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std::format

From cppreference.com
< cpp‎ | utility‎ | format
 
C++
 
Text processing library
 
Formatting library
Standard format specification
Formatting functions
format
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
Format strings
(C++20)(C++20)(C++20)
(C++26)
Formatting concepts
(C++23)
Formatter
(C++20)
(C++23)
(C++23)
(C++23)
(C++23)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
(C++23)
(C++23)
Formatting arguments
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++20) (deprecated in C++26)
(C++20)(C++20)
Format error
(C++20)
 
Defined in header <format>
template< class... Args >
std::string format( std::format_string<Args...> fmt, Args&&... args );
 (1) (since C++20)
template< class... Args >
std::wstring format( std::wformat_string<Args...> fmt, Args&&... args );
 (2) (since C++20)
template< class... Args >

std::string format( const std::locale& loc,

                    std::format_string<Args...> fmt, Args&&... args );
 (3) (since C++20)
template< class... Args >

std::wstring format( const std::locale& loc,

                     std::wformat_string<Args...> fmt, Args&&... args );
 (4) (since C++20)

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

1) Equivalent to return std::vformat(fmt.get(), std::make_format_args(args...));.
2) Equivalent to return std::vformat(fmt.get(), std::make_wformat_args(args...));.
3) Equivalent to return std::vformat(loc, fmt.get(), std::make_format_args(args...));.
4) Equivalent to return std::vformat(loc, fmt.get(), std::make_wformat_args(args...));.

The format string fmt is checked at compile time unless it is initialized from the result of std::runtime_format(since C++26). If, at compile time, the format string is found to be invalid for the types of the arguments to be formatted, a compilation error will be emitted.

The following requirements apply to each type T in Args, where CharT is char for overloads (1,3), wchar_t for overloads (2,4):

Contents

[edit] Parameters

fmt -
[edit]
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 -