std::basic_format_context

 

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Formatting library

Standard format specification

Formatting functions

format (C++20)

format_to (C++20)

format_to_n (C++20)

formatted_size (C++20)

vformat (C++20)

vformat_to (C++20)

Format strings

basic_format_stringformat_stringwformat_string (C++20)(C++20)(C++20)

runtime_format (C++26)

Formatting concepts

formattable (C++23)

Formatter

formatter (C++20)

formatter<pair-or-tuple> (C++23)

formatter<range> (C++23)

range_formatter (C++23)

enable_nonlocking_formatter_optimization (C++23)

basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)

basic_format_contextformat_contextwformat_context (C++20)(C++20)(C++20)

range_format (C++23)

format_kind (C++23)

Formatting arguments

basic_format_arg (C++20)

basic_format_arg::handle (C++20)

basic_format_argsformat_argswformat_args (C++20)(C++20)(C++20)

visit_format_arg (C++20) (deprecated in C++26)

make_format_argsmake_wformat_args (C++20)(C++20)

Format error

format_error (C++20)

 

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.

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