std::runtime_format

 

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format (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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Defined in header <format>
/*runtime-format-string*/<char> runtime_format( std::string_view fmt ) noexcept;	(1)	(since C++26)
/*runtime-format-string*/<wchar_t> runtime_format( std::wstring_view fmt ) noexcept;	(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 of the exposition-only type:

Class template runtime-format-string <CharT>

template< class CharT > struct /*runtime-format-string*/;		(exposition only*)

Member objects

The returned object contains an exposition-only non-static data member str of type std::basic_string_view<CharT>.

Constructors and assignments

/*runtime-format-string*/( std::basic_string_view<CharT> s ) noexcept;	(1)
/*runtime-format-string*/( const /*runtime-format-string*/& ) = delete;	(2)
/*runtime-format-string*/& operator=( const /*runtime-format-string*/& ) = delete;	(3)

1) Initializes str with s.

2) Copy constructor is explicitly deleted. The type is neither copyable nor movable.

3) The assignment is explicitly deleted.

Notes

Since the return type of runtime_format is neither copyable nor movable, an attempt of passing runtime_fmt as glvalue inhibits the construction of std::basic_format_string which results in program ill-formed. To construct std::basic_format_string with runtime_format, the returned value of runtime_format is passed directly on std::basic_format_string as prvalue where copy elision is guaranteed.

auto runtime_fmt = std::runtime_format("{}");
 
auto s0 = std::format(runtime_fmt, 1); // error
auto s1 = std::format(std::move(runtime_fmt), 1); // still error
auto s2 = std::format(std::runtime_format("{}"), 1); // ok

Feature-test macro	Value	Std	Feature
__cpp_lib_format	202311L	(C++26)	Runtime format strings

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)