# std::basic_format_context

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< cpp‎ | utility‎ | format

Defined in header `<format>` |  |
template< class OutputIt, class CharT > class basic_format_context; | (1) | (since C++20)
using format_context = basic_format_context</* unspecified */, char>; | (2) | (since C++20)
using wformat_context = basic_format_context</* unspecified */, wchar_t>; | (3) | (since C++20)

Provides access to formatting state consisting of the formatting arguments and the output iterator.

2) The unspecified template argument is an output iterator that appends to std::string, such as std::back_insert_iterator < std::string >. Implementations typically use an iterator to type-erased buffer type that supports appending to any contiguous and resizable container.
3) The unspecified template argument is an output iterator that appends to std::wstring.

The behavior is undefined if `OutputIt` does not model std::output_iterator <const CharT&>.

A program that declares an explicit or partial specialization of `std::basic_format_context` is ill-formed, no diagnostic required.

`std::basic_format_context` objects can only be created by the implementation. User codes are only allowed to modify the format context via the `format` function of std::formatter specializations.

### Member types

Type | Definition
`iterator` | `OutputIt`
`char_type` | `CharT`

### Member alias templates

Type | Definition
formatter_type<T> | std::formatter <T, CharT>

### Member functions

(constructor) [deleted] | `basic_format_context` cannot be constructed by user code (public member function)
operator= [deleted] | `basic_format_context` is not assignable (public member function)
arg | returns the argument at the given index (public member function)
locale | returns the locale used for locale-specific formatting (public member function)
out | returns the iterator to output buffer (public member function)
advance_to | advances the output iterator to the given position (public member function)

## std::basic_format_context::arg

std::basic_format_arg <basic_format_context> arg( std::size_t id ) const; |  |

Returns a `std::basic_format_arg` holding the `id`-th argument in `args`, where `args` is the parameter pack or `std::basic_format_args` object passed to the formatting function.

If `id` is not less than the number of formatting arguments, returns a default-constructed `std::basic_format_arg` (holding a std::monostate object).

## std::basic_format_context::locale

std::locale locale(); |  |

Returns the locale passed to the formatting function, or a default-constructed std::locale if the formatting function does not take a locale.

## std::basic_format_context::out

iterator out(); |  |

Returns the iterator to the output buffer. The result is move-constructed from the stored iterator.

## std::basic_format_context::advance_to

void advance_to( iterator it ); |  |

Move assigns `it` to the stored output iterator. After a call to `advance_to`, the next call to `out()` will return an iterator with the value that `it` had before the assignment.

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3567 | C++20 | `basic_format_context` does not work move-only iterator types | made to move iterators
LWG 3975 | C++20 | user specialization of `basic_format_context` was allowed | disallowed
LWG 4061 | C++20 | `basic_format_context` was constructible and assignable by user code | made neither constructible nor assignable

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