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

NAME
       std::move_if_noexcept - std::move_if_noexcept

Synopsis
	  Defined in header <utility>
	  template< class T >

	  typename std::conditional<
	      !std::is_nothrow_move_constructible<T>::value		    &&
       (since C++11)
	  std::is_copy_constructible<T>::value,
       (until C++14)
	      const T&,
	      T&&

	  >::type move_if_noexcept( T& x ) noexcept;
	  template< class T >

	  constexpr typename std::conditional<
	      !std::is_nothrow_move_constructible<T>::value &&
	  std::is_copy_constructible<T>::value,
       (since C++14)
	      const T&,
	      T&&

	  >::type move_if_noexcept( T& x ) noexcept;

	  std::move_if_noexcept	obtains	an rvalue reference to its argument if
       its move
	  constructor does not throw exceptions	or if there is	no  copy  con-
       structor	(move-only
	  type),  otherwise obtains an lvalue reference	to its argument. It is
       typically used
	  to combine move semantics with strong	exception guarantee.

Parameters
	  x - the object to be moved or	copied

Return value
	  std::move(x) or x, depending on exception guarantees.

Complexity
	  Constant.

Notes
	  This is used,	for example, by	std::vector::resize, which may have to
       allocate	new
	  storage and then move	or copy	elements from old storage to new stor-
       age. If an
	  exception occurs during this operation,  std::vector::resize	undoes
       everything it did
	  to  this  point, which is only possible if std::move_if_noexcept was
       used to decide
	  whether to use move construction or copy construction	 (unless  copy
       constructor is
	  not available, in which case move constructor	is used	either way and
       the strong
	  exception guarantee may be waived).

Example
       // Run this code

	#include <iostream>
	#include <utility>

	struct Bad
	{
	    Bad() {}
	    Bad(Bad&&) // may throw
	    {
		std::cout << "Throwing move constructor	called\n";
	    }
	    Bad(const Bad&) // may throw as well
	    {
		std::cout << "Throwing copy constructor	called\n";
	    }
	};

	struct Good
	{
	    Good() {}
	    Good(Good&&) noexcept // will NOT throw
	    {
		std::cout << "Non-throwing move	constructor called\n";
	    }
	    Good(const Good&) noexcept // will NOT throw
	    {
		std::cout << "Non-throwing copy	constructor called\n";
	    }
	};

	int main()
	{
	    Good g;
	    Bad	b;
	    [[maybe_unused]] Good g2 = std::move_if_noexcept(g);
	    [[maybe_unused]] Bad b2 = std::move_if_noexcept(b);
	}

Output:
	Non-throwing move constructor called
	Throwing copy constructor called

See also
	  forward forwards a function argument
	  (C++11) (function template)
	  move	  obtains an rvalue reference
	  (C++11) (function template)

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

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