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Showing content from https://en.cppreference.com/w/cpp/language/../keywords/../utility/format/basic_format_string.html below:

std::basic_format_string, std::format_string, std::wformat_string - cppreference.com

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).

[edit] Member functions constructs a basic_format_string, raising compile error if the argument is not a format string
(public member function) 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.

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 }.

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

std::basic_format_string::get

Returns the stored string view.

[edit] 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.

[edit] 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 P2508R1 C++20 there's no user-visible name for this facility the name basic_format_string is exposed

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