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std::runtime_format(3)	      C++ Standard Libary	std::runtime_format(3)

NAME
       std::runtime_format - std::runtime_format

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

	  Returns  an  object that stores a runtime format string directly us-
       able 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 template runtime-format-string <CharT>

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

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

	   Constructors	and assignments

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

	  1) Initializes str with s.
	  2) Copy constructor is explicitly deleted. The type is neither copy-
       able nor	movable.
	  3) The assignment is explicitly deleted.

Notes
	  Since	the return type	of runtime_format is neither copyable nor mov-
       able, 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	      stores formatted representation of the arguments
       in a new	string
	  (C++20)	      (function	template)
	  vformat	      non-template variant of std::format using	 type-
       erased argument
	  (C++20)	      representation
			      (function)
	  basic_format_string
	  format_string	      class template that performs compile-time	format
       string checks
	  wformat_string      at construction time
	  (C++20)	      (class template)
	  (C++20)
	  (C++20)

http://cppreference.com		  2024.06.10		std::runtime_format(3)

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