toml11

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

compatible to the latest version of TOML v0.5.0 after version 2.0.0.

It passes the language agnostic test suite for TOML parsers by BurntSushi. Not only the test suite itself, a TOML reader/encoder also runs on CircleCI. You can see the error messages about invalid files and serialization results of valid files at CircleCI.

Example

#include <toml11/toml.hpp>
#include <iostream>

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

    // title = "an example toml file"
    std::string title = toml::get<std::string>(data.at("title"));
    std::cout << "the title is " << title << std::endl;

    // nums = [1, 2, 3, 4, 5]
    std::vector<int> nums  = toml::get<std::vector<int>>(data.at("nums"));
    std::cout << "the length of `nums` is" << nums.size() << std::endl;

    return 0;
}

Table of Contents

• Integration
• Decoding a toml file
  • In the case of syntax error
• Getting a toml value
  • In the case of type error
• Getting an array
• Getting a table
  • Dotted keys
• Getting an array of tables
• Cost of conversion
• Getting datetime and its variants
• Getting with a fallback
• Expecting conversion
• Finding a value from a table
• Checking value type
• Visiting a toml::value
• TOML literal
• Conversion between toml value and arbitrary types
• Invalid UTF-8 Codepoints
• Formatting user-defined error messages
• Serializing TOML data
• Underlying types
• Running Tests
• Contributors
• Licensing Terms

Integration

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

#include <toml11/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::get<std::string>(data.at("title"));
    std::cout << "the title is " << title << std::endl;
    return 0;
}

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::table data = toml::parse(fname);

If it encounters a file open error, 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, it is recommended to pass the filename with the stream.

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

Note that on Windows, 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. To use a file stream with toml::parse on Windows, don't forget to open it in binary mode.

In the case of syntax error

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

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

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!

Getting a toml value

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

# sample.toml
answer  = 42
pi      = 3.14
numbers = [1,2,3]
time    = 1979-05-27T07:32:00Z
[tab]
key = "value"

const auto data      = toml::parse("sample.toml");
const auto answer    = toml::get<std::int64_t    >(data.at("answer"));
const auto pi        = toml::get<double          >(data.at("pi"));
const auto numbers   = toml::get<std::vector<int>>(data.at("numbers"));
const auto timepoint = toml::get<std::chrono::system_clock::time_point>(data.at("time"));
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 a reference through which you can modify the content.

auto  data   = toml::parse("sample.toml");
auto& answer = toml::get<toml::integer>(data["answer"]);     // get reference
answer = 6 * 9;                                              // write to data.answer
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.

NOTE: To enable to get a reference, conversions between Float and Integer are not supported.

In the case of type error

If you pass an invalid type to toml::get, toml::type_error will be thrown. 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 pointers 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.

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

Surprisingly, you can also get std::pairs and std::tuples from toml::array.

const auto tp = toml::get<std::tuple<short, int, unsigned int>>(data.at("numbers"));

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

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 aofa = toml::get<
    std::pair<std::vector<int>, std::vector<std::string>>
    >(data.at("array_of_arrays"));

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 aofa  = toml::get<toml::array>(data.at("array_of_arrays"));
const auto first = toml::get<std::vector<int>>(aofa.at(0));

See also expecting conversion and checking-value-type.

Getting a table

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.

toml::table data = toml::parse("example.toml");
if(data.count("title") != 0)
{
    data["title"] = std::string("TOML example");
}

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.

[tab]
key1 = "foo" # all the values are
key2 = "bar" # toml String

const auto data = toml::parse("sample.toml");
const auto tab = toml::get<std::map<std::string, std::string>>(data.at("tab"));
std::cout << tab["key1"] << std::endl; // foo
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.

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

This is equivalent to the following.

[physical]
color = "orange"
shape = "round"

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

const auto physical = toml::get<toml::table>(data.at("physical"));
const auto color    = toml::get<std::string>(physical.at("color"));

Getting 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.

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

[[array_of_tables]]
key = "value4"
[[array_of_tables]]
key = "value5"
[[array_of_tables]]
key = "value6"

const auto data = toml::parse("sample.toml");
const auto aot1 = toml::get<std::vector<toml::table>>(data.at("array_of_inline_tables"));
const auto aot2 = toml::get<std::vector<toml::table>>(data.at("array_of_tables"));

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 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::get<std::vector<int>>(data.at("foo"));

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

const auto& tab     = toml::get<toml::array>(data.at("tab"));
const auto& numbers = toml::get<toml::table>(data.at("numbers"));

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>(numbers.at(0));
const auto& num1 = toml::get<toml::integer>(numbers.at(1));
const auto& num2 = toml::get<toml::integer>(numbers.at(2));

Getting 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.

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

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

toml11 contains datetime as its own struct. You can see the definitions in toml/datetime.hpp.

Getting with a fallback

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.

toml::get_or automatically deduces 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>(data.at("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>(data.at("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);

Finding a value from a 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.

const auto data = toml::parse("example.toml");
const auto num  = toml::find<int>(data, "num", /*for err msg*/"example.toml");

If the value does not exist, it throws std::out_of_range with an error message.

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.

const auto data = toml::parse("example.toml");
const auto num  = toml::find<int>(data.at("table"), "num");
// expecting the following example.toml
// [table]
// 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 an error message including table location.

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

If it's not a toml::table, the same error as "invalid type" would be thrown.

There is another utility function, toml::find_or. It is almost same as toml::find, but returns a default value if the value is not found or has a different type, like toml::get_or.

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

Checking value type

You can check what type of value does toml::value contains by is_* function.

toml::value v = /* ... */;
if(v.is_integer() && toml::get<int>(v) == 42)
{
    std::cout << "value is 42" << std::endl;
}

The complete list of the functions is below.

const toml::value v(/*...*/);
v.is_boolean();
v.is_integer();
v.is_float();
v.is_string();
v.is_offset_datetime();
v.is_local_datetime();
v.is_local_date();
v.is_local_time();
v.is_array();
v.is_table();
v.is_uninitialized();

Also, you can get enum class value from toml::value.

switch(data.at("something").type())
{
    case toml::value_t::Integer: /*do some stuff*/ ; break;
    case toml::value_t::Float  : /*do some stuff*/ ; break;
    case toml::value_t::String : /*do some stuff*/ ; break;
    default : throw std::runtime_error(
        "unexpected type : " + toml::stringize(data.at("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(v.is(toml::value_t::Boolean)) // ...

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.

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"
    [table]
    key = "value"
)"_toml;
// 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 since it allows a bare value without a key, it is difficult to distinguish arrays and table definitions. Currently, it parses [1] as a table definition if there are no commas. 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 is ambiguous.
                  // Currently, it becomes a table taht 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.

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 some stuff.

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

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

const foo f = toml::get<ext::foo>(data.at("foo"));

There are 2 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");
        return;
    }
};
} // ext

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

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

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

namespace toml
{
template<>
struct from<ext::foo>
{
    ext::foo from_toml(const toml::value& v)
    {
        ext::foo f;
        f.a = toml::find<int        >(v, "a");
        f.b = toml::find<double     >(v, "b");
        f.c = toml::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.

---

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::table into_toml() const // you need to mark it const.
    {
        return toml::table{{"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
{
template<>
struct into<ext::foo>
{
    toml::table into_toml(const ext::foo& f)
    {
        return toml::table{{"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. toml::table, toml::array, is okay to return from into_toml.

Invalid UTF-8 codepoints

toml11 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]

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",
                                    data.at("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",
                                    data.at("min"), "minimum number here",
                                    data.at("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

Serializing TOML data

toml11 (after v2.1.0) enables you to serialize data into toml format.

const auto data = toml::table{{"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 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.

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

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

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);

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
Boolean	bool	toml::value_t::Boolean
Integer	std::int64_t	toml::value_t::Integer
Float	double	toml::value_t::Float
String	toml::string	toml::value_t::String
LocalDate	toml::local_date	toml::value_t::LocalDate
LocalTime	toml::local_time	toml::value_t::LocalTime
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<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.

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 Datetimes as std::chrono classes through toml::get.

Running Tests

To run test codes, you need to clone toml-lang/toml repository under build/ directory because some of the test codes read a file in the repository.

$ mkdir build
$ cd build
$ git clone https://github.com/toml-lang/toml.git
$ cmake ..
$ 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.

Contributors

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

Licensing terms

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

• Copyright (c) 2017 Toru Niina

All rights reserved.