Question

Forgive me if this has been asked before, I am sure it has but I couldn’t find an answer I was happy with.

I am coming to cpp from a heavy Java background and would like to understand when to return a reference/pointer to an object rather than a copy.

for the following class definition:

class SpaceShip {
    string name;
    WeaponSystem weaponSystem; //represents some object, this is just an example, I dont have this type of object at all in my program 
    int hull;

    string GetName() const {
       return name;
    }

    WeaponSystem GetWeaponSystem() const {
       return weaponSystem;
    }

    int GetHull() const {
       return hull;
    }
};

I know that returning a copy of things is expensive, I would think this means I want to avoid returning something like a string or weaponSystem by value, but an int by value is ok.

Is this right? I also know that I need to be aware of where things live in memory, does returning a reference to something in this class mean danger down the line if this object is destroyed and something still owns a reference to it’s name?

Answer 1

On your last point, you definitely need to be a lot more careful about resource management in C++ than in Java. In particular, you need to decide when an object is no longer needed. Returning by reference has an effect of aliasing to the returned object. It is not noticeable when the object you are sharing is immutable, but unlike Java’s Strings, C++ string are mutable. Therefore if you return name by value and then rename your SpaceShip, the caller would see the old name even after the renaming. If you return by reference, however, the caller will see a change as soon as ShaceShip is renamed.

When you deal with copying complex objects, you can decide how much is copied by providing a custom implementation of a copy constructor. If you decide to provide a copy constructor, don’t forget the rule of three, and override the other two.