chore: fix README

without this, the first line of the serialized result becomes too wide

PROPOSAL.md

@@ -1,59 +0,0 @@
-
-### encoding user's data
-
-You can encode your data to toml format.
-
-```cpp
-const toml::value integer(1);
-const toml::value array{3.1, 3.14, 3.141, 3.1415};
-const toml::value table{{"answer", 42}, {"pi", 3.14}, {"string", "foobar"}};
-
-std::cout << toml::format("integer", integer) << std::endl;
-std::cout << toml::format("array",   array)   << std::endl;
-std::cout << toml::format("table",   table)   << std::endl;
-```
-
-this program will output as below.
-
-```toml
-integer = 1
-array = [3.1, 3.14, 3.141, 3.1415]
-[table]
-answer = 42
-pi = 3.14
-string = "foobar"
-```
-
-Without key name, you can make string formatted as toml.
-
-```cpp
-const std::string integer_ = toml::format(integer); // "1"
-const std::string array_   = toml::format(array);   // "[3.1, 3.14, 3.141, 3.1415]"
-const std::string table_   = toml::format(table);   // "answer = 42\npi=3.14\nstring=foobar"
-```
-
-### inlinize
-
-You can make `toml::Table` inline.
-
-```cpp
-const toml::value table{{"answer", 42}, {"pi", 3.14}, {"string", "foobar"}};
-// if the inline-table format length is less than 80, the table will be inlined
-std::cout << toml::format("table", table, toml::make_inline(80)) << std::endl;
-// In any case, the table will be inlined.
-std::cout << toml::format("table", table, toml::forceinline) << std::endl;
-```
-
-```toml
-table = {answer = 42, pi = 3.14, string = "foobar"}
-```
-
-And there are some stream manipulators for toml format.
-
-```cpp
-const toml::value table{{"answer", 42}, {"pi", 3.14}, {"string", "foobar"}};
-// if the inline-table format length is less than 80, the table will be inlined
-std::cout << toml::make_inline(80) << table << std::endl;
-// In any case, the table will be inlined.
-std::cout << toml::forceinline << table << std::endl;
-```

README.md

@@ -9,9 +9,13 @@ toml11
 
 C++11 header-only toml parser 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.
+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.
 
-Are you looking for pre-C++11 compatible toml parser? Try [Boost.toml](https://github.com/ToruNiina/Boost.toml)! It has almost the same functionality as this library and works with C++98 & Boost.
+Are you looking for pre-C++11 compatible toml parser?
+Try [Boost.toml](https://github.com/ToruNiina/Boost.toml)!
+It has almost the same functionality as this library and works with C++98 & Boost.
 
 ## How to use
 
@@ -55,13 +59,16 @@ assert(ifs.good());
 const auto data = toml::parse(ifs /*, "filename" (optional)*/);
 ```
 
-To show a better error message, it is recommended to pass a filename with `istream`. See also [in the case of syntax error](#in-the-case-of-syntax-error) and [passing invalid type to toml::get](#passing-invalid-type-to-tomlget).
+To show a better error message, it is recommended to pass a filename with `istream`.
+See also [in the case of syntax error](#in-the-case-of-syntax-error)
+and [passing invalid type to toml::get](#passing-invalid-type-to-tomlget).
 
 ### In the case of syntax error
 
 If there is a syntax error in a toml file, `toml::parse` will throw `toml::syntax_error`.
 
-toml11 now has clean and informative error messages inspired by Rust and it looks like the following (comment after hash sign are actually not shown).
+toml11 now has clean and informative error messages inspired by Rust and
+it looks like the following (comment after hash sign are actually not shown).
 
 ```console
 terminate called after throwing an instance of 'toml::syntax_error'
@@ -71,7 +78,8 @@ terminate called after throwing an instance of 'toml::syntax_error'
    |        ^------ expected newline, but got '='.        # error reason
 ```
 
-If you (mistakenly) duplicate tables and got an error, you may want to see where the other is. toml11 shows both at the same time.
+If you (mistakenly) duplicate tables and got an error, you may want to see
+where the other is. toml11 shows both at the same time.
 
 ```console
 terminate called after throwing an instance of 'toml::syntax_error'
@@ -84,11 +92,14 @@ terminate called after throwing an instance of 'toml::syntax_error'
    | ~~~~~~~ table defined twice
 ```
 
-Since the error message generation is generally a difficult task, the current status is not ideal. toml11 needs your help. If you encounter a weird error message, please let us know and contribute to improve the quality!
+Since the error message generation is generally a difficult task, the current
+status is not ideal. toml11 needs your help. If you encounter a weird error message,
+please let us know and contribute to improve the quality!
 
 ### Getting a toml value
 
-After parsing successfully, you can obtain the values from the result of `toml::parse` (here, `data`) using `toml::get` function.
+After parsing successfully, you can obtain the values from the result of
+`toml::parse` (here, `data`) using `toml::get` function.
 
 ```toml
 answer  = 42
@@ -108,18 +119,22 @@ const auto tab       = toml::get<toml::Table>(data.at("tab"));
 const auto key       = toml::get<std::string>( tab.at("key"));
 ```
 
-When you pass an exact TOML type that does not require type conversion, `toml::get` returns also a reference through which you can modify the content.
+When you pass an exact TOML type that does not require type conversion,
+`toml::get` returns also a reference through which you can modify the content.
 
 ```cpp
 toml::get<toml::integer>(data["answer"]) = 6 * 9;
 std::cout << toml::get<int>(data.at("answer")) << std::endl; // 54
 ```
 
-If the specified type requires conversion, you can't take a reference to the value. See also [underlying types](#underlying-types).
+If the specified type requires conversion, you can't take a reference to the value.
+See also [underlying types](#underlying-types).
 
 #### Passing invalid type to toml::get
 
-If you choose the invalid type, `toml::type_error` will be thrown. Similar to the `syntax_error`, toml11 also displays informative error messages. The error message when you choose `int` to get `string` value would be like this.
+If you choose the invalid type, `toml::type_error` will be thrown.
+Similar to the `syntax_error`, toml11 also displays informative error messages.
+The error message when you choose `int` to get `string` value would be like this.
 
 ```console
 terminate called after throwing an instance of 'toml::type_error'
@@ -129,7 +144,9 @@ terminate called after throwing an instance of 'toml::type_error'
    |         ~~~~~~~~~~~~~~ the actual type is string
 ```
 
-NOTE: In order to show this kind of error message, all the toml values have 1 shared_ptr that points the corresponding byte sequence and 2 iterators that point the range. It is recommended to destruct all the `toml::value` classes after configuring your application to save memory resources.
+NOTE: In order to show this kind of error message, all the toml values have 1
+`shared_ptr` that points the corresponding byte sequence and 2 iterators that point the range.
+It is recommended to destruct all the `toml::value` classes after configuring your application to save memory resources.
 
 ### Getting arrays
 
@@ -164,20 +181,25 @@ const auto aofa = toml::get<
     >(data.at("aofa"));
 ```
 
-If you don't know what the type is inside the array, you can use `toml::array`, which is a `std::vector` of `toml::value`, instead.
+If you don't know what the type is inside the array, you can use `toml::array`,
+which is a `std::vector` of `toml::value`, instead.
 
 ```cpp
 const auto aofa  = toml::get<toml::array>(data.at("aofa"));
 const auto first = toml::get<toml::array>(aofa.at(0));
 ```
 
-See also [expecting conversion](#expecting-conversion) and [checking-value-type](#checking-value-type).
+See also [expecting conversion](#expecting-conversion)
+and [checking-value-type](#checking-value-type).
 
 ### Getting tables
 
-`toml::table` is a key component of this library, which is an alias of a `std::unordered_map` from `toml::key (a.k.a. std::string)` to `toml::value`. `toml::parse` returns this as a result.
+`toml::table` is a key component of this library, which is an alias of
+a `std::unordered_map` from `toml::key (a.k.a. std::string)` to `toml::value`.
+`toml::parse` returns this.
 
-Since it is just an alias of `std::unordered_map`, it has all the functionalities that `std::unordered_map` has, e.g. `operator[]`, `count`, and `find`.
+Since it is just an alias of `std::unordered_map`, it has all the functionalities
+that `std::unordered_map` has, e.g. `operator[]`, `count`, and `find`.
 
 ```cpp
 toml::table data = toml::parse("example.toml");
@@ -187,7 +209,8 @@ if(data.count("title") != 0)
 }
 ```
 
-When all the values of the table have the same type, toml11 allows you to convert a `toml::table` to a `map` that contains the convertible type.
+When all the values of the table have the same type, toml11 allows you to
+convert a `toml::table` to a `map` that contains the convertible type.
 
 ```toml
 [tab]
@@ -203,7 +226,8 @@ std::cout << tab["key2"] << std::endl; // bar
 
 ### Dotted keys
 
-TOML v0.5.0 has a new feature named "dotted keys". You can chain keys to represent the structure of the data.
+TOML v0.5.0 has a new feature named "dotted keys".
+You can chain keys to represent the structure of the data.
 
 ```toml
 physical.color = "orange"
@@ -227,7 +251,8 @@ const auto color    = toml::get<std::string>(physical.at("color"));
 
 ### An array of tables
 
-An array of tables is just an array of tables. You can get it completely in the same way as the other arrays and tables.
+An array of tables is just an array of tables.
+You can get it completely in the same way as the other arrays and tables.
 
 ```toml
 array_of_inline_table = [{key = "value1"}, {key = "value2"}, {key = "value3"}]
@@ -247,7 +272,9 @@ const auto aot2 = toml::get<std::vector<toml::table>>(data.at("array_of_table"))
 
 ### Cost of conversion
 
-Although `toml::get` is convenient, it has additional copy-cost because it copies data contained in `toml::value` to the user-specified type. Of course in some case this overhead is not ignorable.
+Although `toml::get` is convenient, it has additional copy-cost because
+it copies data contained in `toml::value` to the user-specified type.
+Of course in some case this overhead is not ignorable.
 
 By passing the exact types, `toml::get` returns reference that has nealy zero overhead.
 
@@ -256,7 +283,8 @@ const auto& tab     = toml::get<toml::array>(data.at("tab"));
 const auto& numbers = toml::get<toml::table>(data.at("numbers"));
 ```
 
-Unfortunately, in this case you need to call `toml::get` each time you access to the element of `toml::array` because `toml::array` is an array of `toml::value`.
+Unfortunately, in this case you need to call `toml::get` each time you access to
+the element of `toml::array` because `toml::array` is an array of `toml::value`.
 
 ```cpp
 const auto& num0 = toml::get<toml::integer>(numbers.at(0));
@@ -266,7 +294,9 @@ const auto& num2 = toml::get<toml::integer>(numbers.at(2));
 
 ### Datetime and its variants
 
-TOML v0.5.0 has 4 different datetime objects, `local_date`, `local_time`, `local_datetime`, and `offset_datetime`. With toml11, you can convert `local_time` to your favorite `std::chrono::duration` and others to `std::chrono::system_clock::time_point`.
+TOML v0.5.0 has 4 different datetime objects, `local_date`, `local_time`,
+`local_datetime`, and `offset_datetime`. With toml11, you can convert `local_time`
+to your favorite `std::chrono::duration` and others to `std::chrono::system_clock::time_point`.
 
 ```toml
 time = 12:30:00
@@ -291,7 +321,8 @@ const auto value = toml::get_or(data, "key", 42); // value => int 42.
 
 ### Expecting conversion
 
-By using `toml::expect`, you will get your expected value or an error message without throwing `toml::type_error`.
+By using `toml::expect`, you will get your expected value or an error message
+without throwing `toml::type_error`.
 
 ```cpp
 const auto value = toml::expect<std::string>(data.at("title"));
@@ -314,7 +345,9 @@ const auto value = toml::expect<int>(data.at("number"))
 
 ### Finding value from table
 
-toml11 provides utility function to find a value from `toml::table`. Of course, you can do this in your own way with `toml::get` because it just searches an  `unordered_map` and returns a value if it exists.
+toml11 provides utility function to find a value from `toml::table`.
+Of course, you can do this in your own way with `toml::get` because
+it just searches an  `unordered_map` and returns a value if it exists.
 
 ```cpp
 const auto data = toml::parse("example.toml");
@@ -328,7 +361,8 @@ terminate called after throwing an instance of 'std::out_of_range'
   what():  [error] key "num" not found in example.toml
 ```
 
-You can use this with a `toml::value` that is expected to be a `toml::table`. It automatically casts the value to table.
+You can use this with a `toml::value` that is expected to be a `toml::table`.
+It automatically casts the value to table.
 
 ```cpp
 const auto data = toml::parse("example.toml");
@@ -338,7 +372,8 @@ const auto num  = toml::find<int>(data.at("table"), "num");
 // num = 42
 ```
 
-In this case, because the value `data.at("table")` knows the locatoin of itself, you don't need to pass where you find the value. `toml::find` will show you a great error message.
+In this case, because the value `data.at("table")` knows the locatoin of itself,
+you don't need to pass where you find the value. `toml::find` will show you a great error message.
 
 ```console
 terminate called after throwing an instance of 'std::out_of_range'
@@ -388,9 +423,26 @@ int i = 0;
 toml::from_toml(i, data.at("something"));
 ```
 
+### visiting toml::value
+
+TOML v2.1.0+ provides `toml::visit` to apply a function to `toml::value` in the
+same way as `std::variant`.
+
+```cpp
+const toml::value v(3.14);
+toml::visit([](const auto& val) -> void {
+        std::cout << val << std::endl;
+    }, v);
+```
+
+The function object that would be passed to `toml::visit` must be able to
+recieve all the possible TOML types. Also, the result types should be the same
+each other.
+
 ### Sanitizing UTF-8 codepoints
 
-toml11 shows warning if a value of an escape sequence used to represent unicode character exceeds the unicode range.
+toml11 shows warning if a value of an escape sequence used
+to represent unicode character exceeds the unicode range.
 
 ```console
 [warning] input codepoint (0011FFFF) is too large to decode as a unicode character. The result may not be able to render to your screen.
@@ -462,6 +514,97 @@ you will get an error message like this.
    |       ~~ maximum number here
 ```
 
+### Serializing TOML data
+
+toml11 v2.1.0 enables you to serialize data into toml format.
+
+```cpp
+const auto data = toml::table{{"foo", 42}, {"bar", "baz"}};
+
+const std::string serial = toml::format(data);
+// serial == "{bar=\"baz\",foo=42}"
+
+std::cout << data << std::endl;
+// bar = "baz"
+// foo = 42
+```
+
+toml11 automatically makes a tiny table and array inline.
+You can specify the width to make them inline by `std::setw` for streams.
+
+```cpp
+const auto data = toml::table{
+    {"qux",    toml::table{{"foo", 42}, {"bar", "baz"}}},
+    {"quux",   toml::array{"small", "array", "of", "strings"}},
+    {"foobar", toml::array{"this", "array", "of", "strings", "is", "too", "long",
+                           "to", "print", "into", "single", "line", "isn't", "it?"}},
+};
+
+// the threshold becomes 80.
+std::cout << std::setw(80) << data << std::endl;
+// foobar = [
+// "this","array","of","strings","is","too","long","to","print","into",
+// "single","line","isn't","it?",
+// ]
+// quux = ["small","array","of","strings"]
+// qux = {bar="baz",foo=42}
+
+
+// the width is 0. nothing become inline.
+std::cout << std::setw(0) << data << std::endl;
+// foobar = [
+// "this",
+// ... (snip)
+// "it?",
+// ]
+// quux = [
+// "small",
+// "array",
+// "of",
+// "strings",
+// ]
+// [qux]
+// bar = "baz"
+// foo = 42
+```
+
+It is recommended to set width before printing data. Some I/O functions changes
+width to 0, and it makes all the stuff (including `toml::array`) multiline.
+The resulting files becomes too long.
+
+`toml::format` receives optional second argument to set the width.
+By default, it is 80.
+
+```cpp
+const auto data = toml::table{
+    {"qux", toml::table{{"foo", 42}, {"bar", "baz"}}}
+};
+
+const std::string serial = toml::format(data, /*width = */ 0);
+// [qux]
+// bar = "baz"
+// foo = 42
+```
+
+To control the precision of floating point numbers, you need to pass
+`std::setprecision` to stream or pass `int` to the optional third argument of
+`toml::format` (by default, it is `std::numeric_limits<double>::max_digit10`).
+
+```cpp
+const auto data = toml::table{
+    {"pi", 3.141592653589793},
+    {"e",  2.718281828459045}
+};
+std::cout << std::setprecision(17) << data << std::endl;
+// e = 2.7182818284590451
+// pi = 3.1415926535897931
+std::cout << std::setprecision( 7) << data << std::endl;
+// e = 2.718282
+// pi = 3.141593
+
+const std::string serial = toml::format(data, /*width = */ 0, /*prec = */ 17);
+```
+
 ## Underlying types
 
 The toml types (can be used as `toml::*` in this library) and corresponding `enum` names are listed in the table below.
@@ -477,11 +620,16 @@ The toml types (can be used as `toml::*` in this library) and corresponding `enu
 | LocalDatetime  | `toml::local_datetime`                       | `toml::value_t::LocalDatetime`  |
 | OffsetDatetime | `toml::offset_datetime`                      | `toml::value_t::offsetDatetime` |
 | Array          | `std::vector<toml::value>`                   | `toml::value_t::Array`          |
-| Table          | `std::unordered_map<std::string, toml::key>` | `toml::value_t::Table`          |
+| Table          | `std::unordered_map<toml::key, toml::value>` | `toml::value_t::Table`          |
 
-`toml::string` is effectively the same as `std::string` but has an additional flag that represents a kind of a string, `string_t::basic` and `string_t::literal`. Although `std::string` is not an exact toml type, still you can get a reference that points to internal `std::string` by using `toml::get<std::string>()` for convenience.
+`toml::string` is effectively the same as `std::string` but has an additional
+flag that represents a kind of a string, `string_t::basic` and `string_t::literal`.
+Although `std::string` is not an exact toml type, still you can get a reference
+that points to internal `std::string` by using `toml::get<std::string>()` for convenience.
 
-`Datetime` variants are `struct` that are defined in this library. Because `std::chrono::system_clock::time_point` is a __time point__, not capable of representing a Local Time independent from a specific day.
+`Datetime` variants are `struct` that are defined in this library.
+Because `std::chrono::system_clock::time_point` is a __time point__,
+not capable of representing a Local Time independent from a specific day.
 
 It is recommended to get `Datetime`s as `std::chrono` classes through `toml::get`.
 

tests/CMakeLists.txt

@@ -25,6 +25,7 @@ set(TEST_NAMES
     test_to_toml
     test_from_toml
     test_parse_file
+    test_serialize_file
     test_parse_unicode
     test_error_detection
 )

tests/test_serialize_file.cpp

@@ -0,0 +1,54 @@
+#define BOOST_TEST_MODULE "test_serialize_file"
+#ifdef UNITTEST_FRAMEWORK_LIBRARY_EXIST
+#include <boost/test/unit_test.hpp>
+#else
+#define BOOST_TEST_NO_LIB
+#include <boost/test/included/unit_test.hpp>
+#endif
+#include <toml.hpp>
+#include <iostream>
+#include <fstream>
+
+
+BOOST_AUTO_TEST_CASE(test_example)
+{
+    const auto data = toml::parse("toml/tests/example.toml");
+    {
+        std::ofstream ofs("tmp1.toml");
+        ofs << std::setw(80) << data;
+    }
+
+    auto serialized = toml::parse("tmp1.toml");
+    {
+        auto& owner = toml::get<toml::table>(serialized.at("owner"));
+        auto& bio   = toml::get<std::string>(owner.at("bio"));
+        const auto CR = std::find(bio.begin(), bio.end(), '\r');
+        if(CR != bio.end())
+        {
+            bio.erase(CR);
+        }
+    }
+    BOOST_CHECK(data == serialized);
+}
+
+BOOST_AUTO_TEST_CASE(test_fruit)
+{
+    const auto data = toml::parse("toml/tests/fruit.toml");
+    {
+        std::ofstream ofs("tmp2.toml");
+        ofs << std::setw(80) << data;
+    }
+    const auto serialized = toml::parse("tmp2.toml");
+    BOOST_CHECK(data == serialized);
+}
+
+BOOST_AUTO_TEST_CASE(test_hard_example)
+{
+    const auto data = toml::parse("toml/tests/hard_example.toml");
+    {
+        std::ofstream ofs("tmp3.toml");
+        ofs << std::setw(80) << data;
+    }
+    const auto serialized = toml::parse("tmp3.toml");
+    BOOST_CHECK(data == serialized);
+}

toml.hpp

@@ -34,6 +34,7 @@
 #endif
 
 #include "toml/parser.hpp"
+#include "toml/serializer.hpp"
 #include "toml/to_toml.hpp"
 #include "toml/from_toml.hpp"
 #include "toml/get.hpp"

toml/datetime.hpp

@@ -14,6 +14,36 @@
 namespace toml
 {
 
+// To avoid non-threadsafe std::localtime. In C11 (not C++11!), localtime_s is
+// provided in the absolutely same purpose, but C++11 is actually not compatible
+// with C11. We need to dispatch the function depending on the OS.
+namespace detail
+{
+// TODO: find more sophisticated way to handle this
+#if _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE || _POSIX_SOURCE
+inline std::tm localtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::localtime_r(src, &dst);
+    if(!result)
+    {
+        throw std::runtime_error("localtime_r failed.");
+    }
+    return dst;
+}
+#else
+// XXX: On Windows, std::localtime is thread-safe because they uses thread-local
+// storage to store the instance of std::tm. On the other platforms, it may not
+// be thread-safe.
+inline std::tm localtime_s(const std::time_t* src)
+{
+    const auto result = std::localtime(src);
+    if(!result) {throw std::runtime_error("localtime failed.");}
+    return *result;
+}
+#endif
+} // detail
+
 enum class month_t : std::int8_t
 {
     Jan =  0,
@@ -50,10 +80,8 @@ struct local_date
 
     explicit local_date(const std::chrono::system_clock::time_point& tp)
     {
-        const auto t = std::chrono::system_clock::to_time_t(tp);
-        const auto tmp = std::localtime(&t); //XXX: not threadsafe!
-        assert(tmp); // if std::localtime fails, tmp is nullptr
-        const std::tm time = *tmp;
+        const auto t    = std::chrono::system_clock::to_time_t(tp);
+        const auto time = detail::localtime_s(&t);
         *this = local_date(time);
     }
 
@@ -330,10 +358,8 @@ struct local_datetime
     explicit local_datetime(const std::chrono::system_clock::time_point& tp)
     {
         const auto t = std::chrono::system_clock::to_time_t(tp);
-        const auto tmp = std::localtime(&t); //XXX: not threadsafe!
-        assert(tmp); // if std::localtime fails, tmp is nullptr
+        std::tm time = detail::localtime_s(&t);
 
-        std::tm time = *tmp;
         this->date = local_date(time);
         this->time = local_time(time);
 
@@ -482,10 +508,8 @@ struct offset_datetime
     static time_offset get_local_offset()
     {
          // get current timezone
-        const auto  tmp1 = std::time(nullptr);
-        const auto  tmp2 = std::localtime(&tmp1); // XXX not threadsafe!
-        assert(tmp2);
-        std::tm t = *tmp2;
+        const auto tmp1 = std::time(nullptr);
+        const auto t    = detail::localtime_s(&tmp1);
 
         std::array<char, 6> buf;
         const auto result = std::strftime(buf.data(), 6, "%z", &t); // +hhmm\0

toml/serializer.hpp

@@ -0,0 +1,511 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_SERIALIZER_HPP
+#define TOML11_SERIALIZER_HPP
+#include "value.hpp"
+#include "lexer.hpp"
+#include <limits>
+
+namespace toml
+{
+
+struct serializer
+{
+    serializer(const std::size_t w              = 80,
+               const int         float_prec     = std::numeric_limits<toml::floating>::max_digits10,
+               const bool        can_be_inlined = false,
+               std::vector<toml::key> ks        = {})
+        : can_be_inlined_(can_be_inlined), float_prec_(float_prec), width_(w),
+          keys_(std::move(ks))
+    {}
+    ~serializer() = default;
+
+    std::string operator()(const toml::boolean& b) const
+    {
+        return b ? "true" : "false";
+    }
+    std::string operator()(const integer i) const
+    {
+        return std::to_string(i);
+    }
+    std::string operator()(const toml::floating f) const
+    {
+        std::string token = [=] {
+            // every float value needs decimal point (or exponent).
+            std::ostringstream oss;
+            oss << std::setprecision(float_prec_) << std::showpoint << f;
+            return oss.str();
+        }();
+
+        if(token.back() == '.') // 1. => 1.0
+        {
+            token += '0';
+        }
+        const auto e = std::find_if(token.cbegin(), token.cend(),
+            [](const char c) -> bool {
+                return c == 'E' || c == 'e';
+            });
+        if(e == token.cend())
+        {
+            return token; // there is no exponent part. just return it.
+        }
+
+        // zero-prefix in an exponent is not allowed in TOML.
+        // remove it if it exists.
+        bool        sign_exists = false;
+        std::size_t zero_prefix = 0;
+        for(auto iter = std::next(e), iend = token.cend(); iter != iend; ++iter)
+        {
+            if(*iter == '+' || *iter == '-'){sign_exists = true; continue;}
+            if(*iter == '0'){zero_prefix += 1;}
+            else {break;}
+        }
+        if(zero_prefix != 0)
+        {
+            const auto offset = std::distance(token.cbegin(), e) +
+                               (sign_exists ? 2 : 1);
+            token.erase(offset, zero_prefix);
+        }
+        return token;
+    }
+    std::string operator()(const string& s) const
+    {
+        if(s.kind == string_t::basic)
+        {
+            if(std::find(s.str.cbegin(), s.str.cend(), '\n') != 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;
+            }
+
+            // no linefeed. try to make it oneline-string.
+            std::string oneline = this->escape_basic_string(s.str);
+            if(oneline.size() + 2 < width_ || width_ < 2)
+            {
+                const std::string quote("\"");
+                return quote + oneline + quote;
+            }
+
+            // the line is too long compared to the specified width.
+            // split it into multiple lines.
+            std::string token("\"\"\"\n");
+            while(!oneline.empty())
+            {
+                if(oneline.size() < width_)
+                {
+                    token += oneline;
+                    oneline.clear();
+                }
+                else if(oneline.at(width_-2) == '\\')
+                {
+                    token += oneline.substr(0, width_-2);
+                    token += "\\\n";
+                    oneline.erase(0, width_-2);
+                }
+                else
+                {
+                    token += oneline.substr(0, width_-1);
+                    token += "\\\n";
+                    oneline.erase(0, width_-1);
+                }
+            }
+            return token + std::string("\\\n\"\"\"");
+        }
+        else // the string `s` is literal-string.
+        {
+            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");
+                const std::string close("'''");
+                return open + s.str + close;
+            }
+            else
+            {
+                const std::string quote("'");
+                return quote + s.str + quote;
+            }
+        }
+    }
+
+    std::string operator()(const local_date& d) const
+    {
+        std::ostringstream oss;
+        oss << d;
+        return oss.str();
+    }
+    std::string operator()(const local_time& t) const
+    {
+        std::ostringstream oss;
+        oss << t;
+        return oss.str();
+    }
+    std::string operator()(const local_datetime& dt) const
+    {
+        std::ostringstream oss;
+        oss << dt;
+        return oss.str();
+    }
+    std::string operator()(const offset_datetime& odt) const
+    {
+        std::ostringstream oss;
+        oss << odt;
+        return oss.str();
+    }
+
+    std::string operator()(const array& v) const
+    {
+        if(!v.empty() && v.front().is(value_t::Table))// v is an array of tables
+        {
+            // if it's not inlined, we need to add `[[table.key]]`.
+            // but if it can be inlined, we need `table.key = [...]`.
+            if(this->can_be_inlined_)
+            {
+                std::string token;
+                if(!keys_.empty())
+                {
+                    token += this->serialize_key(keys_.back());
+                    token += " = ";
+                }
+                bool width_exceeds = false;
+                token += "[\n";
+                for(const auto& item : v)
+                {
+                    const auto t =
+                        this->make_inline_table(item.cast<value_t::Table>());
+
+                    if(t.size() + 1 > width_ || // +1 for the last comma {...},
+                       std::find(t.cbegin(), t.cend(), '\n') != t.cend())
+                    {
+                        width_exceeds = true;
+                        break;
+                    }
+                    token += t;
+                    token += ",\n";
+                }
+                if(!width_exceeds)
+                {
+                    token += "]\n";
+                    return token;
+                }
+                // if width_exceeds, serialize it as [[array.of.tables]].
+            }
+
+            std::string token;
+            for(const auto& item : v)
+            {
+                token += "[[";
+                token += this->serialize_dotted_key(keys_);
+                token += "]]\n";
+                token += this->make_multiline_table(item.cast<value_t::Table>());
+            }
+            return token;
+        }
+        if(v.empty())
+        {
+            return std::string("[]");
+        }
+
+        // not an array of tables. normal array. first, try to make it inline.
+        {
+            const auto inl = this->make_inline_array(v);
+            if(inl.size() < this->width_ &&
+               std::find(inl.cbegin(), inl.cend(), '\n') == inl.cend())
+            {
+                return inl;
+            }
+        }
+
+        // if the length exceeds this->width_, print multiline array
+        std::string token;
+        std::string current_line;
+        token += "[\n";
+        for(const auto& item : v)
+        {
+            auto next_elem = toml::visit(*this, item);
+            // newline between array-value and comma is not allowed
+            if(next_elem.back() == '\n'){next_elem.pop_back();}
+
+            if(current_line.size() + next_elem.size() + 1 < this->width_)
+            {
+                current_line += next_elem;
+                current_line += ',';
+            }
+            else if(current_line.empty())
+            {
+                // the next elem cannot be within the width.
+                token += next_elem;
+                token += ",\n";
+                // keep current line empty
+            }
+            else // current_line has some tokens and it exceeds width
+            {
+                assert(current_line.back() == ',');
+                token += current_line;
+                token += '\n';
+                current_line = next_elem;
+                current_line += ',';
+            }
+        }
+        if(!current_line.empty())
+        {
+            if(current_line.back() != '\n') {current_line += '\n';}
+            token += current_line;
+        }
+        token += "]\n";
+        return token;
+    }
+
+    std::string operator()(const table& v) const
+    {
+        if(this->can_be_inlined_)
+        {
+            std::string token;
+            if(!this->keys_.empty())
+            {
+                token += this->serialize_key(this->keys_.back());
+                token += " = ";
+            }
+            token += this->make_inline_table(v);
+            if(token.size() < this->width_)
+            {
+                return token;
+            }
+        }
+
+        std::string token;
+        if(!keys_.empty())
+        {
+            token += '[';
+            token += this->serialize_dotted_key(keys_);
+            token += "]\n";
+        }
+        token += this->make_multiline_table(v);
+        return token;
+    }
+
+  private:
+
+    std::string serialize_key(const toml::key& key) const
+    {
+        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("\"");
+        token += this->escape_basic_string(key);
+        token += "\"";
+        return token;
+    }
+
+    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.
+        std::string retval;
+        for(const char 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;}
+                default  : {retval += c;      break;}
+            }
+        }
+        return retval;
+    }
+
+    std::string escape_ml_basic_string(const std::string& s) const
+    {
+        std::string retval;
+        for(auto i=s.cbegin(), e=s.cend(); i!=e; ++i)
+        {
+            switch(*i)
+            {
+                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':
+                {
+                    if(std::next(i) != e && *std::next(i) == '\n')
+                    {
+                        retval += "\r\n";
+                        ++i;
+                    }
+                    else
+                    {
+                        retval += "\\r";
+                    }
+                    break;
+                }
+                default: {retval += *i; break;}
+            }
+        }
+        return retval;
+    }
+
+    std::string make_inline_array(const array& v) const
+    {
+        std::string token;
+        token += '[';
+        bool is_first = true;
+        for(const auto& item : v)
+        {
+            if(is_first) {is_first = false;} else {token += ',';}
+            token += visit(serializer(std::numeric_limits<std::size_t>::max(),
+                                      this->float_prec_, true), item);
+        }
+        token += ']';
+        return token;
+    }
+
+    std::string make_inline_table(const table& v) const
+    {
+        assert(this->can_be_inlined_);
+        std::string token;
+        token += '{';
+        bool is_first = true;
+        for(const auto& kv : v)
+        {
+            // 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 += '=';
+            token += visit(serializer(std::numeric_limits<std::size_t>::max(),
+                                      this->float_prec_, true), kv.second);
+        }
+        token += '}';
+        return token;
+    }
+
+    std::string make_multiline_table(const table& v) const
+    {
+        std::string token;
+
+        // 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)
+        {
+            if(kv.second.is(value_t::Table) || is_array_of_tables(kv.second))
+            {
+                continue;
+            }
+
+            const auto key_and_sep    = this->serialize_key(kv.first) + " = ";
+            const auto residual_width = (this->width_ > key_and_sep.size()) ?
+                                        this->width_ - key_and_sep.size() : 0;
+            token += key_and_sep;
+            token += visit(serializer(residual_width, this->float_prec_, true),
+                           kv.second);
+            if(token.back() != '\n')
+            {
+                token += '\n';
+            }
+        }
+
+        // normal tables / array of tables
+
+        // after multiline table appeared, the other tables cannot be inline
+        // because the table would be assigned into the table.
+        // [foo]
+        // ...
+        // bar = {...} # <- bar will be a member of [foo].
+        bool multiline_table_printed = false;
+        for(const auto& kv : v)
+        {
+            if(!kv.second.is(value_t::Table) && !is_array_of_tables(kv.second))
+            {
+                continue; // other stuff are already serialized. skip them.
+            }
+
+            std::vector<toml::key> ks(this->keys_);
+            ks.push_back(kv.first);
+
+            auto tmp = visit(serializer(
+                this->width_, this->float_prec_, !multiline_table_printed, ks),
+                kv.second);
+
+            if((!multiline_table_printed) &&
+               std::find(tmp.cbegin(), tmp.cend(), '\n') != tmp.cend())
+            {
+                multiline_table_printed = true;
+            }
+            else
+            {
+                // still inline tables only.
+                tmp += '\n';
+            }
+            token += tmp;
+        }
+        return token;
+    }
+
+    bool is_array_of_tables(const value& v) const
+    {
+        if(!v.is(value_t::Array)) {return false;}
+
+        const auto& a = v.cast<value_t::Array>();
+        return !a.empty() && a.front().is(value_t::Table);
+    }
+
+  private:
+
+    bool        can_be_inlined_;
+    int         float_prec_;
+    std::size_t width_;
+    std::vector<toml::key> keys_;
+};
+
+inline std::string
+format(const value& v, std::size_t w = 80,
+       int fprec = std::numeric_limits<toml::floating>::max_digits10)
+{
+    return visit(serializer(w, fprec, true), v);
+}
+inline std::string
+format(const table& t, std::size_t w = 80,
+       int fprec = std::numeric_limits<toml::floating>::max_digits10)
+{
+    return serializer(w, fprec, true)(t);
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const value& v)
+{
+    // get status of std::setw().
+    const std::size_t w     = os.width();
+    const int         fprec = os.precision();
+    os.width(0);
+    // the root object can't be an inline table. so pass `false`.
+    os << visit(serializer(w, fprec, false), v);
+    return os;
+}
+
+} // toml
+#endif// TOML11_SERIALIZER_HPP

toml/value.hpp

@@ -820,5 +820,77 @@ inline std::string format_error(const std::string& err_msg,
                                              std::move(hints));
 }
 
+template<typename Visitor>
+detail::return_type_of_t<Visitor, const toml::boolean&>
+visit(Visitor&& visitor, const toml::value& v)
+{
+    switch(v.type())
+    {
+        case value_t::Boolean       : {return visitor(v.cast<value_t::Boolean       >());}
+        case value_t::Integer       : {return visitor(v.cast<value_t::Integer       >());}
+        case value_t::Float         : {return visitor(v.cast<value_t::Float         >());}
+        case value_t::String        : {return visitor(v.cast<value_t::String        >());}
+        case value_t::OffsetDatetime: {return visitor(v.cast<value_t::OffsetDatetime>());}
+        case value_t::LocalDatetime : {return visitor(v.cast<value_t::LocalDatetime >());}
+        case value_t::LocalDate     : {return visitor(v.cast<value_t::LocalDate     >());}
+        case value_t::LocalTime     : {return visitor(v.cast<value_t::LocalTime     >());}
+        case value_t::Array         : {return visitor(v.cast<value_t::Array         >());}
+        case value_t::Table         : {return visitor(v.cast<value_t::Table         >());}
+        case value_t::Empty         : break;
+        case value_t::Unknown       : break;
+        default: break;
+    }
+    throw std::runtime_error(format_error("[error] toml::visit: toml::value "
+            "does not have any valid value.", v, "here"));
+}
+
+template<typename Visitor>
+detail::return_type_of_t<Visitor, toml::boolean&>
+visit(Visitor&& visitor, toml::value& v)
+{
+    switch(v.type())
+    {
+        case value_t::Boolean       : {return visitor(v.cast<value_t::Boolean       >());}
+        case value_t::Integer       : {return visitor(v.cast<value_t::Integer       >());}
+        case value_t::Float         : {return visitor(v.cast<value_t::Float         >());}
+        case value_t::String        : {return visitor(v.cast<value_t::String        >());}
+        case value_t::OffsetDatetime: {return visitor(v.cast<value_t::OffsetDatetime>());}
+        case value_t::LocalDatetime : {return visitor(v.cast<value_t::LocalDatetime >());}
+        case value_t::LocalDate     : {return visitor(v.cast<value_t::LocalDate     >());}
+        case value_t::LocalTime     : {return visitor(v.cast<value_t::LocalTime     >());}
+        case value_t::Array         : {return visitor(v.cast<value_t::Array         >());}
+        case value_t::Table         : {return visitor(v.cast<value_t::Table         >());}
+        case value_t::Empty         : break;
+        case value_t::Unknown       : break;
+        default: break;
+    }
+    throw std::runtime_error(format_error("[error] toml::visit: toml::value "
+            "does not have any valid value.", v, "here"));
+}
+
+template<typename Visitor>
+detail::return_type_of_t<Visitor, toml::boolean&>
+visit(Visitor&& visitor, toml::value&& v)
+{
+    switch(v.type())
+    {
+        case value_t::Boolean       : {return visitor(std::move(v.cast<value_t::Boolean       >()));}
+        case value_t::Integer       : {return visitor(std::move(v.cast<value_t::Integer       >()));}
+        case value_t::Float         : {return visitor(std::move(v.cast<value_t::Float         >()));}
+        case value_t::String        : {return visitor(std::move(v.cast<value_t::String        >()));}
+        case value_t::OffsetDatetime: {return visitor(std::move(v.cast<value_t::OffsetDatetime>()));}
+        case value_t::LocalDatetime : {return visitor(std::move(v.cast<value_t::LocalDatetime >()));}
+        case value_t::LocalDate     : {return visitor(std::move(v.cast<value_t::LocalDate     >()));}
+        case value_t::LocalTime     : {return visitor(std::move(v.cast<value_t::LocalTime     >()));}
+        case value_t::Array         : {return visitor(std::move(v.cast<value_t::Array         >()));}
+        case value_t::Table         : {return visitor(std::move(v.cast<value_t::Table         >()));}
+        case value_t::Empty         : break;
+        case value_t::Unknown       : break;
+        default: break;
+    }
+    throw std::runtime_error(format_error("[error] toml::visit: toml::value "
+            "does not have any valid value.", v, "here"));
+}
+
 }// toml
 #endif// TOML11_VALUE