NAME¶

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

Synopsis¶

Defined in header <format>
template< class CharT, class... Args > (1) (since C++20)
struct basic_format_string;
template< class... Args >

using format_string = (2) (since C++20)

basic_format_string<char, std::type_identity_t<Args>...>;
template< class... Args >

using wformat_string = (3) (since C++20)

basic_format_string<wchar_t, std::type_identity_t<Args>...>;

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
constructor 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 > (1)
consteval basic_format_string( const T& s );
basic_format_string( /* runtime-format-string */<CharT> s ) (2) (since C++26)
noexcept;

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¶

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

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
arg-id - 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.
s - the format specification defined by the std::formatter
format-spec - 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. (since C++23)
* 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.

* 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 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 the name basic_format_string is
this facility exposed

Category:¶

* Todo no example