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

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Formatting library
Standard format specification
Formatting functions
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Format strings
basic_format_string format_string wformat_string
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(C++26)
Formatting concepts
(C++23)
Formatter
(C++20)
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(C++20) (C++20) (C++20)
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Formatting arguments
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(C++20) (deprecated in C++26)
(C++20) (C++20)
Format error
(C++20)
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 , the construction is ill-formed.
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 . It does not perform format string checks upon construction.

Parameters

s - 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.

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 } .

(since C++23)
(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 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.

Example

This section is incomplete
Reason: no 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
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