默认情况下应调用移动构造函数

Question

In following case where i have created move ctor in Integer class, i am expecting that it should be called by default on rvalue reference while creating Product object but i am getting call of copy constructor only. Gcc - 7.5.0 on Ubuntu 18

#include<iostream>
using namespace std;

class Integer 
{
    int *dInt = nullptr;
public: 
    Integer(int xInt)  {
        dInt = new int(xInt);
        cout<<"Integer Created"<<endl;
    } 
    Integer(const Integer &xObj)
    {
        cout<<"Copy called"<<endl;
        dInt = new int(xObj.mGetInt());
    }

    Integer(Integer &&xObj)
    {
        cout<<"Move called"<<endl;
        dInt = xObj.dInt;
        xObj.dInt = nullptr;
    }

    Integer& operator=(const Integer &xObj)
    {
        cout<<"Assignment operator called"<<endl;
        *dInt = xObj.mGetInt();
        return *this;
    }

    Integer& operator=(Integer &&xObj)
    {
        cout<<"Move Assignment operator called"<<endl;
        delete dInt;
        dInt = xObj.dInt;
        xObj.dInt = nullptr;
        return *this;   
    }
    ~Integer() 
    {
        cout<<"Integer destroyed"<<endl;
        delete dInt;
    }

    int mGetInt() const {return *dInt;}
};

class Product 
{
    Integer dId;
public: 
    Product(Integer &&xId)
    :dId(xId)
    {

    }
};
int main () 
{
    Product P(10); // Notice implicit conversion of 10 to Integer obj.
}

In above case, move called if i use dId(std::move(xId)) in Product class ctor, I was expecting it should called by default on rvalue reference. In following case i couldn't avoid creating of temporary object of Integer class, Is there any good way to avoid creating of temporary object.

    Product(const Integer &xId)
    :dId(xId)
    {

    }
    
    Product(10); // inside main

My purpose of above question to build my understanding so that i can utilize temporary object memory better.

Answer 1

You need std::move to "propagate" rvalue-reference-ness.

Inside the body of the following function:

void foo(int&& x);

…an expression x is an lvalue int . Not int&& .

References don't really "exist" — even though they are powered by the type system, they are supposed to be treated as aliases (rather than separate entities), so using x inside foo is treated just like using the original, referred-to int inside foo … and doing that would also create a copy, as you know.

---

This will do the job:

Product(Integer&& xId)
    : dId(std::move(xId))
{}

However, I actually encourage you to take Integer by value:

Product(Integer xId)
    : dId(std::move(xId))
{}

That way, you can use the same constructor for passing lvalue Integer too, and a copy will be produced if necessary , whereas a move will happen automatically if not (eg by passing in a literal, which will automatically trigger selection of Integer 's move constructor).