std::basic_format_string,std::format_string,std::wformat_string

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Standard library headers
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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
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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)

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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)

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Defined in header`<format>`
template<class CharT,class...Args> struct basic_format_string;	(1)	(since C++20)
template<class...Args> using format_string= basic_format_string<char,std::type_identity_t<Args>...>;	(2)	(since C++20)
template<class...Args> using wformat_string= basic_format_string<wchar_t,std::type_identity_t<Args>...>;	(3)	(since C++20)

Class templatestd::basic_format_string wraps astd::basic_string_view that will be used by formatting functions.

The constructor ofstd::basic_format_string performs compile-time format string checks unless the constructor argument is returned bystd::runtime_format(since C++26).

[edit]Member functions

(constructor)	constructs a`basic_format_string`, raising compile error if the argument is not a format string (public member function)
get	returns the wrapped string (public member function)

std::basic_format_string::basic_format_string

template<class T> consteval basic_format_string(const T& s);	(1)
basic_format_string(/* runtime-format-string */<CharT> s)noexcept;	(2)	(since C++26)

1) Constructs abasic_format_string object that stores a view of strings. If the argument is not a compile-time constant, or if it cannot be parsed as a format string for the formatting argument typesArgs, the construction is ill-formed.

This overload participates in overload resolution only ifconst T& modelsstd::convertible_to<std::basic_string_view<CharT>>.

2) Constructs abasic_format_string object that stores a view of strings as returned bystd::runtime_format. It does not perform format string checks upon construction.

Parameters

an object that represents the format string. The format string consists of

ordinary characters (except{ and}), which are copied unchanged to the output,
escape sequences{{ and}}, which are replaced with{ and} respectively in the output, and
replacement fields.

Each replacement field has the following format:


`{`arg-id(optional)`}`	(1)

`{`arg-id(optional)`:`format-spec`}`	(2)

1) replacement field without a format specification

2) replacement field with a format specification

arg-id	-	specifies the index of the argument in`args` whose value is to be used for formatting; if it is omitted, the arguments are used in order. Thearg-id s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.
format-spec	-	the format specification defined by thestd::formatter specialization for the corresponding argument. Cannot start with}.

For basic types and standard string types, the format specification is interpreted asstandard format specification.
For chrono types, the format specification is interpreted aschrono format specification.

For range types, the format specification is interpreted asrange format specification.
Forstd::pair andstd::tuple, the format specification is interpreted astuple format specification.
Forstd::thread::id andstd::stacktrace_entry, seethread id format specification andstacktrace entry format specification.
Forstd::basic_stacktrace, no format specifier is allowed.

(since C++23)

Forstd::filesystem::path, seepath format specification.

(since C++26)

For other formattable types, the format specification is determined by user-definedformatter specializations.

std::basic_format_string::get

constexprstd::basic_string_view<CharT> get()constnoexcept;

Returns the stored string view.

[edit]Notes

The alias templatesformat_string andwformat_string usestd::type_identity_t to inhibit template argument deduction. Typically, when they appear as a function parameter, their template arguments are deduced from other function arguments.

template<class...Args>std::string format(std::format_string<Args...> fmt, Args&&...args); auto s= format("{} {}",1.0,2);// Calls format<double, int>. Args are deduced from 1.0, 2// Due to the use of type_identity_t in format_string, template argument deduction// does not consider the type of the format string.