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

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 template std::basic_format_string wraps a std::basic_string_view that will be used by formatting functions.

The constructor of std::basic_format_string performs compile-time format string checks unless the constructor argument is returned by std::runtime_format (since C++26)

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 a basic_format_string object that stores a view of string s. If the argument is not a compile-time constant, or if it cannot be parsed as a format string for the formatting argument types Args

This overload participates in overload resolution only if const T& models std::convertible_to < std::basic_string_view <CharT>>

2) Constructs a basic_format_string object that stores a view of string s as returned by std::runtime_format

Parameters

s

-

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

ordinary characters (except { and }
escape sequences {{ and }}, which are replaced with { 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. The arg-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 the std::formatter specialization for the corresponding argument. Cannot start with }

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

For range types, the format specification is interpreted as range format specification.
For std::pair and std::tuple, the format specification is interpreted as tuple format specification
For std::thread::id and std::stacktrace_entry, see thread id format specification and stacktrace entry format specification
For std::basic_stacktrace, no format specifier is allowed.

(since C++23)

For std::filesystem::path, see path format specification

(since C++26)

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

std::basic_format_string::get

constexpr std::basic_string_view <CharT> get( ) const noexcept ;

Returns the stored string view.

Notes

The alias templates format_string and wformat_string use std::type_identity_t

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.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2508R1	C++20	there's no user-visible name for this facility	the name `basic_format_string` is exposed

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 support library (C++26)
Technical specifications
Symbols index
External libraries

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)