C++11: 'decltype class instance declaration' with std::move( ) doesn't call 'move constructor.' Why?

Question

I recently started using c++ and I chose to learn c++11 features. But how c++ codes run is sometimes not so tangible.

below is my code. At the part with decltype(std::move(sample)) sample2 = std::move(sample); I'm not sure why this line doesn't call move constructor. Could you explain why?

#include <iostream>

class AAA
{
   public:
      AAA() { std::cout << "default constructor" << std::endl; }
      AAA(const AAA& cs) { std::cout << "copy constructor" << std::endl; }
      AAA(AAA&& cs) { std::cout << "move constructor" << std::endl; }
      ~AAA() { std::cout << "destructor" << std::endl; }
};

int main(int argc, char* args[])
{
    AAA sample;
// ((right here))
    decltype(std::move(sample)) sample2 = std::move(sample); 
    return 0;
}

It is compiled on [ ubuntu 16.04 LTS ] with [ gcc 5.4.0 ]

original code : https://ide.geeksforgeeks.org/tALvLuSNbN

Answer 1

The function std::move<T> returns a T &&, so for std::move(sample) it returns AAA &&. This is an rvalue reference and behave a lot like an lvalue reference (a type like AAA & would be an lvalue reference) in that they both are alias to objects that already exist.

It's important to understand that std::move does not in itself cause anything to be moved. It simply returns an rvalue reference to the argument it's given. For example std::move(foo); alone does absolutely nothing. It's only when the result is used to initialize or assign to an object that it becomes useful.

For example auto bar = std::move(foo); will return an rvalue reference to foo and use that reference to call bar's constructor.

To answer the question, since std::move(sample) returns a AAA &&, the line in question is the same as AAA && sample2 = std::move(sample);. The behavior is practically the same as AAA & sample2 = sample;. In both cases, you are initializing a reference to an existing object and no new object needs to be constructed.

If your goal is to move sample into a new AAA, the correct line would be auto sample2 = std::move(sample); like you do with sample3. Though beware that the line sample3 is moving from an already moved-from sample.