< cpp‎ | utility‎ | format
Utilities library
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Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
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Formatting library (C++20)
Relational operators (deprecated in C++20)
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Elementary string conversions

Defined in header <format>
(1) (since C++20)
(2) (since C++20)
std::string vformat( const std::locale& loc,
                     std::string_view fmt, std::format_args args );
(3) (since C++20)
std::wstring vformat( const std::locale& loc,
                      std::wstring_view fmt, std::wformat_args args );
(4) (since C++20)

Format arguments held by args according to the format string fmt, and return the result as a string. If present, loc is used for locale-specific formatting.


[edit] Parameters

fmt - 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)
(since C++26)
  • For other formattable types, the format specification is determined by user-defined formatter specializations.
args - arguments to be formatted
loc - std::locale used for locale-specific formatting

[edit] Return value

A string object holding the formatted result.

[edit] Exceptions

Throws std::format_error if fmt is not a valid format string for the provided arguments, or std::bad_alloc on allocation failure. Also propagates any exception thrown by formatter or iterator operations.

[edit] Example

#include <format>
#include <iostream>
template<typename... Args>
inline void println(const std::format_string<Args...> fmt, Args&&... args)
    std::cout << std::vformat(fmt.get(), std::make_format_args(args...)) << '\n';
int main()
    println("{}{} {}{}", "Hello", ',', "C++", -1 + 2 * 3 * 4);


Hello, C++23

[edit] See also