Passing pointer of an object to another class in its constructor

Question

This question is somewhat related but still doesn't solves my problem. Consider two classes A and B:

class A{
public:
    A(int id, B* coordinator);
    virtual ~A();
    B *coordinator;
}
class B{
public:
    B(int id, int param2);
    virtual ~B();
    run();
}

void b::run(){
    while(true){
      A(1, this);
      A(2, this);
      //something else create which may create more A
    }
}

int main(int argc, char* argv[]) {
    B bObj(atoi(argv[1]), atoi(argv[2]));
}

In main function I create an object of class B type called as bObj. Constructor of B calls run() and B.run() goes in an infinite loop. Where it creates multiple objects of A. Now I want to pass a pointer of bObj to newly created objects of class A, so that they can access other public variables and functions of bObj. So I am passing this pointer while creating objects of A from B. And I have declared a class variable coordinator of B* type in class A. But I am getting this: Compiler error -> ISO C++ forbids declaration of ‘B’ with no type.

Please help me with this. I am fairly new to C++.

Chor Sipahi · Accepted Answer · 2021-07-03 17:35:21Z

1

Did you try forward declaration of class B? That should atleast partially solve your problem.

answered Jul 3, 2021 at 17:35

Chor Sipahi

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1 Comment

PHcoDer Over a year ago

I got it solved I was including B.h in A. That was creating the problem. I removed that and its compiling. But now when I am calling the functions of B from A using the pointer. I am getting these two errors: forward declaration of ‘struct B’ & invalid use of incomplete type ‘struct B' . I have shared whole code here: coliru.stacked-crooked.com/a/017ec9516add9e7e

WhiZTiM · Accepted Answer · 2016-09-19 02:00:07Z

1

In your current code, the compiler sees the entity B in your constructor, but doesn't know it as a type since it is yet to encounter such type declaration.

To solve your problem, You should simply forward declare B. And remember the ; semi-colon at the end of each class definition

class B;                        //forward declaration

class A{
public:
    A(int id, B* coordinator);
    virtual ~A();
    B *coordinator;
};                              //remember the semi-colon!

edited Sep 19, 2016 at 2:00

answered Sep 19, 2016 at 1:57

WhiZTiM

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6 Comments

PHcoDer Over a year ago

Thanks! Where should I do this declaration?

WhiZTiM Over a year ago

Above the declaration of the class A. See the snippet in my answer

PHcoDer Over a year ago

Thanks, I did that. But Now I am getting error everywhere where I have declared variables of type A. Like there is a vector in B which contains instances of A created in B. When I create vector<A> allAs;. I get error as "template argument1 is invalid". I am initializing B* passed to A in A's constructors' initializers list.

WhiZTiM Over a year ago

Can you post a fully working code? perhaps on coliru.stacked-crooked.com and click share, then copy the site link and paste here

PHcoDer Over a year ago

I got it solved I was including B.h in A. That was creating the problem. I removed that and its compiling. But now when I am calling the functions of B from A using the pointer. I am getting these two errors: forward declaration of ‘struct B’ & invalid use of incomplete type ‘struct B' . I will share the whole code soon.

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