std::basic_format_arg<Context>::handle

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Text processing library

Localization library

Regular expressions library(C++11)

Formatting library(C++20)

Null-terminated sequence utilities

Byte strings

Multibyte strings

Wide strings

Primitive numeric conversions

to_chars (C++17) to_chars_result (C++17) from_chars (C++17) from_chars_result (C++17) chars_format (C++17)

Text encoding identifications

text_encoding (C++26)

 

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 Context > class basic_format_arg<Context>::handle;		(since C++20)

A type-erased wrapper that allows formatting an object of a user-defined type.

handle objects are typically created by std::make_format_args and accessed through std::visit_format_arg or the visit member functions of std::basic_format_arg(since C++26).

Data members

A typical implementation of handle is TriviallyCopyable and only stores two non-static data members:

• a constvoid* pointer to the object to be formatted, and
• a void(*)(std::basic_format_parse_context<Context::char_type>&, Context&, constvoid*) function pointer to the function performing needed operations in the format member function (see below).

Member functions

format (C++20)

formats the referenced object with the given contexts (public member function)

std::basic_format_arg<Context>::handle::format

void format(std::basic_format_parse_context<Context::char_type>& parse_ctx,              Context& format_ctx )const;		(since C++20)

Let

• T be the type of the formatting argument,
• TD be std::remove_const_t<T>,
• TQ be const TD if const TD satisfies __formattable_with<Context> or TD otherwise, and
• ref be a reference to the formatting argument.

Equivalent to: typename Context::template formatter_type<TD> f;
parse_ctx.advance_to(f.parse(parse_ctx));
format_ctx.advance_to(f.format(const_cast<TQ&>(static_cast<const TD&>(ref)), format_ctx));

Notes

A handle has reference semantics for the formatted argument and does not extend its lifetime. It is the programmer's responsibility to ensure that the argument outlives the handle. Usually, a handle is only used within formatting functions.

See also

basic_format_arg (C++20)	class template that provides access to a formatting argument for user-defined formatters (class template)
make_format_argsmake_wformat_args (C++20)(C++20)	creates a type-erased object referencing all formatting arguments, convertible to format_args (function template)