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std::basic_format_context

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< cpp‎ | utility‎ | format
 
 
Text processing library
 
Formatting library
Standard format specification
Formatting functions
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
Format strings
Formatting concepts
(C++23)
Formatter
(C++20)
basic_format_contextformat_contextwformat_context
(C++20)(C++20)(C++20)
(C++23)
(C++23)
Formatting arguments
(C++20) (deprecated in C++26)
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.

Contents

[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

(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.

[edit] Example

This section is incomplete
Reason: no 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
LWG 4061 C++20 basic_format_context was constructible and assignable by user code made neither constructible nor assignable

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