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Header-only TOML parser and serializer for modern C++.
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C++ TOML MIT license Releases Mentioned in Awesome C++ CircleCI

  • Header-only
  • TOML v0.5.0, plus optional support for some unreleased TOML language features
  • C++17 (plus some C++20 features where available, e.g. experimental support for char8_t strings)
  • Proper UTF-8 handling (incl. BOM)
  • Works with or without exceptions
  • Doesn't require RTTI
  • First-class support for serializing to JSON
  • Tested on Clang, GCC and MSVC (VS2019)
  • Tested on x64, x86 and ARM


Given a TOML file configuration.toml containing the following:

name = "toml++"
authors = ["Mark Gillard <>"]

cpp = 17

Reading it in C++ is easy with toml++:

auto config = toml::parse_file( "configuration.toml" );

// get key-value pairs
std::string_view library_name = config["library"]["name"].value_or(""sv);
std::string_view library_author = config["library"]["authors"][0].value_or(""sv);
int64_t depends_on_cpp_version = config["dependencies"]["cpp"].value_or(0);

// modify the data
config.insert_or_assign("alternatives", toml::array{

// iterate & visit over the data
for (auto [k, v] : config)
    v.visit([](auto& node) noexcept
        std::cout << node << std:endl;
        if constexpr (toml::is_string<decltype(node)>)

// re-serialize as TOML
std::cout << config << std::endl;

// re-serialize as JSON
std::cout << toml::json_formatter{ config } << std::endl;

You'll find some more code examples in the examples directory, and plenty more as part of the API documentation.

Adding toml++ to your project

toml++ comes in two flavours: Regular and Single-header.

Regular mode

  1. Add tomlplusplus/include to your include paths
  2. #include <toml++/toml.h>

Single-header mode

  1. Drop toml.hpp wherever you like in your source tree
  2. There is no step two

The API is the same regardless of how you consume the library.


A number of configurable options are exposed in the form of preprocessor #defines. Most likely you won't need to mess with these at all, but if you do, set them before including toml++.

Option Type Default Description
TOML_ALL_INLINE boolean 1 Disable this to explicitly control where toml++'s implementation is compiled (e.g. as part of a library).
TOML_API define undefined API annotation to add to public symbols (e.g. __declspec(dllexport) on Windows).
TOML_ASSERT(expr) function macro assert(expr)
(or undefined)
Sets the assert function used by the library.
TOML_CHAR_8_STRINGS boolean 0 Uses C++20 char8_t-based strings as the toml string data type. Experimental!
TOML_CONFIG_HEADER string literal undefined Includes the given header file before the rest of the library.
TOML_EXCEPTIONS boolean per your compiler's settings Sets whether the library uses exceptions.
TOML_IMPLEMENTATION define undefined Define this to enable compilation of the library's implementation. Meaningless if TOML_ALL_INLINE is 1.
TOML_LARGE_FILES boolean 0 Uses 32-bit integers for line and column indices (instead of 16-bit).
TOML_OPTIONAL_TYPE type name undefined Overrides the optional<T> type used by the library if you need something better than std::optional.
TOML_SMALL_FLOAT_TYPE type name undefined If your codebase has an additional 'small' float type (e.g. half-precision), this tells toml++ about it.
TOML_SMALL_INT_TYPE type name undefined If your codebase has an additional 'small' integer type (e.g. 24-bits), this tells toml++ about it.
TOML_UNDEF_MACROS boolean 1 #undefs the library's internal macros at the end of the header.
TOML_UNRELEASED_FEATURES boolean 1 Enables support for unreleased TOML language features not yet part of a numbered version.

A number of these have ABI implications; the library uses inline namespaces to prevent you from accidentally linking incompatible combinations together.

TOML Language Support

At any given time toml++ aims to implement whatever the numbered version of TOML is, with the addition of unreleased features from the TOML master and some sane cherry-picks from the TOML issues list where the discussion strongly indicates inclusion in a near-future release.

The library advertises the most recent numbered language version it fully supports via the preprocessor defines TOML_LANG_MAJOR, TOML_LANG_MINOR and TOML_LANG_PATCH.

🔸Unreleased TOML features:

  • #356: Allow leading zeros in the exponent part of a float
  • #516: Allow newlines and trailing commas in inline tables
  • #562: Allow hex floatingpoint values
  • #567: Clarify that control characters are not permitted in comments
  • #571: Allow raw tabs inside strings
  • #644: Support + in key names
  • #665: Make arrays heterogeneous
  • #671: Local time of day format should support 09:30 as opposed to 09:30:00
  • #687: Relax bare key restrictions to allow additional unicode characters
  • #709: Include an \xHH escape code sequence

These can be disabled (and thus strict TOML v0.5.0 compliance enforced) by specifying TOML_UNRELEASED_FEATURES = 0 (see Configuration).

🔹TOML v0.5.0 and earlier:

  • All features supported.


Contributions are very welcome! Either by reporting issues or submitting pull requests. If you wish to submit a pull request, please see CONTRIBUTING for all the details you need to get going.

License and Attribution

toml++ is licensed under the terms of the MIT license - see LICENSE.

UTF-8 decoding is performed using a state machine based on Bjoern Hoehrmann's 'Flexible and Economical UTF-8 Decoder'.

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