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

Defined in header <format>

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)

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 and performs compile-time format string checks at construction time.

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 );

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>>.

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. The arg-ids 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.

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 std::thread::id, the format specification is interpreted as thread id format specification.
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

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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