Reference initialization in C++ when private unique_ptr is involved in class definition

Question

How the compile error below can be explained?

#include <iostream>
#include <memory>
#include <vector>

using namespace std;

class A{
    unique_ptr<vector<short>> v;
public:
    A(){
        v = unique_ptr<vector<short>>(new vector<short>());
        cout << "A()" << endl;
    }
    A(const A& a) : A(){
        cout << "A(const A& a)" << endl;
    }
};

int main() {

    A a1; // prints A()
    A a2 = a1; // prints A() then A(const A& a)
    A& a3 = a1; // prints nothing
    a3 = a1; // compile time error: use of deleted function ‘A& A::operator=(const A&)

    return 0;
}

Actually, why A& a3 = a1 is ok and a3 = a1 is not? Also, which overloaded version of the operator= is being used and how could it be properly implemented to avoid this kind of problem?

Answer 1

In the line

a3 = a1;

you are invoking a deleted operator= (remember that unique_ptr is non-copyable). g++ spits out the error for you:

error: use of deleted function 'A& A::operator=(const A&)'

Whereas in the line

A& a3 = a1; 

there is no copy made, you only initialize a reference.

You may want to move your pointer, like

a3 = std::move(a1); 

Unfortunately, it won't work, since you explicitly declared a copy constructor, and this prevents the compiler from generating the default move constructor and assignment operator. Solution: declare the move assignment operator and move constructors as =default; ,

A& operator=(A&&) = default;
A(A&&) = default;

and the line a3 = std::move(a1) above will work.