc> invoke(std::basic_string<Char2, Traits2, Alloc2> s)
+    {
+        std::basic_string<Char, Traits, Alloc> retval;
+        std::transform(s.begin(), s.end(), std::back_inserter(retval),
+            [](const Char2 c) {return static_cast<Char>(c);});
+        return retval;
+    }
+    template<std::size_t N>
+    static std::basic_string<Char, Traits, Alloc> invoke(const Char2 (&s)[N])
+    {
+        std::basic_string<Char, Traits, Alloc> retval;
+        // "string literal" has null-char at the end. to skip it, we use prev.
+        std::transform(std::begin(s), std::prev(std::end(s)), std::back_inserter(retval),
+            [](const Char2 c) {return static_cast<Char>(c);});
+        return retval;
+    }
+};
+
+template<typename Char,  typename Traits, typename Alloc>
+struct string_conv_impl<Char, Traits, Alloc, Char, Traits, Alloc>
+{
+    static_assert(sizeof(Char) == sizeof(char), "");
+
+    static std::basic_string<Char, Traits, Alloc> invoke(std::basic_string<Char, Traits, Alloc> s)
+    {
+        return s;
+    }
+    template<std::size_t N>
+    static std::basic_string<Char, Traits, Alloc> invoke(const Char (&s)[N])
+    {
+        return std::basic_string<Char, Traits, Alloc>(s);
+    }
+};
+
+template<typename S, typename Char2, typename Traits2, typename Alloc2>
+cxx::enable_if_t<is_std_basic_string<S>::value, S>
+string_conv(std::basic_string<Char2, Traits2, Alloc2> s)
+{
+    using C = typename S::value_type;
+    using T = typename S::traits_type;
+    using A = typename S::allocator_type;
+    return string_conv_impl<C, T, A, Char2, Traits2, Alloc2>::invoke(std::move(s));
+}
+template<typename S, std::size_t N>
+cxx::enable_if_t<is_std_basic_string<S>::value, S>
+string_conv(const char (&s)[N])
+{
+    using C = typename S::value_type;
+    using T = typename S::traits_type;
+    using A = typename S::allocator_type;
+    using C2 = char;
+    using T2 = std::char_traits<C2>;
+    using A2 = std::allocator<C2>;
+
+    return string_conv_impl<C, T, A, C2, T2, A2>::template invoke<N>(s);
+}
+
 } // namespace detail
 } // namespace toml
 #endif // TOML11_UTILITY_HPP

include/toml11/version.hpp

@@ -2,8 +2,8 @@
 #define TOML11_VERSION_HPP
 
 #define TOML11_VERSION_MAJOR 4
-#define TOML11_VERSION_MINOR 0
-#define TOML11_VERSION_PATCH 3
+#define TOML11_VERSION_MINOR 1
+#define TOML11_VERSION_PATCH 0
 
 #ifndef __cplusplus
 #    error "__cplusplus is not defined"

include/toml_fwd.hpp

@@ -40,6 +40,7 @@ enum class string_format : std::uint8_t;
 enum class datetime_delimiter_kind : std::uint8_t;
 enum class array_format : std::uint8_t;
 enum class table_format : std::uint8_t;
+enum class key_format : std::uint8_t;
 
 struct boolean_format_info;
 struct integer_format_info;
@@ -51,6 +52,7 @@ struct local_date_format_info;
 struct local_time_format_info;
 struct array_format_info;
 struct table_format_info;
+struct key_format_info;
 
 template<typename Key, typename Val, typename Cmp, typename Allocator>
 class ordered_map;

src/serializer.cpp

@@ -10,25 +10,25 @@ namespace toml
 struct type_config;
 struct ordered_type_config;
 
-template std::string
+template typename basic_value<type_config>::string_type
 format<type_config>(const basic_value<type_config>&, const spec);
 
-template std::string
+template typename basic_value<type_config>::string_type
 format<type_config>(const typename basic_value<type_config>::key_type& k,
                     const basic_value<type_config>& v, const spec);
 
-template std::string
+template typename basic_value<type_config>::string_type
 format<type_config>(const std::vector<typename basic_value<type_config>::key_type>& ks,
                     const basic_value<type_config>& v, const spec s);
 
-template std::string
+template typename basic_value<type_config>::string_type
 format<ordered_type_config>(const basic_value<ordered_type_config>&, const spec);
 
-template std::string
+template typename basic_value<type_config>::string_type
 format<ordered_type_config>(const typename basic_value<ordered_type_config>::key_type& k,
                             const basic_value<ordered_type_config>& v, const spec);
 
-template std::string
+template typename basic_value<type_config>::string_type
 format<ordered_type_config>(const std::vector<typename basic_value<ordered_type_config>::key_type>& ks,
                             const basic_value<ordered_type_config>& v, const spec s);
 

tests/test_find.cpp

@@ -627,14 +627,29 @@ TEST_CASE("testing toml::find string conversion")
         toml::find<std::string>(v, "key") += "bar";
         CHECK_EQ("foobar", toml::find<std::string>(v, "key"));
     }
+}
 
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
+#if defined(TOML11_HAS_CHAR8_T)
+TEST_CASE("testing toml::find<string-like>")
+{
+    using value_type = toml::value;
     {
         value_type v = toml::table{{"key", "foo"}};
-        CHECK_EQ("foo", toml::find<std::string_view>(v, "key"));
+        CHECK_EQ(u8"foo", toml::find<std::u8string>(v, "key"));
     }
-#endif
 }
+#endif
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
+TEST_CASE("testing toml::get<string_view>")
+{
+    using value_type = toml::value;
+    {
+        value_type v("foo");
+        CHECK_EQ("foo", toml::get<std::string_view>(v));
+    }
+}
+#endif
 
 TEST_CASE("testing toml::find array conversion")
 {

tests/test_format_integer.cpp

@@ -4,6 +4,7 @@
 #include <toml11/parser.hpp>
 #include <toml11/serializer.hpp>
 #include <toml11/types.hpp>
+#include <toml11/literal.hpp>
 
 TEST_CASE("testing decimal")
 {
@@ -25,19 +26,40 @@ TEST_CASE("testing decimal")
 
 TEST_CASE("testing hex")
 {
-    const auto hex_fmt = [](std::size_t w, std::size_t s) {
+    const auto hex_fmt = [](bool u, std::size_t w, std::size_t s) {
         toml::integer_format_info fmt;
         fmt.fmt = toml::integer_format::hex;
+        fmt.uppercase = u;
         fmt.width = w;
         fmt.spacer = s;
         return fmt;
     };
 
-    CHECK_EQ("0xdeadbeef",  toml::format(toml::value(0xDEADBEEF, hex_fmt(8, 0))));
-    CHECK_EQ("0xdead_beef", toml::format(toml::value(0xDEADBEEF, hex_fmt(8, 4))));
-    CHECK_EQ("0xff",        toml::format(toml::value(0xFF,       hex_fmt(2, 0))));
-    CHECK_EQ("0x00ff",      toml::format(toml::value(0xFF,       hex_fmt(4, 0))));
-    CHECK_EQ("0x0000ff",    toml::format(toml::value(0xFF,       hex_fmt(6, 0))));
+    CHECK_EQ("0xdeadbeef",  toml::format(toml::value(0xDEADBEEF, hex_fmt(false, 8, 0))));
+    CHECK_EQ("0xdead_beef", toml::format(toml::value(0xDEADBEEF, hex_fmt(false, 8, 4))));
+    CHECK_EQ("0xff",        toml::format(toml::value(0xFF,       hex_fmt(false, 2, 0))));
+    CHECK_EQ("0x00ff",      toml::format(toml::value(0xFF,       hex_fmt(false, 4, 0))));
+    CHECK_EQ("0x0000ff",    toml::format(toml::value(0xFF,       hex_fmt(false, 6, 0))));
+
+    CHECK_EQ("0xDEADBEEF",  toml::format(toml::value(0xDEADBEEF, hex_fmt(true, 8, 0))));
+    CHECK_EQ("0xDEAD_BEEF", toml::format(toml::value(0xDEADBEEF, hex_fmt(true, 8, 4))));
+    CHECK_EQ("0xFF",        toml::format(toml::value(0xFF,       hex_fmt(true, 2, 0))));
+    CHECK_EQ("0x00FF",      toml::format(toml::value(0xFF,       hex_fmt(true, 4, 0))));
+    CHECK_EQ("0x0000FF",    toml::format(toml::value(0xFF,       hex_fmt(true, 6, 0))));
+
+    using namespace toml::literals::toml_literals;
+
+    CHECK_EQ("0xdeadbeef",  toml::format("0xdeadbeef"_toml ));
+    CHECK_EQ("0xdead_beef", toml::format("0xdead_beef"_toml));
+    CHECK_EQ("0xff",        toml::format("0xff"_toml       ));
+    CHECK_EQ("0x00ff",      toml::format("0x00ff"_toml     ));
+    CHECK_EQ("0x0000ff",    toml::format("0x0000ff"_toml   ));
+
+    CHECK_EQ("0xDEADBEEF",  toml::format("0xDEADBEEF"_toml ));
+    CHECK_EQ("0xDEAD_BEEF", toml::format("0xDEAD_BEEF"_toml));
+    CHECK_EQ("0xFF",        toml::format("0xFF"_toml       ));
+    CHECK_EQ("0x00FF",      toml::format("0x00FF"_toml     ));
+    CHECK_EQ("0x0000FF",    toml::format("0x0000FF"_toml   ));
 }
 
 TEST_CASE("testing oct")

tests/test_get.cpp

@@ -240,8 +240,19 @@ TEST_CASE("testing toml::get<floating-like>")
     }
 }
 
-#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
+#if defined(TOML11_HAS_CHAR8_T)
 TEST_CASE("testing toml::get<string-like>")
+{
+    using value_type = toml::value;
+    {
+        value_type v("foo");
+        CHECK_EQ(u8"foo", toml::get<std::u8string>(v));
+    }
+}
+#endif
+
+#if TOML11_CPLUSPLUS_STANDARD_VERSION >= 201703L
+TEST_CASE("testing toml::get<string_view>")
 {
     using value_type = toml::value;
     {

tests/test_parse_integer.cpp

@@ -34,23 +34,29 @@ TEST_CASE("testing decimal_value")
 TEST_CASE("testing hex_value")
 {
     toml::detail::context<toml::type_config> ctx(toml::spec::v(1,0,0));
-    const auto hex_fmt = [](std::size_t w, std::size_t s) {
+    const auto hex_fmt = [](bool u, std::size_t w, std::size_t s) {
         toml::integer_format_info fmt;
         fmt.fmt = toml::integer_format::hex;
+        fmt.uppercase = u;
         fmt.width = w;
         fmt.spacer = s;
         return fmt;
     };
 
-    toml11_test_parse_success<toml::value_t::integer>("0xDEADBEEF",  0xDEADBEEF, comments(), hex_fmt(8, 0), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0xdeadbeef",  0xDEADBEEF, comments(), hex_fmt(8, 0), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0xDEADbeef",  0xDEADBEEF, comments(), hex_fmt(8, 0), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0xDEAD_BEEF", 0xDEADBEEF, comments(), hex_fmt(8, 4), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0xdead_beef", 0xDEADBEEF, comments(), hex_fmt(8, 4), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0xdead_BEEF", 0xDEADBEEF, comments(), hex_fmt(8, 4), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0xFF",        0xFF,       comments(), hex_fmt(2, 0), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0x00FF",      0xFF,       comments(), hex_fmt(4, 0), ctx);
-    toml11_test_parse_success<toml::value_t::integer>("0x0000FF",    0xFF,       comments(), hex_fmt(6, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xDEADBEEF",  0xDEADBEEF, comments(), hex_fmt(true,  8, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xdeadbeef",  0xDEADBEEF, comments(), hex_fmt(false, 8, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xDEADbeef",  0xDEADBEEF, comments(), hex_fmt(true,  8, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xDEAD_BEEF", 0xDEADBEEF, comments(), hex_fmt(true,  8, 4), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xdead_beef", 0xDEADBEEF, comments(), hex_fmt(false, 8, 4), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xdead_BEEF", 0xDEADBEEF, comments(), hex_fmt(true,  8, 4), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xFF",        0xFF,       comments(), hex_fmt(true,  2, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0x00FF",      0xFF,       comments(), hex_fmt(true,  4, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0x0000FF",    0xFF,       comments(), hex_fmt(true,  6, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0xff",        0xFF,       comments(), hex_fmt(false, 2, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0x00ff",      0xFF,       comments(), hex_fmt(false, 4, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0x0000ff",    0xFF,       comments(), hex_fmt(false, 6, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0x00",        0,          comments(), hex_fmt(true,  2, 0), ctx);
+    toml11_test_parse_success<toml::value_t::integer>("0x12345",     0x12345,    comments(), hex_fmt(true,  5, 0), ctx);
 }
 
 TEST_CASE("testing oct_value")

tests/test_user_defined_conversion.cpp

@@ -49,6 +49,25 @@ struct foobar
     int a;
     std::string b;
 };
+
+struct corge
+{
+    int a;
+    std::string b;
+
+    void from_toml(const toml::value& v)
+    {
+        this->a = toml::find<int>(v, "a");
+        this->b = toml::find<std::string>(v, "b");
+        return ;
+    }
+
+    template <typename TC>
+    toml::basic_value<TC> into_toml() const
+    {
+        return toml::basic_value<TC>(typename toml::basic_value<TC>::table_type{{"a", this->a}, {"b", this->b}});
+    }
+};
 } // extlib
 
 namespace toml
@@ -215,6 +234,19 @@ TEST_CASE("test_conversion_by_member_methods")
         CHECK(v == v2);
     }
 
+    
+    {
+        const toml::value v(toml::table{{"a", 42}, {"b", "baz"}});
+
+        const auto corge = toml::get<extlib::corge>(v);
+        CHECK_EQ(corge.a, 42);
+        CHECK_EQ(corge.b, "baz");
+
+        const toml::value v2(corge);
+
+        CHECK_EQ(v, v2);
+    }
+
     {
         const toml::ordered_value v(toml::ordered_table{{"a", 42}, {"b", "baz"}});
 

tests/test_value.cpp

@@ -587,6 +587,105 @@ TEST_CASE("testing constructor (string_view)")
 }
 #endif
 
+#ifdef TOML11_HAS_CHAR8_T
+TEST_CASE("testing constructor (u8string)")
+{
+    toml::string_format_info fmt;
+    fmt.fmt    = toml::string_format::basic;
+
+    const std::string eq("hoge");
+    const std::string ne("fuga");
+
+    const std::u8string ref(u8"hoge");
+
+    {
+        toml::value x(ref);
+        test_is_type(x, toml::value_t::string);
+        test_as_type_throws(x, toml::value_t::string);
+        test_as_type_fmt_throws(x, toml::value_t::string);
+
+        test_as_type<toml::value_t::string>(x, "hoge", "fuga");
+    }
+    // -----------------------------------------------------------------------
+    {
+        toml::value x_with_comments(ref, std::vector<std::string>{"foo", "bar"});
+
+        test_is_type           (x_with_comments, toml::value_t::string);
+        test_as_type_throws    (x_with_comments, toml::value_t::string);
+        test_as_type_fmt_throws(x_with_comments, toml::value_t::string);
+
+        test_as_type<toml::value_t::string>(x_with_comments, eq, ne);
+
+        CHECK_EQ(x_with_comments.comments().size(), 2);
+        CHECK_EQ(x_with_comments.comments().at(0), "foo");
+        CHECK_EQ(x_with_comments.comments().at(1), "bar");
+
+        CHECK_EQ(x_with_comments.location().is_ok(), false);
+    }
+    // -----------------------------------------------------------------------
+    {
+        toml::value x_with_format(eq, fmt);
+
+        test_is_type           (x_with_format, toml::value_t::string);
+        test_as_type_throws    (x_with_format, toml::value_t::string);
+        test_as_type_fmt_throws(x_with_format, toml::value_t::string);
+
+        test_as_type    <toml::value_t::string>(x_with_format, eq, ne);
+        test_as_type_fmt<toml::value_t::string>(x_with_format, fmt);
+
+        CHECK_EQ(x_with_format.comments().size(), 0);
+        CHECK_EQ(x_with_format.location().is_ok(), false);
+    }
+    // -----------------------------------------------------------------------
+    {
+        toml::value x_with_com_fmt(ref, fmt,
+                std::vector<std::string>{"foo", "bar"});
+
+        test_is_type           (x_with_com_fmt, toml::value_t::string);
+        test_as_type_throws    (x_with_com_fmt, toml::value_t::string);
+        test_as_type_fmt_throws(x_with_com_fmt, toml::value_t::string);
+
+        test_as_type    <toml::value_t::string>(x_with_com_fmt, eq, ne);
+        test_as_type_fmt<toml::value_t::string>(x_with_com_fmt, fmt);
+
+        CHECK_EQ(x_with_com_fmt.comments().size(), 2);
+        CHECK_EQ(x_with_com_fmt.comments().at(0), "foo");
+        CHECK_EQ(x_with_com_fmt.comments().at(1), "bar");
+
+        CHECK_EQ(x_with_com_fmt.location().is_ok(), false);
+    }
+    // -----------------------------------------------------------------------
+    {
+        toml::value x_assign(ne, fmt);
+        x_assign = ref;
+
+        test_is_type           (x_assign, toml::value_t::string);
+        test_as_type_throws    (x_assign, toml::value_t::string);
+        test_as_type_fmt_throws(x_assign, toml::value_t::string);
+
+        test_as_type    <toml::value_t::string>(x_assign, eq, ne);
+        test_as_type_fmt<toml::value_t::string>(x_assign, fmt);
+
+        CHECK_EQ(x_assign.comments().size(), 0);
+        CHECK_EQ(x_assign.location().is_ok(), false);
+    }
+    // -----------------------------------------------------------------------
+    {
+        toml::value x_assign_different_type(true);
+        x_assign_different_type = ref;
+
+        test_is_type           (x_assign_different_type, toml::value_t::string);
+        test_as_type_throws    (x_assign_different_type, toml::value_t::string);
+        test_as_type_fmt_throws(x_assign_different_type, toml::value_t::string);
+
+        test_as_type<toml::value_t::string>(x_assign_different_type, eq, ne);
+
+        CHECK_EQ(x_assign_different_type.comments().size(), 0);
+        CHECK_EQ(x_assign_different_type.location().is_ok(), false);
+    }
+}
+#endif
+
 TEST_CASE("testing constructor (local_date)")
 {
     toml::local_date_format_info fmt;

tests/utility.cpp

@@ -29,10 +29,11 @@ namespace toml
 std::ostream& operator<<(std::ostream& os, const integer_format_info& fmt)
 {
     os << "integer_format_info{";
-    os << "fmt = "      << fmt.fmt    << ", ";
-    os << "width = "    << fmt.width  << ", ";
-    os << "spacer = "   << fmt.spacer << ", ";
-    os << "suffix = \"" << fmt.suffix << "\"}";
+    os << "fmt = "       << fmt.fmt       << ", ";
+    os << "uppercase = " << fmt.uppercase << ", ";
+    os << "width = "     << fmt.width     << ", ";
+    os << "spacer = "    << fmt.spacer    << ", ";
+    os << "suffix = \""  << fmt.suffix    << "\"}";
     return os;
 }
 std::ostream& operator<<(std::ostream& os, const floating_format_info& fmt)

tests/utility.hpp

@@ -86,13 +86,6 @@ void toml11_test_parse_failure(F fn, std::string in, toml::detail::context<TC> c
 
 namespace toml
 {
-template<typename TC>
-std::ostream& operator<<(std::ostream& os, const toml::basic_value<TC>& v)
-{
-    os << toml::format(v);
-    return os;
-}
-
 std::ostream& operator<<(std::ostream& os, const integer_format_info&);
 std::ostream& operator<<(std::ostream& os, const floating_format_info&);
 std::ostream& operator<<(std::ostream& os, const string_format_info&);