Question

Here is what I am trying to do, I want the class A’s constructor to call B’s constructor for an object of B, like this main()->A->B

A.cpp:(included B.h)

A::A(){
  B foo;
}

B.h:

class B{
 B(){//some code};
};

but GCC won’t compile and says A::B foo has initializer but incomplete type. I am guessing the compiler saw no local class of B defined in A, so it complained and didn’t know the Class B was from another file. My question is how to construct an B’s object in A’s constructor like above.I am sure I am missing some fundamentals about C++, please bear with me. Thank you in advance.

Answer 1

You do not have any class of type A::B. From your comment, it looks like you are trying to use a pointer to B by calling it A::B *. This is incorrect. A pointer to B is always B *, regardless of where it appears. From what you said, it looks like you want something like this:

a.hpp

#ifndef A_HPP_
#define A_HPP_

class B;
class A {
public:
    A(B * b);
private:
    B * my_very_own_b;
};

#endif    // A_HPP_

a.cpp

#include "a.hpp"
#include "b.hpp"

A::A(B * b):
    my_very_own_b(b)
    {
}

b.hpp

#ifndef B_HPP_
#define B_HPP_

class B {
public:
    B();
private:
    int x;
};

#endif    // B_HPP_

b.cpp

#include "b.hpp"
B::B():
    x(0)
    {
}

main.cpp

#include "a.hpp"
#include "b.hpp"

int main() {
    B b;
    A a(&b);
    return 0;
}