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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;

	  move_if_noexcept obtains an rvalue reference to its argument if  its
       move constructor
	  does	not throw exceptions or	if there is no copy constructor	(move-
       only type),
	  otherwise obtains an lvalue reference	to its argument. It  is	 typi-
       cally 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.

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;
	    Good g2 = std::move_if_noexcept(g);
	    Bad	b2 = std::move_if_noexcept(b);
	}

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

Complexity
	  Constant

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

http://cppreference.com		  2022.07.31	      std::move_if_noexcept(3)

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