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

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< cpp‎ | utility‎ | 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)

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-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. 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.
- 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.

(since C++23)

- 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 this facility | the name `basic_format_string` is exposed

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