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toml11 is a C++11 (or later) header-only toml parser/encoder depending only on C++ standard library.

  • It is compatible to the latest version of TOML v1.0.0-rc.2.
  • It is one of the most TOML standard compliant libraries, tested with the language agnostic test suite for TOML parsers by BurntSushi.
  • It shows highly informative error messages. You can see the error messages about invalid files at CircleCI.
  • 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.


#include <toml.hpp>
#include <iostream>

int main()
    auto data = toml::parse("example.toml");

    // find a value with the specified type from a table
    std::string title = toml::find<std::string>(data, "title");

    // convert the whole array into any container automatically
    std::vector<int> nums = toml::find<std::vector<int>>(data, "nums");

    // access with STL-like manner
        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;

Table of Contents


Just include the file after adding it to the include path.

#include <toml.hpp> // that's all! now you can use it.
#include <iostream>

int main()
    const auto data  = toml::parse("example.toml");
    const auto title = toml::find<std::string>(data, "title");
    std::cout << "the title is " << title << std::endl;
    return 0;

The convenient way is to add this repository as a git-submodule or to install it in your system by CMake.

Note for MSVC: We recommend to set /Zc:__cplusplus to detect C++ version correctly.

Decoding a toml file

To parse a toml file, the only thing you have to do is to pass a filename to the toml::parse function.

const std::string fname("sample.toml");
const toml::value data = toml::parse(fname);

As required by the TOML specification, the top-level value is always a table. You can find a value inside it, cast it into a table explicitly, and insert it as a value into other toml::value.

If it encounters an error while opening a file, it will throw std::runtime_error.

You can also pass a std::istream to the toml::parse function. To show a filename in an error message, however, it is recommended to pass the filename with the stream.

std::ifstream ifs("sample.toml", std::ios_base::binary);
const auto data = toml::parse(ifs, /*optional -> */ "sample.toml");

Note: When you are on Windows, open a file in binary mode. If a file is opened in text-mode, CRLF ("\r\n") will automatically be converted to LF ("\n") and this causes inconsistency between file size and the contents that would be read. This causes weird error.

In the case of syntax error

If there is a syntax error in a toml file, toml::parse will throw toml::syntax_error that inherits std::exception.

toml11 has clean and informative error messages inspired by Rust and it looks like the following.

terminate called after throwing an instance of 'toml::syntax_error'
  what():  [error] toml::parse_table: invalid line format # error description
 --> example.toml                                         # file name
 3 | a = 42 = true                                        # line num and content
   |        ^------ expected newline, but got '='.        # error reason

If you (mistakenly) duplicate tables and got an error, it is helpful to see where they are. toml11 shows both at the same time like the following.

terminate called after throwing an instance of 'toml::syntax_error'
  what():  [error] toml::insert_value: table ("table") already exists.
 --> duplicate-table.toml
 1 | [table]
   | ~~~~~~~ table already exists here
 3 | [table]
   | ~~~~~~~ table defined twice

When toml11 encounters a malformed value, it tries to detect what type it is. Then it shows hints to fix the format. An error message while reading one of the malformed files in the language agnostic test suite. is shown below.

what(): [error] bad time: should be HH:MM:SS.subsec
 --> ./datetime-malformed-no-secs.toml
 1 | no-secs = 1987-07-05T17:45Z
   |                     ^------- HH:MM:SS.subsec
Hint: pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999
Hint: fail: 1979-05-27T7:32:00, 1979-05-27 17:32

You can find other examples in a job named output_result on CircleCI.

Since the error message generation is generally a difficult task, the current status is not ideal. If you encounter a weird error message, please let us know and contribute to improve the quality!

Invalid UTF-8 codepoints

It throws syntax_error if a value of an escape sequence representing unicode character is not a valid UTF-8 codepoint.

  what():  [error] toml::read_utf8_codepoint: input codepoint is too large.
 --> utf8.toml
 1 | exceeds_unicode = "\U0011FFFF example"
   |                              ^--------- should be in [0x00..0x10FFFF]

Finding a toml value

After parsing successfully, you can obtain the values from the result of toml::parse using toml::find function.

# sample.toml
answer  = 42
pi      = 3.14
numbers = [1,2,3]
time    = 1979-05-27T07:32:00Z
const auto data      = toml::parse("sample.toml");
const auto answer    = toml::find<std::int64_t    >(data, "answer");
const auto pi        = toml::find<double          >(data, "pi");
const auto numbers   = toml::find<std::vector<int>>(data, "numbers");
const auto timepoint = toml::find<std::chrono::system_clock::time_point>(data, "time");

By default, toml::find returns a toml::value.

const toml::value& answer = toml::find(data, "answer");

When you pass an exact TOML type that does not require type conversion, toml::find returns a reference without copying the value.

const auto  data   = toml::parse("sample.toml");
const auto& answer = toml::find<toml::integer>(data, "answer");

If the specified type requires conversion, you can't take a reference to the value. See also underlying types.

NOTE: For some technical reason, automatic conversion between integer and floating is not supported. If you want to get a floating value even if a value has integer value, you need to convert it manually after obtaining a value, like the followings.

const auto vx = toml::find(data, "x");
double x = vx.is_floating() ? vx.as_floating(std::nothrow) :
           static_cast<double>(vx.as_integer()); // it throws if vx is neither
                                                 // floating nor integer.

Finding a value in a table

There are several way to get a value defined in a table. First, you can get a table as a normal value and find a value from the table.

name = "apple"
color = "red"
shape = "round"
const auto  data  = toml::parse("fruit.toml");
const auto& fruit = toml::find(data, "fruit");
const auto  name  = toml::find<std::string>(fruit, "name");

const auto& physical = toml::find(fruit, "physical");
const auto  color    = toml::find<std::string>(physical, "color");
const auto  shape    = toml::find<std::string>(physical, "shape");

Here, variable fruit is a toml::value and can be used as the first argument of toml::find.

Second, you can pass as many arguments as the number of subtables to toml::find.

const auto data  = toml::parse("fruit.toml");
const auto color = toml::find<std::string>(data, "fruit", "physical", "color");
const auto shape = toml::find<std::string>(data, "fruit", "physical", "shape");

Finding a value in an array

You can find n-th value in an array by toml::find.

values = ["foo", "bar", "baz"]
const auto data   = toml::parse("sample.toml");
const auto values = toml::find(data, "values");
const auto bar    = toml::find<std::string>(values, 1);

toml::find can also search array recursively.

const auto data = toml::parse("fruit.toml");
const auto bar  = toml::find<std::string>(data, "values", 1);

Before calling toml::find, you can check if a value corresponding to a key exists. You can use both bool toml::value::contains(const key&) const and std::size_t toml::value::count(const key&) const. Those behaves like the std::map::contains and std::map::count.

const auto data = toml::parse("fruit.toml");
if(data.contains("fruit") &&"fruit").count("physical") != 0)
    // ...

In case of error

If the value does not exist, toml::find throws std::out_of_range with the location of the table.

terminate called after throwing an instance of 'std::out_of_range'
  what():  [error] key "answer" not found
 --> example.toml
 6 | [tab]
   | ~~~~~ in this table

If the specified type differs from the actual value contained, it throws toml::type_error that inherits std::exception.

Similar to the case of syntax error, toml11 also displays clean error messages. The error message when you choose int to get string value would be like this.

terminate called after throwing an instance of 'toml::type_error'
  what():  [error] toml::value bad_cast to integer
 --> example.toml
 3 | title = "TOML Example"
   |         ~~~~~~~~~~~~~~ the actual type is string

NOTE: In order to show this kind of error message, all the toml values have a pointer to represent its range in a file. The entire contents of a file is shared by toml::values and remains on the heap memory. It is recommended to destruct all the toml::value classes after configuring your application if you have a large TOML file compared to the memory resource.

Dotted keys

TOML v0.5.0 has a new feature named "dotted keys". You can chain keys to represent the structure of the data.

physical.color = "orange"
physical.shape = "round"

This is equivalent to the following.

color = "orange"
shape = "round"

You can get both of the above tables with the same c++ code.

const auto physical = toml::find(data, "physical");
const auto color    = toml::find<std::string>(physical, "color");

The following code does not work for the above toml file.

// XXX this does not work!
const auto color = toml::find<std::string>(data, "physical.color");

The above code works with the following toml file.

"physical.color" = "orange"
# equivalent to {"physical.color": "orange"},
# NOT {"physical": {"color": "orange"}}.

Casting a toml value


toml::parse returns toml::value. toml::value is a union type that can contain one of the following types.

  • toml::boolean (bool)
  • toml::integer (std::int64_t)
  • toml::floating (double)
  • toml::string (a type convertible to std::string)
  • toml::local_date
  • toml::local_time
  • toml::local_datetime
  • toml::offset_datetime
  • toml::array (by default, std::vector<toml::value>)
  • toml::table (by default, std::unordered_map<toml::key, toml::value>)

To get a value inside, you can use toml::get<T>(). The usage is the same as toml::find<T> (actually, toml::find internally uses toml::get after casting a value to toml::table).

const toml::value  data    = toml::parse("sample.toml");
const toml::value  answer_ = toml::get<toml::table >(data).at("answer");
const std::int64_t answer  = toml::get<std::int64_t>(answer_);

When you pass an exact TOML type that does not require type conversion, toml::get returns a reference through which you can modify the content (if the toml::value is const, it returns const reference).

toml::value   data    = toml::parse("sample.toml");
toml::value   answer_ = toml::get<toml::table >(data).at("answer");
toml::integer& answer = toml::get<toml::integer>(answer_);
answer = 6 * 9; // write to data.answer. now `answer_` contains 54.

If the specified type requires conversion, you can't take a reference to the value. See also underlying types.

It also throws a toml::type_error if the type differs.


You can also use a member function to cast a value.

const std::int64_t answer = data.as_table().at("answer").as_integer();

It also throws a toml::type_error if the type differs. If you are sure that the value v contains a value of the specified type, you can suppress checking by passing std::nothrow.

const auto& answer = data.as_table().at("answer");
if(answer.is_integer() && answer.as_integer(std::nothrow) == 42)
    std::cout << "value is 42" << std::endl;

If std::nothrow is passed, the functions are marked as noexcept.

By casting a toml::value into an array or a table, you can iterate over the elements.

const auto data = toml::parse("example.toml");
std::cout << "keys in the top-level table are the following: \n";
for(const auto& [k, v] : data.as_table())
    std::cout << k << '\n';

const auto& fruits = toml::find(data, "fruits");
for(const auto& v : fruits.as_array())
    std::cout << toml::find<std::string>(v, "name") << '\n';

The full list of the functions is below.

namespace toml {
class value {
    // ...
    const boolean&         as_boolean()         const&;
    const integer&         as_integer()         const&;
    const floating&        as_floating()        const&;
    const string&          as_string()          const&;
    const offset_datetime& as_offset_datetime() const&;
    const local_datetime&  as_local_datetime()  const&;
    const local_date&      as_local_date()      const&;
    const local_time&      as_local_time()      const&;
    const array&           as_array()           const&;
    const table&           as_table()           const&;
    // --------------------------------------------------------
    // non-const version
    boolean&               as_boolean()         &;
    // ditto...
    // --------------------------------------------------------
    // rvalue version
    boolean&&              as_boolean()         &&;
    // ditto...

    // --------------------------------------------------------
    // noexcept versions ...
    const boolean&         as_boolean(const std::nothrow_t&) const& noexcept;
    boolean&               as_boolean(const std::nothrow_t&) &      noexcept;
    boolean&&              as_boolean(const std::nothrow_t&) &&     noexcept;
    // ditto...
} // toml


You can access to the element of a table and an array by toml::basic_value::at.

const toml::value v{1,2,3,4,5};
std::cout << << std::endl; // 3

const toml::value v{{"foo", 42}, {"bar", 3.14}};
std::cout <<"foo").as_integer() << std::endl; // 42

If an invalid key (integer for a table, string for an array), it throws toml::type_error for the conversion. If the provided key is out-of-range, it throws std::out_of_range.

Note that, although std::string has at() member function, toml::value::at throws if the contained type is a string. Because std::string does not contain toml::value.


You can also access to the element of a table and an array by toml::basic_value::operator[].

const toml::value v{1,2,3,4,5};
std::cout << v[2].as_integer() << std::endl; // 3

const toml::value v{{"foo", 42}, {"bar", 3.14}};
std::cout << v["foo"].as_integer() << std::endl; // 42

When you access to a toml::value that is not initialized yet via operator[](const std::string&), the toml::value will be a table, just like the std::map.

toml::value v; // not initialized as a table.
v["foo"] = 42; // OK. `v` will be a table.

Contrary, if you access to a toml::value that contains an array via operator[], it does not check anything. It converts toml::value without type check and then access to the n-th element without boundary check, just like the std::vector::operator[].

toml::value v; // not initialized as an array
v[2] = 42;     // error! UB

Please make sure that the toml::value has an array inside when you access to its element via operator[].

Checking value type

You can check the type of a value by is_xxx function.

const toml::value v = /* ... */;
    std::cout << "value is an integer" << std::endl;

The complete list of the functions is below.

namespace toml {
class value {
    // ...
    bool is_boolean()         const noexcept;
    bool is_integer()         const noexcept;
    bool is_floating()        const noexcept;
    bool is_string()          const noexcept;
    bool is_offset_datetime() const noexcept;
    bool is_local_datetime()  const noexcept;
    bool is_local_date()      const noexcept;
    bool is_local_time()      const noexcept;
    bool is_array()           const noexcept;
    bool is_table()           const noexcept;
    bool is_uninitialized()   const noexcept;
    // ...
} // toml

Also, you can get enum class value_t from toml::value::type().

    case toml::value_t::integer:  /*do some stuff*/ ; break;
    case toml::value_t::floating: /*do some stuff*/ ; break;
    case toml::value_t::string :  /*do some stuff*/ ; break;
    default : throw std::runtime_error(
        "unexpected type : " + toml::stringize("something").type()));

The complete list of the enums can be found in the section underlying types.

The enums can be used as a parameter of toml::value::is function like the following.

toml::value v = /* ... */;
if( // ...

More about conversion

Since toml::find internally uses toml::get, all the following examples work with both toml::get and toml::find.

Converting an array

You can get any kind of container class from a toml::array except for map-like classes.

// # sample.toml
// numbers = [1,2,3]

const auto numbers = toml::find(data, "numbers");

const auto vc  = toml::get<std::vector<int>  >(numbers);
const auto ls  = toml::get<std::list<int>    >(numbers);
const auto dq  = toml::get<std::deque<int>   >(numbers);
const auto ar  = toml::get<std::array<int, 3>>(numbers);
// if the size of"numbers") is larger than that of std::array,
// it will throw toml::type_error because std::array is not resizable.

Surprisingly, you can convert toml::array into std::pair and std::tuple.

// numbers = [1,2,3]
const auto tp = toml::get<std::tuple<short, int, unsigned int>>(numbers);

This functionality is helpful when you have a toml file like the following.

array_of_arrays = [[1, 2, 3], ["foo", "bar", "baz"]] # toml allows this

What is the corresponding C++ type? Obviously, it is a std::pair of std::vectors.

const auto array_of_arrays = toml::find(data, "array_of_arrays");
const auto aofa = toml::get<
    std::pair<std::vector<int>, std::vector<std::string>>

If you don't know the type of the elements, you can use toml::array, which is a std::vector of toml::value, instead.

const auto a_of_a = toml::get<toml::array>(array_of_arrays);
const auto first  = toml::get<std::vector<int>>(;

You can change the implementation of toml::array with std::deque or some other array-like container. See Customizing containers for detail.

Converting a table

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.

key1 = "foo" # all the values are
key2 = "bar" # toml String
const auto data = toml::parse("sample.toml");
const auto tab = toml::find<std::map<std::string, std::string>>(data, "tab");
std::cout << tab["key1"] << std::endl; // foo
std::cout << tab["key2"] << std::endl; // bar

But since toml::table is just an alias of std::unordered_map<toml::key, toml::value>, normally you don't need to convert it because it has all the functionalities that std::unordered_map has (e.g. operator[], count, and find). In most cases toml::table is sufficient.

toml::table tab = toml::get<toml::table>(data);
if(data.count("title") != 0)
    data["title"] = std::string("TOML example");

You can change the implementation of toml::table with std::map or some other map-like container. See Customizing containers for detail.

Getting an array of tables

An array of tables is just an array of tables. You can get it in completely the same way as the other arrays and tables.

# sample.toml
array_of_inline_tables = [{key = "value1"}, {key = "value2"}, {key = "value3"}]

key = "value4"
key = "value5"
key = "value6"
const auto data = toml::parse("sample.toml");
const auto aot1 = toml::find<std::vector<toml::table>>(data, "array_of_inline_tables");
const auto aot2 = toml::find<std::vector<toml::table>>(data, "array_of_tables");

Cost of conversion

Although conversion through toml::(get|find) is convenient, it has additional copy-cost because it copies data contained in toml::value to the user-specified type. Of course in some cases this overhead is not ignorable.

// the following code constructs a std::vector.
// it requires heap allocation for vector and element conversion.
const auto array = toml::find<std::vector<int>>(data, "foo");

By passing the exact types, toml::get returns reference that has no overhead.

const auto& tab     = toml::find<toml::table>(data, "tab");
const auto& numbers = toml::find<toml::array>(data, "numbers");

Also, as_xxx are zero-overhead because they always return a reference.

const auto& tab     = toml::find(data, "tab"    ).as_table();
const auto& numbers = toml::find(data, "numbers").as_array();

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.

const auto& num0 = toml::get<toml::integer>(;
const auto& num1 = toml::get<toml::integer>(;
const auto& num2 = toml::get<toml::integer>(;

Converting datetime and its variants

TOML v0.5.0 has 4 different datetime objects, local_date, local_time, local_datetime, and offset_datetime.

Since local_date, local_datetime, and offset_datetime represent a time point, you can convert them to std::chrono::system_clock::time_point.

Contrary, local_time does not represents a time point because they lack a date information, but it can be converted to std::chrono::duration that represents a duration from the beginning of the day, 00:00:00.000.

# sample.toml
date = 2018-12-23
time = 12:30:00
l_dt = 2018-12-23T12:30:00
o_dt = 2018-12-23T12:30:00+09:30
const auto data = toml::parse("sample.toml");

const auto date = toml::get<std::chrono::system_clock::time_point>("date"));
const auto l_dt = toml::get<std::chrono::system_clock::time_point>("l_dt"));
const auto o_dt = toml::get<std::chrono::system_clock::time_point>("o_dt"));

const auto time = toml::get<std::chrono::minutes>("time")); // 12 * 60 + 30 min

local_date and local_datetime are assumed to be in the local timezone when they are converted into time_point. On the other hand, offset_datetime only uses the offset part of the data and it does not take local timezone into account.

To contain datetime data, toml11 defines its own datetime types. For more detail, you can see the definitions in toml/datetime.hpp.

Getting with a fallback

toml::find_or returns a default value if the value is not found or has a different type.

const auto data = toml::parse("example.toml");
const auto num  = toml::find_or(data, "num", 42);

Also, toml::get_or returns a default value if toml::get<T> failed.

toml::value v("foo"); // v contains String
const int value = toml::get_or(v, 42); // conversion fails. it returns 42.

These functions automatically deduce what type you want to get from the default value you passed.

To get a reference through this function, take care about the default value.

toml::value v("foo"); // v contains String
toml::integer& i = toml::get_or(v, 42); // does not work because binding `42`
                                        // to `integer&` is invalid
toml::integer opt = 42;
toml::integer& i = toml::get_or(v, opt); // this works.

Expecting conversion

By using toml::expect, you will get your expected value or an error message without throwing toml::type_error.

const auto value = toml::expect<std::string>("title"));
if(value.is_ok()) {
    std::cout << value.unwrap() << std::endl;
} else {
    std::cout << value.unwrap_err() << std::endl;

Also, you can pass a function object to modify the expected value.

const auto value = toml::expect<int>("number"))
    .map(// function that receives expected type (here, int)
    [](const int number) -> double {
        return number * 1.5 + 1.0;
    }).unwrap_or(/*default value =*/ 3.14);

Visiting a toml::value

toml11 provides toml::visit to apply a function to toml::value in the same way as std::variant.

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.

Constructing a toml::value

toml::value can be constructed in various ways.

toml::value v(true);     // boolean
toml::value v(42);       // integer
toml::value v(3.14);     // floating
toml::value v("foobar"); // string
toml::value v(toml::local_date(2019, toml::month_t::Apr, 1)); // date
toml::value v{1, 2, 3, 4, 5};                                 // array
toml::value v{{"foo", 42}, {"bar", 3.14}, {"baz", "qux"}};    // table

When constructing a string, you can choose to use either literal or basic string. By default, it will be a basic string.

toml::value v("foobar", toml::string_t::basic  );
toml::value v("foobar", toml::string_t::literal);

Datetime objects can be constructed from std::tm and std::chrono::system_clock::time_point. But you need to specify what type you use to avoid ambiguity.

const auto now = std::chrono::system_clock::now();
toml::value v(toml::local_date(now));
toml::value v(toml::local_datetime(now));
toml::value v(toml::offset_datetime(now));

Since local time is not equivalent to a time point, because it lacks date information, it will be constructed from std::chrono::duration.

toml::value v(toml::local_time(std::chrono::hours(10)));

You can construct an array object not only from initializer_list, but also from STL containers. In that case, the element type must be convertible to toml::value.

std::vector<int> vec{1,2,3,4,5};
toml::value v(vec);

When you construct an array value, all the elements of initializer_list must be convertible into toml::value.

If a toml::value has an array, you can push_back an element in it.

toml::value v{1,2,3,4,5};

emplace_back also works.

Preserving comments

After toml11 v3, you can choose whether comments are preserved or not.

const auto data1 = toml::parse<toml::discard_comments >("example.toml");
const auto data2 = toml::parse<toml::preserve_comments>("example.toml");

Comments related to a value can be obtained by toml::value::comments(). The return value has the same interface as std::vector<std::string>.

const auto& com = v.comments();
for(const auto& c : com)
    std::cout << c << std::endl;

Comments just before and just after (within the same line) a value are kept in a value.

# this is a comment for v1.
v1 = "foo"

v2 = "bar" # this is a comment for v2.
# Note that this comment is NOT a comment for v2.

# this comment is not related to any value
# because there are empty lines between v3.
# this comment will be ignored even if you set `preserve_comments`.

# this is a comment for v3
# this is also a comment for v3.
v3 = "baz" # ditto.

Each comment line becomes one element of a std::vector.

Hash signs will be removed, but spaces after hash sign will not be removed.

v1.comments().at(0) == " this is a comment for v1."s;

v2.comments().at(1) == " this is a comment for v1."s;

v3.comments().at(0) == " this is a comment for v3."s;
v3.comments().at(1) == " this is also a comment for v3."s;
v3.comments().at(2) == " ditto."s;

Note that a comment just after an opening brace of an array will not be a comment for the array.

# this is a comment for a.
a = [ # this is not a comment for a. this will be ignored.
  1, 2, 3,
  # this is a comment for `42`.
  42, # this is also a comment for `42`.
] # this is a comment for a.

You can also append and modify comments. The interfaces are the same as std::vector<std::string>.

toml::basic_value<toml::preserve_comments> v(42);
v.comments().push_back(" add this comment.");
// # add this comment.
// i = 42

Also, you can pass a std::vector<std::string> when constructing a toml::basic_value<toml::preserve_comments>.

std::vector<std::string> comments{"comment 1", "comment 2"};
const toml::basic_value<toml::preserve_comments> v1(42, std::move(comments));
const toml::basic_value<toml::preserve_comments> v2(42, {"comment 1", "comment 2"});

When toml::discard_comments is chosen, comments will not be contained in a value. value::comments() will always be kept empty. All the modification on comments would be ignored. All the element access in a discard_comments causes the same error as accessing an element of an empty std::vector.

The comments will also be serialized. If comments exist, those comments will be added just before the values.

NOTE: Result types from toml::parse(...) and toml::parse<toml::preserve_comments>(...) are different.

Customizing containers

Actually, toml::basic_value has 3 template arguments.

template<typename Comment, // discard/preserve_comment
         template<typename ...> class Table = std::unordered_map,
         template<typename ...> class Array = std::vector>
class basic_value;

This enables you to change the containers used inside. E.g. you can use std::map to contain a table object instead of std::unordered_map. And also can use std::deque as a array object instead of std::vector.

You can set these parameters while calling toml::parse function.

const auto data = toml::parse<
    toml::preserve_comments, std::map, std::deque

Needless to say, the result types from toml::parse(...) and toml::parse<Com, Map, Cont>(...) are different (unless you specify the same types as default).

Note that, since toml::table and toml::array is an alias for a table and an array of a default toml::value, so it is different from the types actually contained in a toml::basic_value when you customize containers. To get the actual type in a generic way, use typename toml::basic_type<C, T, A>::table_type and typename toml::basic_type<C, T, A>::array_type.

TOML literal

toml11 supports "..."_toml literal. It accept both a bare value and a file content.

using namespace toml::literals::toml_literals;

// `_toml` can convert a bare value without key
const toml::value v = u8"0xDEADBEEF"_toml;
// v is an Integer value containing 0xDEADBEEF.

// raw string literal (`R"(...)"` is useful for this purpose)
const toml::value t = u8R"(
    title = "this is TOML literal"
    key = "value"
// the literal will be parsed and the result will be contained in t

The literal function is defined in the same way as the standard library literals such as std::literals::string_literals::operator""s.

namespace toml
inline namespace literals
inline namespace toml_literals
toml::value operator"" _toml(const char* str, std::size_t len);
} // toml_literals
} // literals
} // toml

Access to the operator can be gained with using namespace toml::literals;, using namespace toml::toml_literals, and using namespace toml::literals::toml_literals.

Note that a key that is composed only of digits is allowed in TOML. And, unlike the file parser, toml-literal allows a bare value without a key. Thus it is difficult to distinguish arrays having integers and definitions of tables that are named as digits. Currently, literal [1] becomes a table named "1". To ensure a literal to be considered as an array with one element, you need to add a comma after the first element (like [1,]).

"[1,2,3]"_toml;   // This is an array
"[table]"_toml;   // This is a table that has an empty table named "table" inside.
"[[1,2,3]]"_toml; // This is an array of arrays
"[[table]]"_toml; // This is a table that has an array of tables inside.

"[[1]]"_toml;     // This literal is ambiguous.
                  // Currently, it becomes a table that has array of table "1".
"1 = [{}]"_toml;  // This is a table that has an array of table named 1.
"[[1,]]"_toml;    // This is an array of arrays.
"[[1],]"_toml;    // ditto.

NOTE: _toml literal returns a toml::value that does not have comments.

Conversion between toml value and arbitrary types

You can also use toml::get and other related functions with the types you defined after you implement a way to convert it.

namespace ext
struct foo
    int         a;
    double      b;
    std::string c;
} // ext

const auto data = toml::parse("example.toml");

// to do this
const foo f = toml::find<ext::foo>(data, "foo");

There are 3 ways to use toml::get with the types that you defined.

The first one is to implement from_toml(const toml::value&) member function.

namespace ext
struct foo
    int         a;
    double      b;
    std::string c;

    void from_toml(const toml::value& v)
        this->a = toml::find<int        >(v, "a");
        this->b = toml::find<double     >(v, "b");
        this->c = toml::find<std::string>(v, "c");
} // ext

In this way, because toml::get first constructs foo without arguments, the type should be default-constructible.

The second is to implement constructor(const toml::value&).

namespace ext
struct foo
    explicit foo(const toml::value& v)
        : a(toml::find<int>(v, "a")), b(toml::find<double>(v, "b")),
          c(toml::find<std::string>(v, "c"))

    int         a;
    double      b;
    std::string c;
} // ext

Note that implicit default constructor declaration will be suppressed when a constructor is defined. If you want to use the struct (here, foo) in a container (e.g. std::vector<foo>), you may need to define default constructor explicitly.

The third is to implement specialization of toml::from for your type.

namespace ext
struct foo
    int         a;
    double      b;
    std::string c;
} // ext

namespace toml
struct from<ext::foo>
    static ext::foo from_toml(const value& v)
        ext::foo f;
        f.a = find<int        >(v, "a");
        f.b = find<double     >(v, "b");
        f.c = find<std::string>(v, "c");
        return f;
} // toml

In this way, since the conversion function is defined outside of the class, you can add conversion between toml::value and classes defined in another library.

Note that you cannot implement both of the functions because the overload resolution of toml::get will be ambiguous.

If you want to convert any versions of toml::basic_value, you need to templatize the conversion function as follows.

struct foo
    template<typename C, template<typename ...> class M, template<typename ...> class A>
    void from_toml(const toml::basic_value<C, M, A>& v)
        this->a = toml::find<int        >(v, "a");
        this->b = toml::find<double     >(v, "b");
        this->c = toml::find<std::string>(v, "c");
// or
namespace toml
struct from<ext::foo>
    template<typename C, template<typename ...> class M, template<typename ...> class A>
    static ext::foo from_toml(const basic_value<C, M, A>& v)
        ext::foo f;
        f.a = find<int        >(v, "a");
        f.b = find<double     >(v, "b");
        f.c = find<std::string>(v, "c");
        return f;
} // toml

The opposite direction is also supported in a similar way. You can directly pass your type to toml::value's constructor by introducing into_toml or toml::into<T>.

namespace ext
struct foo
    int         a;
    double      b;
    std::string c;

    toml::value into_toml() const // you need to mark it const.
        return toml::value{{"a", this->a}, {"b", this->b}, {"c", this->c}};
} // ext

ext::foo    f{42, 3.14, "foobar"};
toml::value v(f);

The definition of toml::into<T> is similar to toml::from<T>.

namespace ext
struct foo
    int         a;
    double      b;
    std::string c;
} // ext

namespace toml
struct into<ext::foo>
    static toml::value into_toml(const ext::foo& f)
        return toml::value{{"a", f.a}, {"b", f.b}, {"c", f.c}};
} // toml

ext::foo    f{42, 3.14, "foobar"};
toml::value v(f);

Any type that can be converted to toml::value, e.g. int, toml::table and toml::array are okay to return from into_toml.

You can also return a custom toml::basic_value from toml::into.

namespace toml
struct into<ext::foo>
    static toml::basic_value<toml::preserve_comments> into_toml(const ext::foo& f)
        toml::basic_value<toml::preserve_comments> v{{"a", f.a}, {"b", f.b}, {"c", f.c}};
        v.comments().push_back(" comment");
        return v;
} // toml

But note that, if this basic_value would be assigned into other toml::value that discards comments, the comments would be dropped.

Formatting user-defined error messages

When you encounter an error after you read the toml value, you may want to show the error with the value.

toml11 provides you a function that formats user-defined error message with related values. With a code like the following,

const auto value = toml::find<int>(data, "num");
if(value < 0)
    std::cerr << toml::format_error("[error] value should be positive",
                          "num"), "positive number required")
              << std::endl;

you will get an error message like this.

[error] value should be positive
 --> example.toml
 3 | num = -42
   |       ~~~ positive number required

When you pass two values to toml::format_error,

const auto min = toml::find<int>(range, "min");
const auto max = toml::find<int>(range, "max");
if(max < min)
    std::cerr << toml::format_error("[error] max should be larger than min",
                          "min"), "minimum number here",
                          "max"), "maximum number here");
              << std::endl;

you will get an error message like this.

[error] max should be larger than min
 --> example.toml
 3 | min = 54
   |       ~~ minimum number here
 4 | max = 42
   |       ~~ maximum number here

You can print hints at the end of the message.

std::vector<std::string> hints;
hints.push_back("positive number means n >= 0.");
hints.push_back("negative number is not positive.");
std::cerr << toml::format_error("[error] value should be positive",
                      "num"), "positive number required", hints)
          << std::endl;
[error] value should be positive
 --> example.toml
 2 | num = 42
   |       ~~ positive number required
Hint: positive number means n >= 0.
Hint: negative number is not positive.

Obtaining location information

You can also format error messages in your own way by using source_location.

struct source_location
    std::uint_least32_t line()      const noexcept;
    std::uint_least32_t column()    const noexcept;
    std::uint_least32_t region()    const noexcept;
    std::string const&  file_name() const noexcept;
    std::string const&  line_str()  const noexcept;
// +-- line()       +--- length of the region (here, region() == 9)
// v            .---+---.
// 12 | value = "foo bar" <- line_str() returns the line itself.
//              ^-------- column() points here

You can get this by

const toml::value           v   = /*...*/;
const toml::source_location loc = v.location();


The following exception classes inherits toml::exception that inherits std::exception.

namespace toml {
struct exception      : public std::exception  {/**/};
struct syntax_error   : public toml::exception {/**/};
struct type_error     : public toml::exception {/**/};
struct internal_error : public toml::exception {/**/};
} // toml

toml::exception has toml::exception::location() member function that returns toml::source_location, in addition to what().

namespace toml {
struct exception : public std::exception
    // ...
    source_location const& location() const noexcept;
} // toml

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 toml::parse and toml::find|get will be colorized. By default, this feature is turned off.

With the following toml file taken from toml-lang/toml/tests/hard_example.toml,

array = [
         "This might most likely happen in multiline arrays",
         Like here,
         "or here,
         and here"
        ]     End of array comment, forgot the #

the error message would be like this.


With the following,

# array = [
#          "This might most likely happen in multiline arrays",
#          Like here,
#          "or here,
#          and here"
#         ]     End of array comment, forgot the #
number = 3.14  pi <--again forgot the #

the error message would be like this.


The message would be messy when it is written to a file, not a terminal because it uses ANSI escape code.

Without TOML11_COLORIZE_ERROR_MESSAGE, you can still colorize user-defined error message by passing true to the toml::format_error function. If you define TOML11_COLORIZE_ERROR_MESSAGE, the value is true by default. If not, the defalut value would be false.

std::cerr << toml::format_error("[error] value should be positive",
                      "num"), "positive number required",
                                hints, /*colorize = */ true) << std::endl;

Note: It colorize [error] in red. That means that it detects [error] prefix at the front of the error message. If there is no [error] prefix, format_error adds it to the error message.

Serializing TOML data

toml11 enables you to serialize data into toml format.

const toml::value data{{"foo", 42}, {"bar", "baz"}};
std::cout << data << std::endl;
// bar = "baz"
// foo = 42

toml11 automatically makes a small table and small array inline. You can specify the width to make them inline by std::setw for streams.

const toml::value data{
    {"qux",    {{"foo", 42}, {"bar", "baz"}}},
    {"quux",   {"small", "array", "of", "strings"}},
    {"foobar", {"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.

To control the precision of floating point numbers, you need to pass std::setprecision to stream.

const toml::value data{
    {"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

There is another way to format toml values, toml::format(). It returns std::string that represents a value.

const toml::value v{{"a", 42}};
const std::string fmt = toml::format(v);
// a = 42

Note that since toml::format formats a value, the resulting string may lack the key value.

const toml::value v{3.14};
const std::string fmt = toml::format(v);
// 3.14

To control the width and precision, toml::format receives optional second and third arguments to set them. By default, the witdh is 80 and the precision is std::numeric_limits<double>::max_digit10.

const auto serial = toml::format(data, /*width = */ 0, /*prec = */ 17);

When you pass a comment-preserving-value, the comment will also be serialized. An array or a table containing a value that has a comment would not be inlined.

Underlying types

The toml types (can be used as toml::* in this library) and corresponding enum names are listed in the table below.

TOML type underlying c++ type enum class
Boolean bool toml::value_t::boolean
Integer std::int64_t toml::value_t::integer
Float double toml::value_t::floating
String toml::string toml::value_t::string
LocalDate toml::local_date toml::value_t::local_date
LocalTime toml::local_time toml::value_t::local_time
LocalDatetime toml::local_datetime toml::value_t::local_datetime
OffsetDatetime toml::offset_datetime toml::value_t::offset_datetime
Array array-like<toml::value> toml::value_t::array
Table map-like<toml::key, toml::value> toml::value_t::table

array-like and map-like are the STL containers that works like a std::vector and std::unordered_map, respectively. By default, std::vector and std::unordered_map are used. See Customizing containers for detail.

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. The most important difference between std::string and toml::string is that toml::string will be formatted as a TOML string when outputed with ostream. This feature is introduced to make it easy to write a custom serializer.

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.

Unreleased TOML features

Since TOML v1.0.0-rc.1 has been released, those features are now activated by default. We no longer need to define TOML11_USE_UNRELEASED_FEATURES.

Note about heterogeneous arrays

Although toml::parse allows heterogeneous arrays, constructor of toml::value does not. Here the reason is explained.

// this won't be compiled
toml::value v{
    "foo", 3.14, 42, {1,2,3,4,5}, {{"key", "value"}}

There is a workaround for this. By explicitly converting values into toml::value, you can initialize toml::value with a heterogeneous array. Also, you can first initialize a toml::value with an array and then push_back into it.

// OK!
toml::value v{
    toml::value("foo"), toml::value(3.14), toml::value(42),
    toml::value{1,2,3,4,5}, toml::value{{"key", "value"}}

// OK!
toml::value v(toml::array{});

// OK!
toml::array a;
toml::value v(std::move(a));

The reason why the first example is not allowed is the following. Let's assume that you are initializing a toml::value with a table.

                    // # expecting TOML table.
toml::value v{      // [v]
    {"answer", 42}, // answer = 42
    {"pi",   3.14}, // pi = 3.14
    {"foo", "bar"}  // foo = "bar"

This is indistinguishable from a (heterogeneous) TOML array definition.

v = [
    ["answer", 42],
    ["pi",   3.14],
    ["foo", "bar"],

This means that the above C++ code makes constructor's overload resolution ambiguous. So a constructor that allows both "table as an initializer-list" and "heterogeneous array as an initializer-list" cannot be implemented.

Thus, although it is painful, we need to explicitly cast values into toml::value when you initialize heterogeneous array in a C++ code.

toml::value v{
    toml::value("foo"), toml::value(3.14), toml::value(42),
    toml::value{1,2,3,4,5}, toml::value{{"key", "value"}}

Breaking Changes from v2

Although toml11 is relatively new library (it's three years old now), it had some confusing and inconvenient user-interfaces because of historical reasons.

Between v2 and v3, those interfaces are rearranged.

Such a big change will not happen in the coming years.

Running Tests

After cloning this repository, run the following command (thank you @jwillikers for automating test set fetching!).

$ mkdir build
$ cd build
$ cmake .. -Dtoml11_BUILD_TEST=ON
$ make
$ make test

To run the language agnostic test suite, you need to compile tests/check_toml_test.cpp and pass it to the tester.


I appreciate the help of the contributors who introduced the great feature to this library.

  • Guillaume Fraux (@Luthaf)
    • Windows support and CI on Appvayor
    • Intel Compiler support
  • Quentin Khan (@xaxousis)
    • Found & Fixed a bug around ODR
    • Improved error messages for invaild keys to show the location where the parser fails
  • Petr BeneŇ° (@wbenny)
    • Fixed warnings on MSVC
  • Ivan Shynkarenka (@chronoxor)
    • Fixed Visual Studio 2019 warnings
  • @khoitd1997
    • Fixed warnings while type conversion
  • @KerstinKeller
    • Added installation script to CMake
  • J.C. Moyer (@jcmoyer)
    • Fixed an example code in the documentation
  • Jt Freeman (@blockparty-sh)
    • Fixed feature test macro around localtime_s
    • 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
  • Scott McCaskill
    • Parse 9 digits (nanoseconds) of fractional seconds in a local_time
  • Shu Wang (@halfelf)
    • fix "Finding a value in an array" example in README
  • @maass-tv and @SeverinLeonhardt
    • Fix MSVC warning C4866
  • OGAWA KenIchi (@kenichiice)
    • Fix include path in README
  • Mohammed Alyousef (@MoAlyousef)
    • Made testing optional in CMake

Licensing terms

This product is licensed under the terms of the MIT License.

  • Copyright (c) 2017-2020 Toru Niina

All rights reserved.

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