Two C++ classes with methods that return the other

Question

I have to classes, a and b, each needs to have a method that returns the other. If I try to compile the following code, I get an error for not defining the latter class ahead:

class a{
public:
    b* change(){
        return new b;
    }
}
class b{
public:
    a* change(){
        return new a;
    }
}

error: 'b' does not name a type

I understand why, of course, but I want to know if there's a way to implement it correctly.

Could you tell us what in particular you want to do? Such a circular dependency may very well be a design mistake. — cadaniluk, Aug 02 '16 at 16:08
@Arkadiy That question is not a duplicate at all. It only answers half of this question. — Maxim Egorushkin, Aug 02 '16 at 16:15
@Maxim: I've read through this duplicate and the answers talk both about forward declaration and out-of-class function bodies. — Ben Voigt, Aug 02 '16 at 16:19
@BenVoigt Oh, this is a different duplicate now, high quality one. — Maxim Egorushkin, Aug 02 '16 at 16:21
It's not a mistake, I want a program that uses a class with a chain of methods, and these dependencies are required in order to swap type, kind of like a state machine — Oded Sayar, Aug 02 '16 at 16:21

Maxim Egorushkin · Accepted Answer · 2016-08-02T16:11:59.710

3

Make the definitions of methods after the declarations of classes:

class b;
class a{
public:
    b* change();
}
class b{
public:
    a* change();
}

b* a::change() { return new b; }
a* b::change() { return new a; }

In other words, when you do new T, the definition of T must be available.

edited Aug 02 '16 at 16:11

answered Aug 02 '16 at 16:06

Maxim Egorushkin

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I'd be pedantic and spell out the 2 things you've shown are required: forward-declaration of the classes to provide incomplete types for the method declarations, and full declaration of the classes to provide complete types for constructing (incidentally to return). The requirement for forward-declarations is left implicit in your answer, but it's absolute. – underscore_d Aug 02 '16 at 16:15
@underscore_d Be my guest and edit. – Maxim Egorushkin Aug 02 '16 at 16:17

score -3 · Answer 2 · answered Aug 02 '16 at 16:07

-3

Firstly,. make an object of class b, and then call it. i.e. class b; and then call it in class a.

answered Aug 02 '16 at 16:07

Malik Nabeel

19
1
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What do you mean? Please show some code illustrating what you're trying to say. _Call_ has 2 possible, very different meanings here, and I think you're getting them mixed up. – underscore_d Aug 02 '16 at 16:08

Two C++ classes with methods that return the other

2 Answers2