C++: is it possible to late-initialize a class member function?

Question

I have the following code:

#include <iostream>

class First {
    public:
        Second* x;
        void make_value(Second* sec_);
        First() {
            // Initialization
        }
};

class Second {
    public:
        First* y;
        Second() {
            // Initialization
        }
};

void First::make_value(Second* sec_) {
    x = sec_;
}

void main() {
    fir = new First();
    sec = new Second();
    fir->make_value(sec);
}

The two classes each have a member variable of the other class, which does not work for obvious reasons.

My question is whether or not there is a way to late-initialize variable x after class Second has been initialized. If not, what alternatives are there?

You can always call some function later that initializes (rather; *assigns to*) stuff, but that's just bad style. — Jesper Juhl, Jan 09 '20 at 19:07
Could you provide any links or articles that would help out? — Diogenis Siganos, Jan 09 '20 at 19:12
You could have one class provide a constructor taking pointer as argument, and the other would pass `this`. But then be very careful about the possibility of using an object before its members are initialized. — aschepler, Jan 09 '20 at 19:13
I am afraid that this solution might not be suitable for the approach I am taking... I'll make some necessary edits. — Diogenis Siganos, Jan 09 '20 at 19:16
@SidS doing that would not help, because class Second would still be referencing class First, which has not been initialized yet. — Diogenis Siganos, Jan 09 '20 at 19:17
you know forward declarations ? (the typical solution for mutual dependency) — 463035818_is_not_an_ai, Jan 09 '20 at 19:31
Nope. Haven't heard of that before... But it seems very useful. Thank you! — Diogenis Siganos, Jan 09 '20 at 19:33
there must be a good dupe, but for now this is the best I could find: https://stackoverflow.com/a/21535351/4117728 — 463035818_is_not_an_ai, Jan 09 '20 at 19:37
Slightly better one: https://stackoverflow.com/questions/16802485/forward-declaration-circular-dependency — user4581301, Jan 09 '20 at 19:38
I have solved the issue thanks to your help. Would you like to post the answer so that I am able to close this? — Diogenis Siganos, Jan 09 '20 at 19:39
Sidenote since it looks like you're new to C++, [Why should C++ programmers minimize use of 'new'?](https://stackoverflow.com/questions/6500313/why-should-c-programmers-minimize-use-of-new) — user4581301, Jan 09 '20 at 19:39
however, forward declaration dont help with everything. Depends on how much you left out from the example. See answer, only references or pointers can be used — 463035818_is_not_an_ai, Jan 09 '20 at 19:39
Oh, thank you! I am indeed new to C++, and I am coming from a Python background, so this is good help! — Diogenis Siganos, Jan 09 '20 at 19:40

score 5 · Accepted Answer · answered Jan 09 '20 at 19:38

For any uses where the compiler doesn't need the definition of a class, a forward declaration will suffice. Pointers and references to types do not require a definition.

class Second; // forward declare

class First {
    public:
        Second* x;
        void make_value(Second* sec_);
        First() {
            // Initialization
        }
};

class Second {
    public:
        First* y;
        Second() {
            // Initialization
        }
};

void First::make_value(Second* sec_) {
    x = sec_;
}

C++: is it possible to late-initialize a class member function?

1 Answers1