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

NAME
     std::any_cast - std::any_cast

Synopsis
	Defined in header <any>
	template< class T >				  (1) (since C++17)
	T any_cast( const any& operand );
	template< class T >				  (2) (since C++17)
	T any_cast( any& operand );
	template< class T >				  (3) (since C++17)
	T any_cast( any&& operand );
	template< class T >				  (4) (since C++17)
	const T* any_cast( const any* operand ) noexcept;
	template< class T >				  (5) (since C++17)
	T* any_cast( any* operand ) noexcept;

	Performs type-safe access to the contained object.

	Let U be std::remove_cv_t<std::remove_reference_t<T>>.

	1)  The program is ill-formed if std::is_constructible_v<T, const U&> is
     false.
	2) The program is ill-formed if std::is_constructible_v<T, U&> is false.
	3) The program is ill-formed if std::is_constructible_v<T, U> is false.
	4,5) The program is ill-formed if std::is_void_v<T> is true.

Parameters
	operand - target any object

Return value
	1,2) Returns static_cast<T>(*std::any_cast<U>(&operand)).
	3) Returns static_cast<T>(std::move(*std::any_cast<U>(&operand))).
	4,5) If operand is not a null pointer, and the typeid of the requested T
     matches
	that of the contents of operand, a pointer to  the  value  contained  by
     operand,
	otherwise a null pointer.

Exceptions
	1-3)  Throws std::bad_any_cast if the typeid of the requested T does not
     match that
	of the contents of operand.

Example
     // Run this code

      #include <any>
      #include <iostream>
      #include <string>
      #include <type_traits>
      #include <utility>

      int main()
      {
	  // Simple example
	  auto a1 = std::any(12);
	  std::cout << "1) a1 is int: " << std::any_cast<int>(a1) << '\n';

	  try
	  {
	      auto s = std::any_cast<std::string>(a1); // throws
	  }
	  catch (const std::bad_any_cast& e)
	  {
	      std::cout << "2) " << e.what() << '\n';
	  }

	  // Pointer example
	  if (int* i = std::any_cast<int>(&a1))
	      std::cout << "3) a1 is int: " << *i << '\n';
	  else if (std::string* s = std::any_cast<std::string>(&a1))
	      std::cout << "3) a1 is std::string: " << *s << '\n';
	  else
	      std::cout << "3) a1 is another type or unset\n";

	  // Advanced example
	  a1 = std::string("hello");
	  auto& ra = std::any_cast<std::string&>(a1); //< reference
	  ra[1] = 'o';

	  std::cout << "4) a1 is string: "
		    << std::any_cast<std::string const&>(a1) << '\n'; //<  const
     reference

	  auto s1 = std::any_cast<std::string&&>(std::move(a1)); //< rvalue ref-
     erence
	  // Note: as1a is a move-constructed std::string:
	  static_assert(std::is_same_v<decltype(s1), std::string>);

	  //  Note:  the  std::string  in  aa1a is left in valid but unspecified
     state
	  std::cout << "5) a1.size(): "
		    << std::any_cast<std::string>(&a1)->size() //< pointer
		    << '\n'
		    << "6) s1: " << s1 << '\n';
      }

Possible output:
      1) a1 is int: 12
      2) bad any_cast
      3) a1 is int: 12
      4) a1 is string: hollo
      5) a1.size(): 0
      6) s1: hollo

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied  to	      Behavior as published		 Correct
     behavior
	LWG 3305 C++17	    the behavior of overloads (4,5) was     the  program
     ill-formed in
			    unclear if T is void		   this case

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