std::runtime_format

 

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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.

Parameters

fmt

-

a string view

Return value

An object holding the runtime format string of the exposition-only type:

Class templateruntime-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);// errorauto s1 =std::format(std::move(runtime_fmt), 1);// still errorauto 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 stringstd::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)