feat: avoid double checking in helper methods

The top-level table has its region at the first character of the file.
That means that, in the case when a key is not found in the top-level
table, the error message points to the first character. If the file has
its first table at the first line, the error message would be like this.
```console
[error] key "a" not found
 --> example.toml
   |
 1 | [table]
   | ^------ in this table
```
It actually points to the top-level table at the first character,
not `[table]`. But it is too confusing. To avoid the confusion, the
error message should explicitly say "key not found in the top-level
table".

.travis.yml

@@ -2,6 +2,30 @@ dist: trusty
 
 matrix:
   include:
+  - os: linux
+    language: cpp
+    compiler: gcc
+    env: COMPILER="g++-4.8" CXX_STANDARD=11
+    addons:
+      apt:
+        sources:
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
+        - sourceline: 'ppa:mhier/libboost-latest'
+        packages:
+        - g++-4.8
+        - boost1.70
+  - os: linux
+    language: cpp
+    compiler: gcc
+    env: COMPILER="g++-4.9" CXX_STANDARD=11
+    addons:
+      apt:
+        sources:
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
+        - sourceline: 'ppa:mhier/libboost-latest'
+        packages:
+        - g++-4.9
+        - boost1.70
   - os: linux
     language: cpp
     compiler: gcc
@@ -269,16 +293,6 @@ matrix:
   - os: osx
     language: cpp
     compiler: clang
-    env: CXX_STANDARD=11
-    cache:
-      directories:
-        - $HOME/Library/Caches/Homebrew
-    addons:
-      homebrew:
-        update: true
-        packages:
-        - cmake
-        - boost
 
 script:
 - |
@@ -288,6 +302,10 @@ script:
     tar xf cmake-3.14.5-Linux-x86_64.tar.gz -C cmake --strip-components=1
     export PATH=${TRAVIS_BUILD_DIR}/cmake/bin:${PATH}
   fi
+- |
+  if [[ "${CXX_STANDARD}" == "" ]]; then
+      export CXX_STANDARD="11"
+  fi
 - |
   if [[ "${TOML_HEAD}" != "ON" ]]; then
     export TOML_HEAD="OFF"
@@ -306,14 +324,8 @@ script:
 - cmake --version
 - mkdir build
 - cd build
-- git clone https://github.com/toml-lang/toml.git
+- echo "COMPILER     = ${COMPILER}"
+- echo "CXX_STANDARD = ${CXX_STANDARD}"
 - cmake -DCMAKE_CXX_COMPILER=$COMPILER -DCMAKE_CXX_STANDARD=$CXX_STANDARD -DTOML11_USE_UNRELEASED_TOML_FEATURES=${TOML_HEAD} -Dtoml11_TEST_WITH_ASAN=${WITH_ASAN} -Dtoml11_TEST_WITH_UBSAN=${WITH_UBSAN} ..
 - make
 - ctest --output-on-failure
-
-# https://stackoverflow.com/a/53331571
-before_cache:
-- |
-  if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then
-    brew cleanup
-  fi

CMakeLists.txt

@@ -1,14 +1,7 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 3.1)
 enable_testing()
 
-project(toml11)
-
-set(toml11_VERSION_MAYOR  3)
-set(toml11_VERSION_MINOR  3)
-set(toml11_VERSION_PATCH  0)
-set(toml11_VERSION
-  "${toml11_VERSION_MAYOR}.${toml11_VERSION_MINOR}.${toml11_VERSION_PATCH}"
-)
+project(toml11 VERSION 3.3.1)
 
 option(toml11_BUILD_TEST "Build toml tests" ON)
 option(toml11_TEST_WITH_ASAN  "use LLVM address sanitizer" OFF)
@@ -16,11 +9,9 @@ option(toml11_TEST_WITH_UBSAN "use LLVM undefined behavior sanitizer" OFF)
 
 include(CheckCXXCompilerFlag)
 if("${CMAKE_VERSION}" VERSION_GREATER 3.1)
-    set(CMAKE_CXX_EXTENSIONS OFF)
-    if(NOT DEFINED CMAKE_CXX_STANDARD)
-        set(CMAKE_CXX_STANDARD 11)
-    endif()
-    set(CXX_STANDARD_REQUIRED ON)
+    set(CMAKE_CXX_STANDARD 11 CACHE STRING "The C++ standard whose features are requested to build all targets.")
+    set(CMAKE_CXX_STANDARD_REQUIRED ON CACHE BOOL "Boolean describing whether the value of CXX_STANDARD is a requirement.")
+    set(CMAKE_CXX_EXTENSIONS OFF CACHE BOOL "Boolean specifying whether compiler specific extensions are requested.")
 else()
     # Manually check for C++11 compiler flag.
     CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)

README.md

@@ -8,16 +8,17 @@ toml11
 [![License](https://img.shields.io/github/license/ToruNiina/toml11.svg?style=flat)](LICENSE)
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.1209136.svg)](https://doi.org/10.5281/zenodo.1209136)
 
-toml11 is a C++11 header-only toml parser/encoder depending only on C++ standard library.
+toml11 is a C++11 (or later) header-only toml parser/encoder depending only on C++ standard library.
 
-compatible to the latest version of
-[TOML v0.5.0](https://github.com/toml-lang/toml/blob/master/versions/en/toml-v0.5.0.md)
-after version 2.0.0.
-
-It passes [the language agnostic test suite for TOML parsers by BurntSushi](https://github.com/BurntSushi/toml-test).
-Not only the test suite itself, a TOML reader/encoder also runs on [CircleCI](https://circleci.com/gh/ToruNiina/toml11).
-You can see the error messages about invalid files and serialization results of valid files at
-[CircleCI](https://circleci.com/gh/ToruNiina/toml11).
+- It is compatible to the latest version of [TOML v0.5.0](https://github.com/toml-lang/toml/blob/master/versions/en/toml-v0.5.0.md).
+- It optionally supports the [unreleased features](#unreleased-toml-features) in the master branch of toml-lang/toml.
+- It is one of the most TOML standard compliant libraries, tested with [the language agnostic test suite for TOML parsers by BurntSushi](https://github.com/BurntSushi/toml-test).
+- It shows highly informative error messages. You can see the error messages about invalid files at [CircleCI](https://circleci.com/gh/ToruNiina/toml11).
+- It has configurable container. You can use any random-access containers and key-value maps as backend containers.
+- It optionally preserves comments without any overhead.
+- It has configurable serializer that supports comments, inline tables, literal strings and multiline strings.
+- It supports user-defined type conversion from/into toml values.
+- It correctly handles UTF-8 sequences, with or without BOM, both on posix and Windows.
 
 ## Example
 
@@ -27,15 +28,25 @@ You can see the error messages about invalid files and serialization results of
 
 int main()
 {
-    const auto data = toml::parse("example.toml");
+    auto data = toml::parse("example.toml");
 
-    // title = "an example toml file"
+    // find a value with the specified type from a table
     std::string title = toml::find<std::string>(data, "title");
-    std::cout << "the title is " << title << std::endl;
 
-    // nums = [1, 2, 3, 4, 5]
-    std::vector<int> nums  = toml::find<std::vector<int>>(data, "nums");
-    std::cout << "the length of `nums` is" << nums.size() << std::endl;
+    // convert the whole array into any container automatically
+    std::vector<int> nums = toml::find<std::vector<int>>(data, "nums");
+
+    // access with STL-like manner
+    if(not data.at("a").contains("b"))
+    {
+        data["a"]["b"] = "c";
+    }
+
+    // pass a fallback
+    std::string name = toml::find_or<std::string>(data, "name", "not found");
+
+    // width-dependent formatting
+    std::cout << std::setw(80) << data << std::endl;
 
     return 0;
 }
@@ -1413,7 +1424,7 @@ const toml::source_location loc = v.location();
 
 ## Exceptions
 
-All the exceptions thrown by toml11 inherits `toml::exception` that inherits
+The following `exception` classes inherits `toml::exception` that inherits
 `std::exception`.
 
 ```cpp
@@ -1440,6 +1451,16 @@ struct exception : public std::exception
 
 It represents where the error occurs.
 
+`syntax_error` will be thrown from `toml::parse` and `_toml` literal.
+`type_error` will be thrown from `toml::get/find`, `toml::value::as_xxx()`, and
+other functions that takes a content inside of `toml::value`.
+
+Note that, currently, from `toml::value::at()` and `toml::find(value, key)`
+may throw an `std::out_of_range` that does not inherits `toml::exception`.
+
+Also, in some cases, most likely in the file open error, it will throw an
+`std::runtime_error`.
+
 ## Colorize Error Messages
 
 By defining `TOML11_COLORIZE_ERROR_MESSAGE`, the error messages from
@@ -1743,13 +1764,12 @@ Such a big change will not happen in the coming years.
 
 ## Running Tests
 
-To run test codes, you need to clone toml-lang/toml repository under `build/` directory
-because some of the test codes read a file in the repository.
+After cloning this repository, run the following command (thank you @jwillikers
+for automating test set fetching!).
 
 ```sh
 $ mkdir build
 $ cd build
-$ git clone https://github.com/toml-lang/toml.git
 $ cmake ..
 $ make
 $ make test
@@ -1783,6 +1803,11 @@ I appreciate the help of the contributors who introduced the great feature to th
   - Suppress warnings in Debug mode
 - OGAWA Kenichi (@kenichiice)
   - Suppress warnings on intel compiler
+- Jordan Williams (@jwillikers)
+  - Fixed clang range-loop-analysis warnings
+  - Fixed feature test macro to suppress -Wundef
+  - Use cache variables in CMakeLists.txt
+  - Automate test set fetching, update and refactor CMakeLists.txt
 
 ## Licensing terms
 

appveyor.yml

@@ -17,10 +17,9 @@ build_script:
   - cd C:\toml11
   - mkdir build
   - cd build
-  - git clone https://github.com/toml-lang/toml.git
-  - file --mime-encoding toml/tests/hard_example_unicode.toml
   - cmake -G"%generator%" -DBOOST_ROOT=C:/Libraries/boost_1_69_0 ..
   - cmake --build . --config "%configuration%"
+  - file --mime-encoding tests/toml/tests/hard_example_unicode.toml
 
 test_script:
   - ctest --build-config "%configuration%" --timeout 300 --output-on-failure

tests/CMakeLists.txt

@@ -1,3 +1,12 @@
+include(ExternalProject)
+ExternalProject_Add(toml
+        SOURCE_DIR ${CMAKE_CURRENT_BINARY_DIR}/toml
+        GIT_REPOSITORY https://github.com/toml-lang/toml
+        GIT_TAG v0.5.0
+        CONFIGURE_COMMAND ""
+        BUILD_COMMAND ""
+        INSTALL_COMMAND "")
+
 set(TEST_NAMES
     test_datetime
     test_string
@@ -48,6 +57,8 @@ CHECK_CXX_COMPILER_FLAG("-Wduplicated-branches" COMPILER_SUPPORTS_WDUPLICATED_BR
 CHECK_CXX_COMPILER_FLAG("-Wlogical-op"          COMPILER_SUPPORTS_WLOGICAL_OP)
 CHECK_CXX_COMPILER_FLAG("-Wuseless-cast"        COMPILER_SUPPORTS_WUSELESS_CAST)
 CHECK_CXX_COMPILER_FLAG("-Wdouble-promotion"    COMPILER_SUPPORTS_WDOUBLE_PROMOTION)
+CHECK_CXX_COMPILER_FLAG("-Wrange-loop-analysis" COMPILER_SUPPORTS_WRANGE_LOOP_ANALYSIS)
+CHECK_CXX_COMPILER_FLAG("-Wundef"               COMPILER_SUPPORTS_WUNDEF)
 
 if(COMPILER_SUPPORTS_WALL)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall")
@@ -82,6 +93,12 @@ endif()
 if(COMPILER_SUPPORTS_WDOUBLE_PROMOTION)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdouble-promotion")
 endif()
+if(COMPILER_SUPPORTS_WRANGE_LOOP_ANALYSIS)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wrange-loop-analysis")
+endif()
+if(COMPILER_SUPPORTS_WUNDEF)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wundef")
+endif()
 
 option(TOML11_USE_UNRELEASED_TOML_FEATURES
     "use features in toml-lang/toml master while testing" OFF)
@@ -150,7 +167,7 @@ foreach(TEST_NAME ${TEST_NAMES})
         endif()
     endif()
 
-    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME} WORKING_DIRECTORY ${PROJECT_BINARY_DIR})
+    add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME} WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 
     # Set the PATH to be able to find Boost DLL
     if(WIN32)

tests/check_toml_test.cpp

@@ -84,9 +84,7 @@ struct json_serializer
         {
             if(!is_first) {std::cout << ", ";}
             is_first = false;
-            std::cout << toml::format(toml::value(elem.first),
-                    std::numeric_limits<std::size_t>::max());
-            std::cout << ':';
+            std::cout << this->format_key(elem.first) << ':';
             toml::visit(*this, elem.second);
         }
         std::cout << '}';
@@ -112,6 +110,12 @@ struct json_serializer
         }
         return retval;
     }
+
+    std::string format_key(const std::string& s) const
+    {
+        const auto quote("\"");
+        return quote + escape_string(s) + quote;
+    }
 };
 
 int main()

tests/test_lex_string.cpp

@@ -57,6 +57,22 @@ BOOST_AUTO_TEST_CASE(test_ml_basic_string)
     TOML11_TEST_LEX_ACCEPT(lex_string,
         "\"\"\"\\\n  The quick brown \\\n\n  fox jumps over \\\n  the lazy dog.\\\n  \"\"\"",
         "\"\"\"\\\n  The quick brown \\\n\n  fox jumps over \\\n  the lazy dog.\\\n  \"\"\"");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "\"\"\"Here are two quotation marks: \"\". Simple enough.\"\"\"",
+        "\"\"\"Here are two quotation marks: \"\". Simple enough.\"\"\"");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "\"\"\"Here are three quotation marks: \"\"\\\".\"\"\"",
+        "\"\"\"Here are three quotation marks: \"\"\\\".\"\"\"");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "\"\"\"Here are fifteen quotation marks: \"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\".\"\"\"",
+        "\"\"\"Here are fifteen quotation marks: \"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\".\"\"\"");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "\"\"\"\"This,\" she said, \"is just a pointless statement.\"\"\"\"",
+        "\"\"\"\"This,\" she said, \"is just a pointless statement.\"\"\"\"");
 }
 
 BOOST_AUTO_TEST_CASE(test_literal_string)
@@ -83,4 +99,16 @@ BOOST_AUTO_TEST_CASE(test_ml_literal_string)
     TOML11_TEST_LEX_ACCEPT(lex_string,
         "'''\nThe first newline is\ntrimmed in raw strings.\n   All other whitespace\n   is preserved.\n'''",
         "'''\nThe first newline is\ntrimmed in raw strings.\n   All other whitespace\n   is preserved.\n'''");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "''''That's still pointless', she said.'''",
+        "''''That's still pointless', she said.'''");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "'''Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".'''",
+        "'''Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".'''");
+
+    TOML11_TEST_LEX_ACCEPT(lex_string,
+        "''''This,' she said, 'is just a pointless statement.''''",
+        "''''This,' she said, 'is just a pointless statement.''''");
 }

tests/test_parse_string.cpp

@@ -29,16 +29,16 @@ BOOST_AUTO_TEST_CASE(test_string)
 
 BOOST_AUTO_TEST_CASE(test_string_value)
 {
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"The quick brown fox jumps over the lazy dog\"",
         toml::value("The quick brown fox jumps over the lazy dog", string_t::basic));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\'The quick brown fox jumps over the lazy dog\'",
         toml::value("The quick brown fox jumps over the lazy dog", string_t::literal));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"\"\"The quick brown fox \\\njumps over the lazy dog\"\"\"",
         toml::value("The quick brown fox jumps over the lazy dog", string_t::basic));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'''The quick brown fox \njumps over the lazy dog'''",
         toml::value("The quick brown fox \njumps over the lazy dog", string_t::literal));
 }
@@ -69,14 +69,17 @@ BOOST_AUTO_TEST_CASE(test_basic_string)
     TOML11_TEST_PARSE_EQUAL(parse_string,
         "\" And when \\\"'s are in the along with # \\\"\"",
         string(" And when \"'s are in the along with # \"", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "\"Here are fifteen apostrophes: '''''''''''''''\"",
+        string("Here are fifteen apostrophes: '''''''''''''''", string_t::basic));
 }
 
 BOOST_AUTO_TEST_CASE(test_basic_string_value)
 {
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"GitHub Cofounder & CEO\\nLikes tater tots and beer.\"",
         value("GitHub Cofounder & CEO\nLikes tater tots and beer.", string_t::basic));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"192.168.1.1\"",
         value("192.168.1.1", string_t::basic));
 #if defined(_MSC_VER) || defined(__INTEL_COMPILER)
@@ -84,16 +87,19 @@ BOOST_AUTO_TEST_CASE(test_basic_string_value)
         "\"\xE4\xB8\xAD\xE5\x9B\xBD\"",
         value("\xE4\xB8\xAD\xE5\x9B\xBD", string_t::basic));
 #else
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"中国\"",
         value("中国", string_t::basic));
 #endif
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"You'll hate me after this - #\"",
         value("You'll hate me after this - #", string_t::basic));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\" And when \\\"'s are in the along with # \\\"\"",
         value(" And when \"'s are in the along with # \"", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "\"Here are fifteen apostrophes: '''''''''''''''\"",
+        value("Here are fifteen apostrophes: '''''''''''''''", string_t::basic));
 }
 
 BOOST_AUTO_TEST_CASE(test_ml_basic_string)
@@ -104,16 +110,41 @@ BOOST_AUTO_TEST_CASE(test_ml_basic_string)
     TOML11_TEST_PARSE_EQUAL(parse_string,
         "\"\"\"\\\n  The quick brown \\\n\n  fox jumps over \\\n  the lazy dog.\\\n  \"\"\"",
         string("The quick brown fox jumps over the lazy dog.", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "\"\"\"Here are two quotation marks: \"\". Simple enough.\"\"\"",
+        string("Here are two quotation marks: \"\". Simple enough.", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "\"\"\"Here are three quotation marks: \"\"\\\".\"\"\"",
+        string("Here are three quotation marks: \"\"\".", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "\"\"\"Here are fifteen quotation marks: \"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\".\"\"\"",
+        string("Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "\"\"\"\"This,\" she said, \"is just a pointless statement.\"\"\"\"",
+        string("\"This,\" she said, \"is just a pointless statement.\"", string_t::basic));
 }
 
 BOOST_AUTO_TEST_CASE(test_ml_basic_string_value)
 {
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"\"\"\nThe quick brown \\\n\n  fox jumps over \\\n  the lazy dog.\"\"\"",
         value("The quick brown fox jumps over the lazy dog.", string_t::basic));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "\"\"\"\\\n  The quick brown \\\n\n  fox jumps over \\\n  the lazy dog.\\\n  \"\"\"",
         value("The quick brown fox jumps over the lazy dog.", string_t::basic));
+
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "\"\"\"Here are two quotation marks: \"\". Simple enough.\"\"\"",
+        value("Here are two quotation marks: \"\". Simple enough.", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "\"\"\"Here are three quotation marks: \"\"\\\".\"\"\"",
+        value("Here are three quotation marks: \"\"\".", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "\"\"\"Here are fifteen quotation marks: \"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\"\"\"\\\".\"\"\"",
+        value("Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".", string_t::basic));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "\"\"\"\"This,\" she said, \"is just a pointless statement.\"\"\"\"",
+        value("\"This,\" she said, \"is just a pointless statement.\"", string_t::basic));
 }
 
 BOOST_AUTO_TEST_CASE(test_literal_string)
@@ -134,16 +165,16 @@ BOOST_AUTO_TEST_CASE(test_literal_string)
 
 BOOST_AUTO_TEST_CASE(test_literal_string_value)
 {
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'C:\\Users\\nodejs\\templates'",
         value("C:\\Users\\nodejs\\templates", string_t::literal));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'\\\\ServerX\\admin$\\system32\\'",
         value("\\\\ServerX\\admin$\\system32\\", string_t::literal));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'Tom \"Dubs\" Preston-Werner'",
         value("Tom \"Dubs\" Preston-Werner", string_t::literal));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'<\\i\\c*\\s*>'",
         value("<\\i\\c*\\s*>", string_t::literal));
 }
@@ -156,16 +187,34 @@ BOOST_AUTO_TEST_CASE(test_ml_literal_string)
     TOML11_TEST_PARSE_EQUAL(parse_string,
         "'''\nThe first newline is\ntrimmed in raw strings.\n   All other whitespace\n   is preserved.\n'''",
         string("The first newline is\ntrimmed in raw strings.\n   All other whitespace\n   is preserved.\n", string_t::literal));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "''''That's still pointless', she said.'''",
+        string("'That's still pointless', she said.", string_t::literal));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "'''Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".'''",
+        string("Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".", string_t::literal));
+    TOML11_TEST_PARSE_EQUAL(parse_string,
+        "''''This,' she said, 'is just a pointless statement.''''",
+        string("'This,' she said, 'is just a pointless statement.'", string_t::literal));
 }
 
 BOOST_AUTO_TEST_CASE(test_ml_literal_string_value)
 {
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'''I [dw]on't need \\d{2} apples'''",
         value("I [dw]on't need \\d{2} apples", string_t::literal));
-    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, 
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
         "'''\nThe first newline is\ntrimmed in raw strings.\n   All other whitespace\n   is preserved.\n'''",
         value("The first newline is\ntrimmed in raw strings.\n   All other whitespace\n   is preserved.\n", string_t::literal));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "''''That's still pointless', she said.'''",
+        value("'That's still pointless', she said.", string_t::literal));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "'''Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".'''",
+        value("Here are fifteen quotation marks: \"\"\"\"\"\"\"\"\"\"\"\"\"\"\".", string_t::literal));
+    TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>,
+        "''''This,' she said, 'is just a pointless statement.''''",
+        value("'This,' she said, 'is just a pointless statement.'", string_t::literal));
 }
 
 BOOST_AUTO_TEST_CASE(test_unicode_escape_sequence)

toml/comments.hpp

@@ -81,6 +81,54 @@ struct preserve_comments
     void assign(std::initializer_list<std::string> ini)  {comments.assign(ini);}
     void assign(size_type n, const std::string& val)     {comments.assign(n, val);}
 
+    // Related to the issue #97.
+    //
+    // It is known that `std::vector::insert` and `std::vector::erase` in
+    // the standard library implementation included in GCC 4.8.5 takes
+    // `std::vector::iterator` instead of `std::vector::const_iterator`.
+    // Because of the const-correctness, we cannot convert a `const_iterator` to
+    // an `iterator`. It causes compilation error in GCC 4.8.5.
+#if defined(__GNUC__) && defined(__GNUC_MINOR__) && defined(__GNUC_PATCHLEVEL__) && !defined(__clang__)
+#  if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) <= 40805
+#    define TOML11_WORKAROUND_GCC_4_8_X_STANDARD_LIBRARY_IMPLEMENTATION
+#  endif
+#endif
+
+#ifdef TOML11_WORKAROUND_GCC_4_8_X_STANDARD_LIBRARY_IMPLEMENTATION
+    iterator insert(iterator p, const std::string& x)
+    {
+        return comments.insert(p, x);
+    }
+    iterator insert(iterator p, std::string&&      x)
+    {
+        return comments.insert(p, std::move(x));
+    }
+    void insert(iterator p, size_type n, const std::string& x)
+    {
+        return comments.insert(p, n, x);
+    }
+    template<typename InputIterator>
+    void insert(iterator p, InputIterator first, InputIterator last)
+    {
+        return comments.insert(p, first, last);
+    }
+    void insert(iterator p, std::initializer_list<std::string> ini)
+    {
+        return comments.insert(p, ini);
+    }
+
+    template<typename ... Ts>
+    iterator emplace(iterator p, Ts&& ... args)
+    {
+        return comments.emplace(p, std::forward<Ts>(args)...);
+    }
+
+    iterator erase(iterator pos) {return comments.erase(pos);}
+    iterator erase(iterator first, iterator last)
+    {
+        return comments.erase(first, last);
+    }
+#else
     iterator insert(const_iterator p, const std::string& x)
     {
         return comments.insert(p, x);
@@ -114,6 +162,7 @@ struct preserve_comments
     {
         return comments.erase(first, last);
     }
+#endif
 
     void swap(preserve_comments& other) {comments.swap(other.comments);}
 

toml/datetime.hpp

@@ -20,7 +20,7 @@ namespace toml
 namespace detail
 {
 // TODO: find more sophisticated way to handle this
-#if _POSIX_C_SOURCE >= 1 || defined(_XOPEN_SOURCE) || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || defined(_POSIX_SOURCE)
+#if (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 1) || defined(_XOPEN_SOURCE) || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || defined(_POSIX_SOURCE)
 inline std::tm localtime_s(const std::time_t* src)
 {
     std::tm dst;
@@ -35,7 +35,7 @@ inline std::tm gmtime_s(const std::time_t* src)
     if (!result) { throw std::runtime_error("gmtime_r failed."); }
     return dst;
 }
-#elif _MSC_VER
+#elif defined(_MSC_VER)
 inline std::tm localtime_s(const std::time_t* src)
 {
     std::tm dst;

toml/get.hpp

@@ -449,10 +449,7 @@ basic_value<C, M, V> const& find(const basic_value<C, M, V>& v, const key& ky)
     const auto& tab = v.as_table();
     if(tab.count(ky) == 0)
     {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), {
-                {std::addressof(detail::get_region(v)), "in this table"}
-            }));
+        detail::throw_key_not_found_error(v, ky);
     }
     return tab.at(ky);
 }
@@ -463,10 +460,7 @@ basic_value<C, M, V>& find(basic_value<C, M, V>& v, const key& ky)
     auto& tab = v.as_table();
     if(tab.count(ky) == 0)
     {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), {
-                {std::addressof(detail::get_region(v)), "in this table"}
-            }));
+        detail::throw_key_not_found_error(v, ky);
     }
     return tab.at(ky);
 }
@@ -477,10 +471,7 @@ basic_value<C, M, V> find(basic_value<C, M, V>&& v, const key& ky)
     typename basic_value<C, M, V>::table_type tab = std::move(v).as_table();
     if(tab.count(ky) == 0)
     {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), {
-                {std::addressof(detail::get_region(v)), "in this table"}
-            }));
+        detail::throw_key_not_found_error(v, ky);
     }
     return basic_value<C, M, V>(std::move(tab.at(ky)));
 }
@@ -542,10 +533,7 @@ find(const basic_value<C, M, V>& v, const key& ky)
     const auto& tab = v.as_table();
     if(tab.count(ky) == 0)
     {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), {
-                {std::addressof(detail::get_region(v)), "in this table"}
-            }));
+        detail::throw_key_not_found_error(v, ky);
     }
     return ::toml::get<T>(tab.at(ky));
 }
@@ -558,10 +546,7 @@ find(basic_value<C, M, V>& v, const key& ky)
     auto& tab = v.as_table();
     if(tab.count(ky) == 0)
     {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), {
-                {std::addressof(detail::get_region(v)), "in this table"}
-            }));
+        detail::throw_key_not_found_error(v, ky);
     }
     return ::toml::get<T>(tab.at(ky));
 }
@@ -574,10 +559,7 @@ find(basic_value<C, M, V>&& v, const key& ky)
     typename basic_value<C, M, V>::table_type tab = std::move(v).as_table();
     if(tab.count(ky) == 0)
     {
-        throw std::out_of_range(detail::format_underline(concat_to_string(
-            "key \"", ky, "\" not found"), {
-                {std::addressof(detail::get_region(v)), "in this table"}
-            }));
+        detail::throw_key_not_found_error(v, ky);
     }
     return ::toml::get<T>(std::move(tab.at(ky)));
 }

toml/lexer.hpp

@@ -154,12 +154,53 @@ using lex_basic_string = sequence<lex_quotation_mark,
                                   repeat<lex_basic_char, unlimited>,
                                   lex_quotation_mark>;
 
+// After toml post-v0.5.0, it is explicitly clarified how quotes in ml-strings
+// are allowed to be used.
+// After this, the following strings are *explicitly* allowed.
+// - One or two `"`s in a multi-line basic string is allowed wherever it is.
+// - Three consecutive `"`s in a multi-line basic string is considered as a delimiter.
+// - One or two `"`s can appear just before or after the delimiter.
+// ```toml
+// str4 = """Here are two quotation marks: "". Simple enough."""
+// str5 = """Here are three quotation marks: ""\"."""
+// str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
+// str7 = """"This," she said, "is just a pointless statement.""""
+// ```
+// In the current implementation (v3.3.0), it is difficult to parse `str7` in
+// the above example. It is difficult to recognize `"` at the end of string body
+// collectly. It will be misunderstood as a `"""` delimiter and an additional,
+// invalid `"`. Like this:
+// ```console
+//   what():  [error] toml::parse_table: invalid line format
+//  --> hoge.toml
+//     |
+//  13 | str7 = """"This," she said, "is just a pointless statement.""""
+//     |                                                               ^- expected newline, but got '"'.
+// ```
+// As a quick workaround for this problem, `lex_ml_basic_string_delim` was
+// splitted into two, `lex_ml_basic_string_open` and `lex_ml_basic_string_close`.
+// `lex_ml_basic_string_open` allows only `"""`. `_close` allows 3-5 `"`s.
+// In parse_ml_basic_string() function, the trailing `"`s will be attached to
+// the string body.
+//
+// Note: This feature is a "clarification". Therefore this change is considered
+//       as a spec that has been defined since the time when the multi-line
+//       basic string was introduced. Although it is a post-v0.5.0 changes,
+//       this change will be activated regardless of the flag,
+//       `TOML11_USE_UNRELEASED_TOML_FEATURES`.
+//
 using lex_ml_basic_string_delim = repeat<lex_quotation_mark, exactly<3>>;
+using lex_ml_basic_string_open  = lex_ml_basic_string_delim;
+using lex_ml_basic_string_close = sequence<
+        repeat<lex_quotation_mark, exactly<3>>,
+        maybe<lex_quotation_mark>, maybe<lex_quotation_mark>
+    >;
+
 #ifdef TOML11_USE_UNRELEASED_TOML_FEATURES
 using lex_ml_basic_unescaped    = exclude<either<in_range<0x00, 0x08>, // 0x09
                                                  in_range<0x0a, 0x1F>, // is tab
-                                                 character<0x5C>,
-                                                 character<0x7F>,
+                                                 character<0x5C>, // backslash
+                                                 character<0x7F>, // DEL
                                                  lex_ml_basic_string_delim>>;
 #else // TOML v0.5.0
 using lex_ml_basic_unescaped    = exclude<either<in_range<0x00,0x1F>,
@@ -176,9 +217,9 @@ using lex_ml_basic_char = either<lex_ml_basic_unescaped, lex_escaped>;
 using lex_ml_basic_body = repeat<either<lex_ml_basic_char, lex_newline,
                                         lex_ml_basic_escaped_newline>,
                                  unlimited>;
-using lex_ml_basic_string = sequence<lex_ml_basic_string_delim,
+using lex_ml_basic_string = sequence<lex_ml_basic_string_open,
                                      lex_ml_basic_body,
-                                     lex_ml_basic_string_delim>;
+                                     lex_ml_basic_string_close>;
 
 using lex_literal_char = exclude<either<in_range<0x00, 0x08>,
                                         in_range<0x10, 0x19>, character<0x27>>>;
@@ -187,7 +228,13 @@ using lex_literal_string = sequence<lex_apostrophe,
                                     repeat<lex_literal_char, unlimited>,
                                     lex_apostrophe>;
 
+// the same reason as above.
 using lex_ml_literal_string_delim = repeat<lex_apostrophe, exactly<3>>;
+using lex_ml_literal_string_open  = lex_ml_literal_string_delim;
+using lex_ml_literal_string_close = sequence<
+        repeat<lex_apostrophe, exactly<3>>,
+        maybe<lex_apostrophe>, maybe<lex_apostrophe>
+    >;
 
 using lex_ml_literal_char = exclude<either<in_range<0x00, 0x08>,
                                            in_range<0x10, 0x1F>,
@@ -195,9 +242,9 @@ using lex_ml_literal_char = exclude<either<in_range<0x00, 0x08>,
                                            lex_ml_literal_string_delim>>;
 using lex_ml_literal_body = repeat<either<lex_ml_literal_char, lex_newline>,
                                    unlimited>;
-using lex_ml_literal_string = sequence<lex_ml_literal_string_delim,
+using lex_ml_literal_string = sequence<lex_ml_literal_string_open,
                                        lex_ml_literal_body,
-                                       lex_ml_literal_string_delim>;
+                                       lex_ml_literal_string_close>;
 
 using lex_string = either<lex_ml_basic_string,   lex_basic_string,
                           lex_ml_literal_string, lex_literal_string>;

toml/parser.hpp

@@ -375,7 +375,7 @@ parse_ml_basic_string(location<Container>& loc)
         std::string retval;
         retval.reserve(token.unwrap().size());
 
-        auto delim = lex_ml_basic_string_delim::invoke(inner_loc);
+        auto delim = lex_ml_basic_string_open::invoke(inner_loc);
         if(!delim)
         {
             throw internal_error(format_underline(
@@ -410,7 +410,26 @@ parse_ml_basic_string(location<Container>& loc)
                     {{std::addressof(inner_loc), "not sufficient token"}}),
                     source_location(std::addressof(inner_loc)));
             }
-            delim = lex_ml_basic_string_delim::invoke(inner_loc);
+            delim = lex_ml_basic_string_close::invoke(inner_loc);
+        }
+        // `lex_ml_basic_string_close` allows 3 to 5 `"`s to allow 1 or 2 `"`s
+        // at just before the delimiter. Here, we need to attach `"`s at the
+        // end of the string body, if it exists.
+        // For detail, see the definition of `lex_ml_basic_string_close`.
+        assert(std::all_of(delim.unwrap().first(), delim.unwrap().last(),
+                           [](const char c) noexcept {return c == '\"';}));
+        switch(delim.unwrap().size())
+        {
+            case 3: {break;}
+            case 4: {retval += "\"";  break;}
+            case 5: {retval += "\"\""; break;}
+            default:
+            {
+                throw internal_error(format_underline(
+                    "parse_ml_basic_string: closing delimiter has invalid length",
+                    {{std::addressof(inner_loc), "end of this"}}),
+                    source_location(std::addressof(inner_loc)));
+            }
         }
         return ok(std::make_pair(toml::string(retval), token.unwrap()));
     }
@@ -485,7 +504,7 @@ parse_ml_literal_string(location<Container>& loc)
     {
         location<std::string> inner_loc(loc.name(), token.unwrap().str());
 
-        const auto open = lex_ml_literal_string_delim::invoke(inner_loc);
+        const auto open = lex_ml_literal_string_open::invoke(inner_loc);
         if(!open)
         {
             throw internal_error(format_underline(
@@ -498,7 +517,7 @@ parse_ml_literal_string(location<Container>& loc)
 
         const auto body = lex_ml_literal_body::invoke(inner_loc);
 
-        const auto close = lex_ml_literal_string_delim::invoke(inner_loc);
+        const auto close = lex_ml_literal_string_close::invoke(inner_loc);
         if(!close)
         {
             throw internal_error(format_underline(
@@ -506,9 +525,29 @@ parse_ml_literal_string(location<Container>& loc)
                 {{std::addressof(inner_loc), "should be '''"}}),
                 source_location(std::addressof(inner_loc)));
         }
-        return ok(std::make_pair(
-                  toml::string(body.unwrap().str(), toml::string_t::literal),
-                  token.unwrap()));
+        // `lex_ml_literal_string_close` allows 3 to 5 `'`s to allow 1 or 2 `'`s
+        // at just before the delimiter. Here, we need to attach `'`s at the
+        // end of the string body, if it exists.
+        // For detail, see the definition of `lex_ml_basic_string_close`.
+
+        std::string retval = body.unwrap().str();
+        assert(std::all_of(close.unwrap().first(), close.unwrap().last(),
+                           [](const char c) noexcept {return c == '\'';}));
+        switch(close.unwrap().size())
+        {
+            case 3: {break;}
+            case 4: {retval += "'";  break;}
+            case 5: {retval += "''"; break;}
+            default:
+            {
+                throw internal_error(format_underline(
+                    "parse_ml_literal_string: closing delimiter has invalid length",
+                    {{std::addressof(inner_loc), "end of this"}}),
+                    source_location(std::addressof(inner_loc)));
+            }
+        }
+        return ok(std::make_pair(toml::string(retval, toml::string_t::literal),
+                                 token.unwrap()));
     }
     else
     {

toml/region.hpp

@@ -55,7 +55,7 @@ struct region_base
     // number of characters in the line after the region
     virtual std::size_t after()    const noexcept {return 0;}
 
-    virtual std::vector<std::string> comments()const {return {};}
+    virtual std::vector<std::string> comments() const {return {};}
     // ```toml
     // # comment_before
     // key = "value" # comment_inline
@@ -511,7 +511,7 @@ inline std::string format_underline(const std::string& message,
         retval << '\n';
         retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ');
         retval << color::bold << color::blue << " | " << color::reset;
-        for(const auto help : helps)
+        for(const auto& help : helps)
         {
             retval << color::bold << "\nHint: " << color::reset;
             retval << help;

toml/serializer.hpp

@@ -22,39 +22,62 @@ namespace toml
 // Since toml restricts characters available in a bare key, generally a string
 // should be escaped. But checking whether a string needs to be surrounded by
 // a `"` and escaping some special character is boring.
-inline std::string format_key(const toml::key& key)
+template<typename charT, typename traits, typename Alloc>
+std::basic_string<charT, traits, Alloc>
+format_key(const std::basic_string<charT, traits, Alloc>& key)
 {
+    // check the key can be a bare (unquoted) key
     detail::location<toml::key> loc(key, key);
     detail::lex_unquoted_key::invoke(loc);
     if(loc.iter() == loc.end())
     {
         return key; // all the tokens are consumed. the key is unquoted-key.
     }
-    std::string token("\"");
+
+    //if it includes special characters, then format it in a "quoted" key.
+    std::basic_string<charT, traits, Alloc> serialized("\"");
     for(const char c : key)
     {
         switch(c)
         {
-            case '\\': {token += "\\\\"; break;}
-            case '\"': {token += "\\\""; break;}
-            case '\b': {token += "\\b";  break;}
-            case '\t': {token += "\\t";  break;}
-            case '\f': {token += "\\f";  break;}
-            case '\n': {token += "\\n";  break;}
-            case '\r': {token += "\\r";  break;}
-            default  : {token += c;      break;}
+            case '\\': {serialized += "\\\\"; break;}
+            case '\"': {serialized += "\\\""; break;}
+            case '\b': {serialized += "\\b";  break;}
+            case '\t': {serialized += "\\t";  break;}
+            case '\f': {serialized += "\\f";  break;}
+            case '\n': {serialized += "\\n";  break;}
+            case '\r': {serialized += "\\r";  break;}
+            default  : {serialized += c;      break;}
         }
     }
-    token += "\"";
-    return token;
+    serialized += "\"";
+    return serialized;
 }
 
-template<typename Comment,
-         template<typename ...> class Table,
-         template<typename ...> class Array>
+template<typename charT, typename traits, typename Alloc>
+std::basic_string<charT, traits, Alloc>
+format_keys(const std::vector<std::basic_string<charT, traits, Alloc>>& keys)
+{
+    std::basic_string<charT, traits, Alloc> serialized;
+    if(keys.empty()) {return serialized;}
+
+    for(const auto& ky : keys)
+    {
+        serialized += format_key(ky);
+        serialized += charT('.');
+    }
+    serialized.pop_back(); // remove the last dot '.'
+    return serialized;
+}
+
+template<typename Value>
 struct serializer
 {
-    using value_type           = basic_value<Comment, Table, Array>;
+    static_assert(detail::is_basic_value<Value>::value,
+                  "toml::serializer is for toml::value and its variants, "
+                  "toml::basic_value<...>.");
+
+    using value_type           = Value;
     using key_type             = typename value_type::key_type            ;
     using comment_type         = typename value_type::comment_type        ;
     using boolean_type         = typename value_type::boolean_type        ;
@@ -119,12 +142,17 @@ struct serializer
             return token; // there is no exponent part. just return it.
         }
 #ifdef TOML11_USE_UNRELEASED_TOML_FEATURES
-        // Although currently it is not released yet, TOML will allow
-        // zero-prefix in an exponent part such as 1.234e+01.
-        // The following code removes the zero prefixes.
-        // If the feature is activated, the following codes can be skipped.
+        // Although currently it is not released yet as a tagged version,
+        // TOML will allow zero-prefix in an exponent part, such as `1.234e+01`.
+        // ```toml
+        // num1 = 1.234e+1  # OK in TOML v0.5.0
+        // num2 = 1.234e+01 # error in TOML v0.5.0 but will be allowed soon
+        // ```
+        // To avoid `e+01`, the following `else` section removes the zero
+        // prefixes in the exponent part.
+        // If the feature is activated, it can be skipped.
         return token;
-#endif
+#else
         // zero-prefix in an exponent is NOT allowed in TOML v0.5.0.
         // remove it if it exists.
         bool        sign_exists = false;
@@ -143,17 +171,29 @@ struct serializer
                         zero_prefix);
         }
         return token;
+#endif
     }
     std::string operator()(const string_type& s) const
     {
         if(s.kind == string_t::basic)
         {
-            if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend())
+            if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
+               std::find(s.str.cbegin(), s.str.cend(), '\"') != s.str.cend())
             {
-                // if linefeed is contained, make it multiline-string.
-                const std::string open("\"\"\"\n");
-                const std::string close("\\\n\"\"\"");
-                return open + this->escape_ml_basic_string(s.str) + close;
+                // if linefeed or double-quote is contained,
+                // make it multiline basic string.
+                const auto escaped = this->escape_ml_basic_string(s.str);
+                std::string open("\"\"\"");
+                std::string close("\"\"\"");
+                if(escaped.find('\n') != std::string::npos ||
+                   this->width_ < escaped.size() + 6)
+                {
+                    // if the string body contains newline or is enough long,
+                    // add newlines after and before delimiters.
+                    open += "\n";
+                    close = std::string("\\\n") + close;
+                }
+                return open + escaped + close;
             }
 
             // no linefeed. try to make it oneline-string.
@@ -194,7 +234,11 @@ struct serializer
             if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
                std::find(s.str.cbegin(), s.str.cend(), '\'') != s.str.cend() )
             {
-                const std::string open("'''\n");
+                std::string open("'''");
+                if(this->width_ + 6 < s.str.size())
+                {
+                    open += '\n'; // the first newline is ignored by TOML spec
+                }
                 const std::string close("'''");
                 return open + s.str + close;
             }
@@ -249,7 +293,7 @@ struct serializer
                 std::string token;
                 if(!keys_.empty())
                 {
-                    token += this->serialize_key(keys_.back());
+                    token += format_key(keys_.back());
                     token += " = ";
                 }
                 bool failed = false;
@@ -305,7 +349,7 @@ struct serializer
                     }
                 }
                 token += "[[";
-                token += this->serialize_dotted_key(keys_);
+                token += format_keys(keys_);
                 token += "]]\n";
                 token += this->make_multiline_table(item.as_table());
             }
@@ -417,7 +461,7 @@ struct serializer
             std::string token;
             if(!this->keys_.empty())
             {
-                token += this->serialize_key(this->keys_.back());
+                token += format_key(this->keys_.back());
                 token += " = ";
             }
             token += this->make_inline_table(v);
@@ -432,7 +476,7 @@ struct serializer
         if(!keys_.empty())
         {
             token += '[';
-            token += this->serialize_dotted_key(keys_);
+            token += format_keys(keys_);
             token += "]\n";
         }
         token += this->make_multiline_table(v);
@@ -441,25 +485,6 @@ struct serializer
 
   private:
 
-    std::string serialize_key(const toml::key& key) const
-    {
-        return ::toml::format_key(key);
-    }
-
-    std::string serialize_dotted_key(const std::vector<toml::key>& keys) const
-    {
-        std::string token;
-        if(keys.empty()){return token;}
-
-        for(const auto& k : keys)
-        {
-            token += this->serialize_key(k);
-            token += '.';
-        }
-        token.erase(token.size() - 1, 1); // remove trailing `.`
-        return token;
-    }
-
     std::string escape_basic_string(const std::string& s) const
     {
         //XXX assuming `s` is a valid utf-8 sequence.
@@ -489,7 +514,9 @@ struct serializer
             switch(*i)
             {
                 case '\\': {retval += "\\\\"; break;}
-                case '\"': {retval += "\\\""; break;}
+                // One or two consecutive "s are allowed.
+                // Later we will check there are no three consecutive "s.
+                //   case '\"': {retval += "\\\""; break;}
                 case '\b': {retval += "\\b";  break;}
                 case '\t': {retval += "\\t";  break;}
                 case '\f': {retval += "\\f";  break;}
@@ -510,6 +537,23 @@ struct serializer
                 default: {retval += *i; break;}
             }
         }
+        // Only 1 or 2 consecutive `"`s are allowed in multiline basic string.
+        // 3 consecutive `"`s are considered as a closing delimiter.
+        // We need to check if there are 3 or more consecutive `"`s and insert
+        // backslash to break them down into several short `"`s like the `str6`
+        // in the following example.
+        // ```toml
+        // str4 = """Here are two quotation marks: "". Simple enough."""
+        // # str5 = """Here are three quotation marks: """."""  # INVALID
+        // 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)
+        {
+            retval.replace(found_3_quotes, 3, "\"\"\\\"");
+            found_3_quotes = retval.find("\"\"\"");
+        }
         return retval;
     }
 
@@ -564,7 +608,7 @@ struct serializer
         {
             // in inline tables, trailing comma is not allowed (toml-lang #569).
             if(is_first) {is_first = false;} else {token += ',';}
-            token += this->serialize_key(kv.first);
+            token += format_key(kv.first);
             token += '=';
             token += visit(serializer(std::numeric_limits<std::size_t>::max(),
                                       this->float_prec_, true), kv.second);
@@ -579,7 +623,7 @@ struct serializer
 
         // print non-table stuff first. because after printing [foo.bar], the
         // remaining non-table values will be assigned into [foo.bar], not [foo]
-        for(const auto kv : v)
+        for(const auto& kv : v)
         {
             if(kv.second.is_table() || is_array_of_tables(kv.second))
             {
@@ -595,7 +639,7 @@ struct serializer
                     token += '\n';
                 }
             }
-            const auto key_and_sep    = this->serialize_key(kv.first) + " = ";
+            const auto key_and_sep    = format_key(kv.first) + " = ";
             const auto residual_width = (this->width_ > key_and_sep.size()) ?
                                         this->width_ - key_and_sep.size() : 0;
             token += key_and_sep;
@@ -677,6 +721,7 @@ format(const basic_value<C, M, V>& v, std::size_t w = 80u,
        int fprec = std::numeric_limits<toml::floating>::max_digits10,
        bool no_comment = false, bool force_inline = false)
 {
+    using value_type = basic_value<C, M, V>;
     // if value is a table, it is considered to be a root object.
     // the root object can't be an inline table.
     if(v.is_table())
@@ -687,10 +732,10 @@ format(const basic_value<C, M, V>& v, std::size_t w = 80u,
             oss << v.comments();
             oss << '\n'; // to split the file comment from the first element
         }
-        oss << visit(serializer<C, M, V>(w, fprec, no_comment, false), v);
+        oss << visit(serializer<value_type>(w, fprec, no_comment, false), v);
         return oss.str();
     }
-    return visit(serializer<C, M, V>(w, fprec, force_inline), v);
+    return visit(serializer<value_type>(w, fprec, force_inline), v);
 }
 
 namespace detail
@@ -726,6 +771,8 @@ template<typename charT, typename traits, typename C,
 std::basic_ostream<charT, traits>&
 operator<<(std::basic_ostream<charT, traits>& os, const basic_value<C, M, V>& v)
 {
+    using value_type = basic_value<C, M, V>;
+
     // get status of std::setw().
     const auto w     = static_cast<std::size_t>(os.width());
     const int  fprec = static_cast<int>(os.precision());
@@ -741,7 +788,7 @@ operator<<(std::basic_ostream<charT, traits>& os, const basic_value<C, M, V>& v)
         os << '\n'; // to split the file comment from the first element
     }
     // the root object can't be an inline table. so pass `false`.
-    os << visit(serializer<C, M, V>(w, fprec, false, no_comment), v);
+    os << visit(serializer<value_type>(w, fprec, false, no_comment), v);
 
     // if v is a non-table value, and has only one comment, then
     // put a comment just after a value. in the following way.

toml/value.hpp

@@ -20,223 +20,145 @@ namespace detail
 {
 
 // to show error messages. not recommended for users.
-template<typename C, template<typename ...> class T, template<typename ...> class A>
-region_base const& get_region(const basic_value<C, T, A>&);
-template<typename Region,
-         typename C, template<typename ...> class T, template<typename ...> class A>
-void change_region(basic_value<C, T, A>&, Region&&);
-
-template<value_t Expected,
-         typename C, template<typename ...> class T, template<typename ...> class A>
-[[noreturn]] inline void
-throw_bad_cast(value_t actual, const ::toml::basic_value<C, T, A>& v)
+template<typename Value>
+inline region_base const& get_region(const Value& v)
 {
-    throw type_error(detail::format_underline(concat_to_string(
-        "toml::value: bad_cast to ", Expected), {
+    return *(v.region_info_);
+}
+
+template<typename Value, typename Region>
+void change_region(Value& v, Region&& reg)
+{
+    using region_type = typename std::remove_reference<
+        typename std::remove_cv<Region>::type
+        >::type;
+
+    std::shared_ptr<region_base> new_reg =
+        std::make_shared<region_type>(std::forward<region_type>(reg));
+    v.region_info_ = new_reg;
+    return;
+}
+
+template<value_t Expected, typename Value>
+[[noreturn]] inline void
+throw_bad_cast(const std::string& funcname, value_t actual, const Value& v)
+{
+    throw type_error(detail::format_underline(
+        concat_to_string(funcname, "bad_cast to ", Expected), {
             {std::addressof(get_region(v)),
              concat_to_string("the actual type is ", actual)}
         }), v.location());
 }
 
-// switch by `value_t` and call the corresponding `value::as_xxx()`. {{{
+// Throw `out_of_range` from `toml::value::at()` and `toml::find()`
+// after generating an error message.
+//
+// The implementation is a bit complicated and there are many edge-cases.
+// If you are not interested in the error message generation, just skip this.
+template<typename Value>
+[[noreturn]] void
+throw_key_not_found_error(const Value& v, const key& ky)
+{
+    // The top-level table has its region at the first character of the file.
+    // That means that, in the case when a key is not found in the top-level
+    // table, the error message points to the first character. If the file has
+    // its first table at the first line, the error message would be like this.
+    // ```console
+    // [error] key "a" not found
+    //  --> example.toml
+    //    |
+    //  1 | [table]
+    //    | ^------ in this table
+    // ```
+    // It actually points to the top-level table at the first character,
+    // not `[table]`. But it is too confusing. To avoid the confusion, the error
+    // message should explicitly say "key not found in the top-level table".
+    const auto& reg = get_region(v);
+    if(reg.line_num() == "1" && reg.size() == 1)
+    {
+        // Here it assumes that top-level table starts at the first character.
+        // The region corresponds to the top-level table will be generated at
+        // `parse_toml_file` function.
+        //     It also assumes that the top-level table size is just one and
+        // the line number is `1`. It is always satisfied. And those conditions
+        // are satisfied only if the table is the top-level table.
+        //
+        // 1. one-character dot-key at the first line
+        // ```toml
+        // a.b = "c"
+        // ```
+        // toml11 counts whole key as the table key. Here, `a.b` is the region
+        // of the table "a". It could be counter intuitive, but it works.
+        // The size of the region is 3, not 1. The above example is the shortest
+        // dot-key example. The size cannot be 1.
+        //
+        // 2. one-character inline-table at the first line
+        // ```toml
+        // a = {b = "c"}
+        // ```
+        // toml11 consideres the inline table body as the table region. Here,
+        // `{b = "c"}` is the region of the table "a". The size of the region
+        // is 9, not 1. The shotest inline table still has two characters, `{`
+        // and `}`. The size cannot be 1.
+        //
+        // 3. one-character table declaration at the first line
+        // ```toml
+        // [a]
+        // ```
+        // toml11 consideres the whole table key as the table region. Here,
+        // `[a]` is the table region. The size is 3, not 1.
+        //
+        throw std::out_of_range(format_underline(concat_to_string(
+            "key \"", ky, "\" not found in the top-level table"), {
+                {std::addressof(reg), "the top-level table starts here"}
+            }));
+    }
+    else
+    {
+        // normal table.
+        throw std::out_of_range(format_underline(concat_to_string(
+            "key \"", ky, "\" not found"), {
+                {std::addressof(reg), "in this table"}
+            }));
+    }
+}
+
+// switch by `value_t` at the compile time.
 template<value_t T>
 struct switch_cast {};
-template<>
-struct switch_cast<value_t::boolean>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::boolean& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_boolean();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::boolean const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_boolean();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::boolean&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_boolean();
-    }
-};
-template<>
-struct switch_cast<value_t::integer>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::integer& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_integer();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::integer const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_integer();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::integer&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_integer();
-    }
-};
-template<>
-struct switch_cast<value_t::floating>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::floating& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_floating();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::floating const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_floating();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::floating&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_floating();
-    }
-};
-template<>
-struct switch_cast<value_t::string>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::string& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_string();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::string const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_string();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::string&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_string();
-    }
-};
-template<>
-struct switch_cast<value_t::offset_datetime>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::offset_datetime& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_offset_datetime();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::offset_datetime const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_offset_datetime();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::offset_datetime&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_offset_datetime();
-    }
-};
-template<>
-struct switch_cast<value_t::local_datetime>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_datetime& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_local_datetime();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_datetime const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_local_datetime();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_datetime&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_local_datetime();
-    }
-};
-template<>
-struct switch_cast<value_t::local_date>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_date& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_local_date();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_date const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_local_date();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_date&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_local_date();
-    }
-};
-template<>
-struct switch_cast<value_t::local_time>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_time& invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_local_time();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_time const& invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_local_time();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static ::toml::local_time&& invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_local_time();
-    }
-};
-template<>
-struct switch_cast<value_t::array>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static typename basic_value<C, T, A>::array_type&
-    invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_array();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static typename basic_value<C, T, A>::array_type const&
-    invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_array();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static typename basic_value<C, T, A>::array_type &&
-    invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_array();
-    }
-};
-template<>
-struct switch_cast<value_t::table>
-{
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static typename basic_value<C, T, A>::table_type&
-    invoke(basic_value<C, T, A>& v) noexcept
-    {
-        return v.as_table();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static typename basic_value<C, T, A>::table_type const&
-    invoke(basic_value<C, T, A> const& v) noexcept
-    {
-        return v.as_table();
-    }
-    template<typename C, template<typename ...> class T, template<typename ...> class A>
-    static typename basic_value<C, T, A>::table_type &&
-    invoke(basic_value<C, T, A>&& v) noexcept
-    {
-        return std::move(v).as_table();
-    }
-}; // }}}
+#define TOML11_GENERATE_SWITCH_CASTER(TYPE) \
+    template<>                                                           \
+    struct switch_cast<value_t::TYPE>                                    \
+    {                                                                    \
+        template<typename Value>                                         \
+        static typename Value::TYPE##_type& invoke(Value& v)             \
+        {                                                                \
+            return v.as_##TYPE();                                        \
+        }                                                                \
+        template<typename Value>                                         \
+        static typename Value::TYPE##_type const& invoke(const Value& v) \
+        {                                                                \
+            return v.as_##TYPE();                                        \
+        }                                                                \
+        template<typename Value>                                         \
+        static typename Value::TYPE##_type&& invoke(Value&& v)           \
+        {                                                                \
+            return std::move(v).as_##TYPE();                             \
+        }                                                                \
+    };                                                                   \
+    /**/
+TOML11_GENERATE_SWITCH_CASTER(boolean)
+TOML11_GENERATE_SWITCH_CASTER(integer)
+TOML11_GENERATE_SWITCH_CASTER(floating)
+TOML11_GENERATE_SWITCH_CASTER(string)
+TOML11_GENERATE_SWITCH_CASTER(offset_datetime)
+TOML11_GENERATE_SWITCH_CASTER(local_datetime)
+TOML11_GENERATE_SWITCH_CASTER(local_date)
+TOML11_GENERATE_SWITCH_CASTER(local_time)
+TOML11_GENERATE_SWITCH_CASTER(array)
+TOML11_GENERATE_SWITCH_CASTER(table)
+
+#undef TOML11_GENERATE_SWITCH_CASTER
 
 }// detail
 
@@ -1255,7 +1177,7 @@ class basic_value
     {
         if(this->type_ != T)
         {
-            detail::throw_bad_cast<T>(this->type_, *this);
+            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
         }
         return detail::switch_cast<T>::invoke(*this);
     }
@@ -1264,7 +1186,7 @@ class basic_value
     {
         if(this->type_ != T)
         {
-            detail::throw_bad_cast<T>(this->type_, *this);
+            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
         }
         return detail::switch_cast<T>::invoke(*this);
     }
@@ -1273,7 +1195,7 @@ class basic_value
     {
         if(this->type_ != T)
         {
-            detail::throw_bad_cast<T>(this->type_, *this);
+            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
         }
         return detail::switch_cast<T>::invoke(std::move(*this));
     }
@@ -1317,13 +1239,14 @@ class basic_value
     // ========================================================================
     // throw version
     // ------------------------------------------------------------------------
-    // const reference
+    // const reference {{{
 
     boolean const& as_boolean() const&
     {
         if(this->type_ != value_t::boolean)
         {
-            detail::throw_bad_cast<value_t::boolean>(this->type_, *this);
+            detail::throw_bad_cast<value_t::boolean>(
+                    "toml::value::as_boolean(): ", this->type_, *this);
         }
         return this->boolean_;
     }
@@ -1331,7 +1254,8 @@ class basic_value
     {
         if(this->type_ != value_t::integer)
         {
-            detail::throw_bad_cast<value_t::integer>(this->type_, *this);
+            detail::throw_bad_cast<value_t::integer>(
+                    "toml::value::as_integer(): ", this->type_, *this);
         }
         return this->integer_;
     }
@@ -1339,7 +1263,8 @@ class basic_value
     {
         if(this->type_ != value_t::floating)
         {
-            detail::throw_bad_cast<value_t::floating>(this->type_, *this);
+            detail::throw_bad_cast<value_t::floating>(
+                    "toml::value::as_floating(): ", this->type_, *this);
         }
         return this->floating_;
     }
@@ -1347,7 +1272,8 @@ class basic_value
     {
         if(this->type_ != value_t::string)
         {
-            detail::throw_bad_cast<value_t::string>(this->type_, *this);
+            detail::throw_bad_cast<value_t::string>(
+                    "toml::value::as_string(): ", this->type_, *this);
         }
         return this->string_;
     }
@@ -1355,7 +1281,8 @@ class basic_value
     {
         if(this->type_ != value_t::offset_datetime)
         {
-            detail::throw_bad_cast<value_t::offset_datetime>(this->type_, *this);
+            detail::throw_bad_cast<value_t::offset_datetime>(
+                    "toml::value::as_offset_datetime(): ", this->type_, *this);
         }
         return this->offset_datetime_;
     }
@@ -1363,7 +1290,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_datetime)
         {
-            detail::throw_bad_cast<value_t::local_datetime>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_datetime>(
+                    "toml::value::as_local_datetime(): ", this->type_, *this);
         }
         return this->local_datetime_;
     }
@@ -1371,7 +1299,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_date)
         {
-            detail::throw_bad_cast<value_t::local_date>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_date>(
+                    "toml::value::as_local_date(): ", this->type_, *this);
         }
         return this->local_date_;
     }
@@ -1379,7 +1308,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_time)
         {
-            detail::throw_bad_cast<value_t::local_time>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_time>(
+                    "toml::value::as_local_time(): ", this->type_, *this);
         }
         return this->local_time_;
     }
@@ -1387,7 +1317,8 @@ class basic_value
     {
         if(this->type_ != value_t::array)
         {
-            detail::throw_bad_cast<value_t::array>(this->type_, *this);
+            detail::throw_bad_cast<value_t::array>(
+                    "toml::value::as_array(): ", this->type_, *this);
         }
         return this->array_.value();
     }
@@ -1395,19 +1326,21 @@ class basic_value
     {
         if(this->type_ != value_t::table)
         {
-            detail::throw_bad_cast<value_t::table>(this->type_, *this);
+            detail::throw_bad_cast<value_t::table>(
+                    "toml::value::as_table(): ", this->type_, *this);
         }
         return this->table_.value();
     }
-
+    // }}}
     // ------------------------------------------------------------------------
-    // nonconst reference
+    // nonconst reference {{{
 
     boolean & as_boolean() &
     {
         if(this->type_ != value_t::boolean)
         {
-            detail::throw_bad_cast<value_t::boolean>(this->type_, *this);
+            detail::throw_bad_cast<value_t::boolean>(
+                    "toml::value::as_boolean(): ", this->type_, *this);
         }
         return this->boolean_;
     }
@@ -1415,7 +1348,8 @@ class basic_value
     {
         if(this->type_ != value_t::integer)
         {
-            detail::throw_bad_cast<value_t::integer>(this->type_, *this);
+            detail::throw_bad_cast<value_t::integer>(
+                    "toml::value::as_integer(): ", this->type_, *this);
         }
         return this->integer_;
     }
@@ -1423,7 +1357,8 @@ class basic_value
     {
         if(this->type_ != value_t::floating)
         {
-            detail::throw_bad_cast<value_t::floating>(this->type_, *this);
+            detail::throw_bad_cast<value_t::floating>(
+                    "toml::value::as_floating(): ", this->type_, *this);
         }
         return this->floating_;
     }
@@ -1431,7 +1366,8 @@ class basic_value
     {
         if(this->type_ != value_t::string)
         {
-            detail::throw_bad_cast<value_t::string>(this->type_, *this);
+            detail::throw_bad_cast<value_t::string>(
+                    "toml::value::as_string(): ", this->type_, *this);
         }
         return this->string_;
     }
@@ -1439,7 +1375,8 @@ class basic_value
     {
         if(this->type_ != value_t::offset_datetime)
         {
-            detail::throw_bad_cast<value_t::offset_datetime>(this->type_, *this);
+            detail::throw_bad_cast<value_t::offset_datetime>(
+                    "toml::value::as_offset_datetime(): ", this->type_, *this);
         }
         return this->offset_datetime_;
     }
@@ -1447,7 +1384,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_datetime)
         {
-            detail::throw_bad_cast<value_t::local_datetime>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_datetime>(
+                    "toml::value::as_local_datetime(): ", this->type_, *this);
         }
         return this->local_datetime_;
     }
@@ -1455,7 +1393,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_date)
         {
-            detail::throw_bad_cast<value_t::local_date>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_date>(
+                    "toml::value::as_local_date(): ", this->type_, *this);
         }
         return this->local_date_;
     }
@@ -1463,7 +1402,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_time)
         {
-            detail::throw_bad_cast<value_t::local_time>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_time>(
+                    "toml::value::as_local_time(): ", this->type_, *this);
         }
         return this->local_time_;
     }
@@ -1471,7 +1411,8 @@ class basic_value
     {
         if(this->type_ != value_t::array)
         {
-            detail::throw_bad_cast<value_t::array>(this->type_, *this);
+            detail::throw_bad_cast<value_t::array>(
+                    "toml::value::as_array(): ", this->type_, *this);
         }
         return this->array_.value();
     }
@@ -1479,19 +1420,22 @@ class basic_value
     {
         if(this->type_ != value_t::table)
         {
-            detail::throw_bad_cast<value_t::table>(this->type_, *this);
+            detail::throw_bad_cast<value_t::table>(
+                    "toml::value::as_table(): ", this->type_, *this);
         }
         return this->table_.value();
     }
 
+    // }}}
     // ------------------------------------------------------------------------
-    // rvalue reference
+    // rvalue reference {{{
 
     boolean && as_boolean() &&
     {
         if(this->type_ != value_t::boolean)
         {
-            detail::throw_bad_cast<value_t::boolean>(this->type_, *this);
+            detail::throw_bad_cast<value_t::boolean>(
+                    "toml::value::as_boolean(): ", this->type_, *this);
         }
         return std::move(this->boolean_);
     }
@@ -1499,7 +1443,8 @@ class basic_value
     {
         if(this->type_ != value_t::integer)
         {
-            detail::throw_bad_cast<value_t::integer>(this->type_, *this);
+            detail::throw_bad_cast<value_t::integer>(
+                    "toml::value::as_integer(): ", this->type_, *this);
         }
         return std::move(this->integer_);
     }
@@ -1507,7 +1452,8 @@ class basic_value
     {
         if(this->type_ != value_t::floating)
         {
-            detail::throw_bad_cast<value_t::floating>(this->type_, *this);
+            detail::throw_bad_cast<value_t::floating>(
+                    "toml::value::as_floating(): ", this->type_, *this);
         }
         return std::move(this->floating_);
     }
@@ -1515,7 +1461,8 @@ class basic_value
     {
         if(this->type_ != value_t::string)
         {
-            detail::throw_bad_cast<value_t::string>(this->type_, *this);
+            detail::throw_bad_cast<value_t::string>(
+                    "toml::value::as_string(): ", this->type_, *this);
         }
         return std::move(this->string_);
     }
@@ -1523,7 +1470,8 @@ class basic_value
     {
         if(this->type_ != value_t::offset_datetime)
         {
-            detail::throw_bad_cast<value_t::offset_datetime>(this->type_, *this);
+            detail::throw_bad_cast<value_t::offset_datetime>(
+                    "toml::value::as_offset_datetime(): ", this->type_, *this);
         }
         return std::move(this->offset_datetime_);
     }
@@ -1531,7 +1479,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_datetime)
         {
-            detail::throw_bad_cast<value_t::local_datetime>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_datetime>(
+                    "toml::value::as_local_datetime(): ", this->type_, *this);
         }
         return std::move(this->local_datetime_);
     }
@@ -1539,7 +1488,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_date)
         {
-            detail::throw_bad_cast<value_t::local_date>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_date>(
+                    "toml::value::as_local_date(): ", this->type_, *this);
         }
         return std::move(this->local_date_);
     }
@@ -1547,7 +1497,8 @@ class basic_value
     {
         if(this->type_ != value_t::local_time)
         {
-            detail::throw_bad_cast<value_t::local_time>(this->type_, *this);
+            detail::throw_bad_cast<value_t::local_time>(
+                    "toml::value::as_local_time(): ", this->type_, *this);
         }
         return std::move(this->local_time_);
     }
@@ -1555,7 +1506,8 @@ class basic_value
     {
         if(this->type_ != value_t::array)
         {
-            detail::throw_bad_cast<value_t::array>(this->type_, *this);
+            detail::throw_bad_cast<value_t::array>(
+                    "toml::value::as_array(): ", this->type_, *this);
         }
         return std::move(this->array_.value());
     }
@@ -1563,10 +1515,12 @@ class basic_value
     {
         if(this->type_ != value_t::table)
         {
-            detail::throw_bad_cast<value_t::table>(this->type_, *this);
+            detail::throw_bad_cast<value_t::table>(
+                    "toml::value::as_table(): ", this->type_, *this);
         }
         return std::move(this->table_.value());
     }
+    // }}}
 
     // accessors =============================================================
     //
@@ -1574,11 +1528,29 @@ class basic_value
     //
     value_type&       at(const key& k)
     {
-        return this->as_table().at(k);
+        if(!this->is_table())
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::at(key): ", this->type_, *this);
+        }
+        if(this->as_table(std::nothrow).count(k) == 0)
+        {
+            detail::throw_key_not_found_error(*this, k);
+        }
+        return this->as_table(std::nothrow).at(k);
     }
     value_type const& at(const key& k) const
     {
-        return this->as_table().at(k);
+        if(!this->is_table())
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::at(key): ", this->type_, *this);
+        }
+        if(this->as_table(std::nothrow).count(k) == 0)
+        {
+            detail::throw_key_not_found_error(*this, k);
+        }
+        return this->as_table(std::nothrow).at(k);
     }
     value_type&       operator[](const key& k)
     {
@@ -1586,49 +1558,78 @@ class basic_value
         {
             *this = table_type{};
         }
-        return this->as_table()[k];
+        else if(!this->is_table()) // initialized, but not a table
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::operator[](key): ", this->type_, *this);
+        }
+        return this->as_table(std::nothrow)[k];
     }
 
     value_type&       at(const std::size_t idx)
     {
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::at(idx): ", this->type_, *this);
+        }
+        if(this->as_array(std::nothrow).size() <= idx)
+        {
+            throw std::out_of_range(detail::format_underline(
+                "toml::value::at(idx): no element corresponding to the index", {
+                    {this->region_info_.get(),
+                     concat_to_string("the length is ", this->as_array(std::nothrow).size(),
+                                      ", and the specified index is ", idx)}
+                }));
+        }
         return this->as_array().at(idx);
     }
     value_type const& at(const std::size_t idx) const
     {
-        return this->as_array().at(idx);
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::at(idx): ", this->type_, *this);
+        }
+        if(this->as_array(std::nothrow).size() <= idx)
+        {
+            throw std::out_of_range(detail::format_underline(
+                "toml::value::at(idx): no element corresponding to the index", {
+                    {this->region_info_.get(),
+                     concat_to_string("the length is ", this->as_array(std::nothrow).size(),
+                                      ", and the specified index is ", idx)}
+                }));
+        }
+        return this->as_array(std::nothrow).at(idx);
     }
 
     value_type&       operator[](const std::size_t idx) noexcept
     {
+        // no check...
         return this->as_array(std::nothrow)[idx];
     }
     value_type const& operator[](const std::size_t idx) const noexcept
     {
+        // no check...
         return this->as_array(std::nothrow)[idx];
     }
 
     void push_back(const value_type& x)
     {
-        if(this->type_ != value_t::array)
+        if(!this->is_array())
         {
-            throw type_error(detail::format_underline(
-                "toml::value::push_back(value): bad_cast to array type", {
-                    {this->region_info_.get(),
-                     concat_to_string("the actual type is ", this->type_)}
-                }), this->location());
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::push_back(value): ", this->type_, *this);
         }
         this->as_array(std::nothrow).push_back(x);
         return;
     }
     void push_back(value_type&& x)
     {
-        if(this->type_ != value_t::array)
+        if(!this->is_array())
         {
-            throw type_error(detail::format_underline(
-                "toml::value::push_back(value): bad_cast to array type", {
-                    {this->region_info_.get(),
-                     concat_to_string("the actual type is ", this->type_)}
-                }), this->location());
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::push_back(value): ", this->type_, *this);
         }
         this->as_array(std::nothrow).push_back(std::move(x));
         return;
@@ -1637,13 +1638,10 @@ class basic_value
     template<typename ... Ts>
     value_type& emplace_back(Ts&& ... args)
     {
-        if(this->type_ != value_t::array)
+        if(!this->is_array())
         {
-            throw type_error(detail::format_underline(
-                "toml::value::emplace_back(value): bad_cast to array type", {
-                    {this->region_info_.get(),
-                     concat_to_string("the actual type is ", this->type_)}
-                }), this->location());
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::emplace_back(...): ", this->type_, *this);
         }
         this->as_array(std::nothrow).emplace_back(std::forward<Ts>(args) ...);
         return this->as_array(std::nothrow).back();
@@ -1655,15 +1653,15 @@ class basic_value
         {
             case value_t::array:
             {
-                return this->as_array().size();
+                return this->as_array(std::nothrow).size();
             }
             case value_t::table:
             {
-                return this->as_table().size();
+                return this->as_table(std::nothrow).size();
             }
             case value_t::string:
             {
-                return this->as_string().str.size();
+                return this->as_string(std::nothrow).str.size();
             }
             default:
             {
@@ -1678,28 +1676,22 @@ class basic_value
 
     std::size_t count(const key_type& k) const
     {
-        if(this->type_ != value_t::table)
+        if(!this->is_table())
         {
-            throw type_error(detail::format_underline(
-                "toml::value::count(key): bad_cast to table type", {
-                    {this->region_info_.get(),
-                     concat_to_string("the actual type is ", this->type_)}
-                }), this->location());
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::count(key): ", this->type_, *this);
         }
-        return this->as_table().count(k);
+        return this->as_table(std::nothrow).count(k);
     }
 
     bool contains(const key_type& k) const
     {
-        if(this->type_ != value_t::table)
+        if(!this->is_table())
         {
-            throw type_error(detail::format_underline(
-                "toml::value::contains(key): bad_cast to table type", {
-                    {this->region_info_.get(),
-                     concat_to_string("the actual type is ", this->type_)}
-                }), this->location());
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::contains(key): ", this->type_, *this);
         }
-        return (this->as_table().count(k) != 0);
+        return (this->as_table(std::nothrow).count(k) != 0);
     }
 
     source_location location() const
@@ -1724,13 +1716,11 @@ class basic_value
     }
 
     // for error messages
-    template<typename C,
-             template<typename ...> class T, template<typename ...> class A>
-    friend region_base const& detail::get_region(const basic_value<C, T, A>&);
+    template<typename Value>
+    friend region_base const& detail::get_region(const Value& v);
 
-    template<typename Region, typename C,
-             template<typename ...> class T, template<typename ...> class A>
-    friend void detail::change_region(basic_value<C, T, A>&, Region&&);
+    template<typename Value, typename Region>
+    friend void detail::change_region(Value& v, Region&& reg);
 
   private:
 
@@ -1760,30 +1750,6 @@ using value = basic_value<discard_comments, std::unordered_map, std::vector>;
 using array = typename value::array_type;
 using table = typename value::table_type;
 
-namespace detail
-{
-template<typename C,
-         template<typename ...> class T, template<typename ...> class A>
-inline region_base const& get_region(const basic_value<C, T, A>& v)
-{
-    return *(v.region_info_);
-}
-
-template<typename Region, typename C,
-         template<typename ...> class T, template<typename ...> class A>
-void change_region(basic_value<C, T, A>& v, Region&& reg)
-{
-    using region_type = typename std::remove_reference<
-        typename std::remove_cv<Region>::type
-        >::type;
-
-    std::shared_ptr<region_base> new_reg =
-        std::make_shared<region_type>(std::forward<region_type>(reg));
-    v.region_info_ = new_reg;
-    return;
-}
-}// detail
-
 template<typename C, template<typename ...> class T, template<typename ...> class A>
 inline bool
 operator==(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)