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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 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 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 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*/& )
     =	    (3)
	delete;

	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 con-
     struct
	std::basic_format_string with runtime_format, the returned value of run-
     time_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, "un-
     used");
	  }
      }

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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