< cpp‎ | utility‎ | format
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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.


[edit] Member types

Type Definition
iterator OutputIt
char_type CharT

[edit] Member alias templates

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

[edit] Member functions

returns the argument at the given index
(public member function)
returns the locale used for locale-specific formatting
(public member function)
returns the iterator to output buffer
(public member function)
advances the output iterator to the given position
(public member function)


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::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.


iterator out();

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


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.

[edit] Example

[edit] 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