How to "return an object" in C++?

Question

I know the title sounds familiar as there are many similar questions, but I'm asking for a different aspect of the problem (I know the difference between having things on the stack and putting them on the heap).

In Java I can always return references to "local" objects

public Thing calculateThing() {
    Thing thing = new Thing();
    // do calculations and modify thing
    return thing;
}

In C++, to do something similar I have 2 options

(1) I can use references whenever I need to "return" an object

void calculateThing(Thing& thing) {
    // do calculations and modify thing
}

Then use it like this

Thing thing;
calculateThing(thing);

(2) Or I can return a pointer to a dynamically allocated object

Thing* calculateThing() {
    Thing* thing(new Thing());
    // do calculations and modify thing
    return thing;
}

Then use it like this

Thing* thing = calculateThing();
delete thing;

Using the first approach I won't have to free memory manually, but to me it makes the code difficult to read. The problem with the second approach is, I'll have to remember to delete thing;, which doesn't look quite nice. I don't want to return a copied value because it's inefficient (I think), so here come the questions

Is there a third solution (that doesn't require copying the value)?
Is there any problem if I stick to the first solution?
When and why should I use the second solution?

user_4215332 · Answer

I don't want to return a copied value because it's inefficient

This may not be true. Compilers can do optimisation to prevent this copying.

For example, GCC does this optimisation. In the following program, neither move constructor nor copy constructor are called, since no copying or moving is done. Also, notice the address of c. Even though the object c is instantiated inside the function f(), c resides in the stack frame of main().

class C {
public:
    int c = 5;
    C() {}
    C(const C& c) { 
        cout << "Copy constructor " << endl;
    }
    C(const C&& c)  noexcept {
        cout << "Move Constructor" << endl;
    }
};

C f() {
    int beforeC;
    C c;
    int afterC;

    cout << &beforeC << endl;   //0x7ffee02f26ac
    cout << &c << endl;         //0x7ffee02f2710 (notice: even though c is instantiated inside f(), c resides in the stack frame of main()
    cout << &afterC << endl;    //0x7ffee02f26a8

    return c;
}

C g() {
    C c = f(); ///neither copy constructor nor move constructor of C are called, since none is done
    cout << &c << endl;  //0x7ffee02f2710
    return c;
}

int main() {
    int beforeC;
    C c = g();    ///neither copy constructor nor move constructor of C are called, since none is done
    int afterC;

    cout << &beforeC << endl; //0x7ffee02f2718 
    cout << &c << endl;       //0x7ffee02f2710 (notice:even though c is returned from f,it resides in the stack frame of main)
    cout << &afterC << endl;  //0x7ffee02f270c
    return 0;
}

How to "return an object" in C++?

8 Answers