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 constructor (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 storage. 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)