移动分配运算符未被调用

Question

I've just been doing some exercises with move semantic, and I can't seem to figure out why my move assignment operator is not being called

My class is as follows (please ignore any bad practice that is not relevant to the question):

private:
    int sz;
    double* elems;

public:
    Vector(int size):sz(size), elems(new double[size])
    {
         for(int i=0; i!=sz; ++i)
         {
             elems[i] = i;
         }
    }

    ~Vector()
    {
         std::cout << "Destruct " << std::endl; delete [] elems;
    }

    double& operator[](int i)
    {
        return elems[i];
    }
    const double& operator[](int i)const 
    {
         return elems[i];
    }

    //operator+ to add vectors
    friend Vector operator+(const Vector& a, const Vector& b)
    {

        Vector ret(a.sz);
        if(a.sz!=b.sz)
            return 0;
        std::cout << "Adding" << std::endl;
        for(int i=0; i!=a.sz; ++i)
        {
            ret[i] = a[i] + b[i];
        }

        return ret;
     }

     //move constructor
     Vector(Vector&& a): sz(a.sz),elems(a.elems)
     {
          std::cout << "Move constructor"<< std::endl;
          a.elems = nullptr;
          a.sz = 0;
     }

     //move assignment
     Vector& operator=(Vector&& a)
     {
        std::cout << "Moveeee" << std::endl;
        delete [] elems;
        elems = a.elems;
        sz = a.sz;
        a.elems = nullptr;
        a.sz = 0;

        return *this;
     }

Now my test case is as follows:

Vector v1(3);
Vector v2(3);
Vector v3(3);
Vector v4 = v1+v2;

The output is as follows:

Adding
Move constructor
Destruct
Destruct
Destruct
Destruct
Destruct

What I expect is line Vector v4 = v1+v2; to call the move assignment, but it doesn't. Instead it calls the move constructor.

I even tried Vector v4 = std::move(v2); , but that also didn't work and called the move constructor.

Any idea why the move assignment is not being called?

Answer 1

Semantically you are not calling assignment

Vector v4 = v1+v2;

is really:

Vector v4(v1+v2);

If there is no copy constructor, but a default constructor and an acceptable assignment exists, then assignment will be called. As commented you would need to do:

Vector v4; v4 = v1 + v2;

to have a true assignment