C++ Utility Library - move_if_noexcept Function

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Description

It returns an rvalue reference to arg, unless copying is a better option than moving to provide at least a strong exception guarantee.

Declaration

Following is the declaration for std::move_if_noexcept function.

template <class T>
  typename conditional < is_nothrow_move_constructible<T>::value ||
                         !is_copy_constructible<T>::value,
                         T&&, const T& >::type move_if_noexcept(T& arg) noexcept;

C++11

template <class T>
  typename conditional < is_nothrow_move_constructible<T>::value ||
                         !is_copy_constructible<T>::value,
                         T&&, const T& >::type move_if_noexcept(T& arg) noexcept;

Parameters

arg − It is an object.

Return Value

It returns an rvalue reference to arg, unless copying is a better option than moving to provide at least a strong exception guarantee.

Exceptions

Basic guarantee − This function never throws exceptions.

Data races

Calling this function introduces no data races.

Example

In below example explains about std::move_if_noexcept function.

#include <iostream>
#include <utility>
 
struct Bad {
   Bad() {}
   Bad(Bad&&) {
      std::cout << "Throwing move constructor called\n";
   }
   Bad(const Bad&) {
      std::cout << "Throwing copy constructor called\n";
   }
};

struct Good {
   Good() {}
   Good(Good&&) noexcept {
      std::cout << "Non-throwing move constructor called\n";
   }
   Good(const Good&) noexcept {
      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);
}

Let us compile and run the above program, this will produce the following result −

Non-throwing move constructor called
Throwing copy constructor called