std::basic_format_context - cppreference.com

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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

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

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

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