toml11/README.md

toml11
======

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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](https://github.com/toml-lang/toml/blob/master/versions/en/toml-v0.5.0.md)
after version 2.0.0.

It passes [the language agnostic test suite for TOML parsers by BurntSushi](https://github.com/BurntSushi/toml-test).
Not only the test suite itself, a TOML reader/encoder also runs on [CircleCI](https://circleci.com/gh/ToruNiina/toml11).
You can see the error messages about invalid files and serialization results of valid files at
[CircleCI](https://circleci.com/gh/ToruNiina/toml11).

## Example

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

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

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

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

    return 0;
}
```

## Table of Contents

- [Integration](#integration)
- [Decoding a toml file](#decoding-a-toml-file)
  - [In the case of syntax error](#in-the-case-of-syntax-error)
  - [Invalid UTF-8 Codepoints](#invalid-utf-8-codepoints)
- [Finding a toml value](#getting-a-toml-value)
  - [In the case of type error](#in-the-case-of-type-error)
  - [Dotted keys](#dotted-keys)
- [Casting a toml value](#casting-a-toml-value)
- [Checking value type](#checking-value-type)
- [More about conversion](#more-about-conversion)
  - [Getting an array](#getting-an-array)
  - [Getting a table](#getting-a-table)
  - [Getting an array of tables](#getting-an-array-of-tables)
  - [Cost of conversion](#cost-of-conversion)
  - [Getting datetime and its variants](#getting-datetime-and-its-variants)
- [Getting with a fallback](#getting-with-a-fallback)
- [Expecting conversion](#expecting-conversion)
- [Visiting a toml::value](#visiting-a-tomlvalue)
- [Constructing a toml::value](#constructing-a-tomlvalue)
- [Preserving Comments](#preserving-comments)
- [Customizing containers](#customizing-containers)
- [TOML literal](#toml-literal)
- [Conversion between toml value and arbitrary types](#conversion-between-toml-value-and-arbitrary-types)
- [Formatting user-defined error messages](#formatting-user-defined-error-messages)
- [Getting comments related to a value](#getting-comments)
- [Serializing TOML data](#serializing-toml-data)
- [Underlying types](#underlying-types)
- [Breaking Changes from v2](#breaking-changes-from-v2)
- [Running Tests](#running-tests)
- [Contributors](#contributors)
- [Licensing Terms](#licensing-terms)

## Integration

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

```cpp
#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::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.

## 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.

```cpp
const std::string fname("sample.toml");
const toml::table data = toml::parse(fname);
```

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, it is recommended to pass the filename
with the stream.

```cpp
std::ifstream ifs("sample.toml", std::ios_base::binary);
assert(ifs.good());
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.

```console
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.

```console
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!

### 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.

```console
  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 from a table

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

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

``` cpp
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");
const auto tab       = toml::find<toml::table>(data, "tab");
const auto key       = toml::find<std::string>(tab,  "key");
```

If the value does not exist, `toml::find` throws an error with the location of
the table.

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

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

```cpp
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](#underlying-types).

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

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

**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.

----

There are several ways to find a value buried in a deep recursion of tables.

First, you can call `toml::find` as many as you need.

```cpp
// # expecting the following example.toml
// answer.to.the.ultimate.question = 42
// # is equivalent to {"answer": {"to":{"the":{"ultimate:{"question":42}}}}}

const toml::table data = toml::parse("example.toml");
const int a = toml::find<int>(toml::find(toml::find(toml::find(toml::find(
    data, "answer"), "to"), "the"), "ultimate"), "question");
```

But it is a bother. Alternatively, you can pass several keys to `toml::find` to
find the value.

```cpp
const toml::value data = toml::parse("example.toml");
const int a = toml::find<int>(data, "answer", "to", "the", "ultimate", "question");
```

### In the case of type error

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.

```console
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::value`s 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.

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

This is equivalent to the following.

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

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

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

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

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

The above code works with the following toml file.

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

## Casting a toml value

### `toml::get`

`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>`)
  - It depends. See [customizing containers](#customizing-containers) for detail.
- `toml::table` (by default, `std::unordered_map<toml::key, toml::value>`)
  - It depends. See [customizing containers](#customizing-containers) for detail.

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`).

``` cpp
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.

```cpp
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](#underlying-types).

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

### `as_xxx`

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

```cpp
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`.

```cpp
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.

The full list of the functions is below.

```cpp
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
```



## Checking value type

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

```cpp
const toml::value v = /* ... */;
if(v.is_integer())
{
    std::cout << "value is an integer" << std::endl;
}
```

The complete list of the functions is below.

```cpp
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()`.

```cpp
switch(data.at("something").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(data.at("something").type()));
}
```

The complete list of the `enum`s can be found in the section
[underlying types](#underlying-types).

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

```cpp
toml::value v = /* ... */;
if(v.is(toml::value_t::boolean)) // ...
```

**NOTE**: BREAKING CHANGES from v2.y.z: `(is|as)_float` has been removed.
Use `(is|as)_floating` instead.
See [Breaking Changes from v2](#breaking-changes-from-v2) for the complete list
of breaking changes.

## 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.

``` cpp
// # 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 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 convert `toml::array` into `std::pair` and `std::tuple`.

```cpp
// 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.

```toml
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::vector`s.

```cpp
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>>
    >(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.

```cpp
const auto a_of_a = toml::get<toml::array>(array_of_arrays);
const auto first  = toml::get<std::vector<int>>(a_of_a.at(0));
```

You can change the implementation of `toml::array` with `std::deque` or some
other array-like container. See [Customizing containers](#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.

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

```cpp
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.

```cpp
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](#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.

```toml
# 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"
```

```cpp
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.

```cpp
// 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.

``` cpp
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.

``` cpp
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`.

```cpp
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));
```

### 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`.

```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
```

```cpp
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 defines its own datetime classes.
You can see the definitions in [toml/datetime.hpp](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.

```cpp
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.

```cpp
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.

```cpp
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`.

```cpp
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.

```cpp
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);
```

## Visiting toml::value

toml11 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.

## Constructing a toml::value

TODO

## Preserving comments

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

```cpp
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>`.

```cpp
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.

```toml
# 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.

```cpp
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.

```toml
# 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`.
  5
] # this is a comment for a.
```

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

```cpp
v.comments().push_back(" add new comment.");
```

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.

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

## Customizing containers

Actually, `toml::basic_value` has 3 template arguments.

```cpp
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.

```cpp
const auto data = toml::parse<
    toml::preserve_comments, std::map, std::deque
    >("example.toml");
```

## TOML literal

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

```cpp
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`.

```cpp
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,]`).

```cpp
"[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.

```cpp
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 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.

```cpp
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.

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

namespace toml
{
template<>
struct from<ext::foo>
{
    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 arbitrary specialization of `toml::basic_value`,
templatize the conversion function as follows.

```cpp
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");
        return;
    }
};
// or
namespace toml
{
template<>
struct from<ext::foo>
{
    template<typename C, template<typename ...> class M, template<typename ...> class A>
    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>`.

```cpp
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>`.

```cpp
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. `int`, `toml::table` and
`toml::array` are okay to return from `into_toml`.

## 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,

```cpp
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.

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

When you pass two values to `toml::format_error`,

```cpp
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.

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

### Obtaining location information

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

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

## Serializing TOML data

toml11 enables you to serialize data into toml format.

```cpp
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.

```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.

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

```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
```

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

```cpp
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.

```cpp
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`.

```cpp
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](#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.

`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`.

## 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.

- `toml::parse` now returns a `toml::value`, not `toml::table`.
- `toml::value` is now an alias of `toml::basic_value<discard_comment, std::vector, std::unordered_map>`.
  - See [Customizing containers](#customizing-containers) for detail.
- The elements of `toml::value_t` are renamed as `snake_case`.
  - See [Underlying types](#underlying-types) for detail.
- Supports for the CamelCaseNames are dropped.
  - See [Underlying types](#underlying-types) for detail.
- `(is|as)_float` has been removed to make the function names consistent with others.
  Since `float` is a keyword, toml11 named a float type as `toml::floating`.
  Also a `value_t` corresponds to `toml::floating` is named `value_t::floating`.
  So `(is|as)_floating` is introduced and `is_float` has been removed.
  - See [Casting a toml::value](#casting-a-tomlvalue) and [Checking value type](#checking-value-type) for detail.
- `toml::find` for `toml::table` has been dropped. Use `toml::value` version instead.
  - See [Finding a toml::value](#finding-a-tomlvalue) for detail.
- Interface around comments.
  - See [Preserving Comments](#preserving-comments) for detail.
- An old `from_toml` has been removed

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

## Running Tests

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

```sh
$ 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
- Petr Beneš (@wbenny)
  - Fixed warnings on MSVC
- Ivan Shynkarenka (@chronoxor)
  - Fixed Visual Studio 2019 warnings
- @khoitd1997
  - Fixed warnings while type conversion

## Licensing terms

This product is licensed under the terms of the [MIT License](LICENSE).

- Copyright (c) 2017-2019 Toru Niina

All rights reserved.