std::runtime_format

 

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format (C++20)

format_to (C++20)

format_to_n (C++20)

formatted_size (C++20)

vformat (C++20)

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basic_format_stringformat_stringwformat_string (C++20)(C++20)(C++20)

runtime_format (C++26)

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formattable (C++23)

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visit_format_arg (C++20) (deprecated in C++26)

make_format_argsmake_wformat_args (C++20)(C++20)

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format_error (C++20)

 

Defined in header <format>
/*runtime-format-string*/<char> runtime_format( std::string_view fmt );	(1)	(since C++26)
/*runtime-format-string*/<wchar_t> runtime_format( std::wstring_view fmt );	(2)	(since C++26)

Returns an object that stores a runtime format string directly usable in user-oriented formatting functions and can be implicitly converted to std::basic_format_string.

1,2) Equivalent to return {fmt};.

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 }. 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.

Return value

An object holding the runtime format string.

Example

Run this code

#include <format>
#include <print>
#include <string>
#include <string_view>
 
int main()
{
    std::print("Hello {}!\n", "world");
 
    std::string fmt;
    for (int i{}; i != 3; ++i)
    {
        fmt += "{} "; // constructs the formatting string
        std::print("{} : ", fmt);
        std::println(std::runtime_format(fmt), "alpha", 'Z', 3.14, "unused");
    }
}

Output:

Hello world!
{}  : alpha
{} {}  : alpha Z
{} {} {}  : alpha Z 3.14

See also

format (C++20)	stores formatted representation of the arguments in a new string (function template)
vformat (C++20)	non-template variant of std::format using type-erased argument representation (function)
basic_format_stringformat_stringwformat_string (C++20)(C++20)(C++20)	class template that performs compile-time format string checks at construction time (class template)