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

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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 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

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.

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)
  • 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