construct an object of B in constructor of class A - c++

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

Try
class A
{
public:
A(); // Don't define A::A() here
// As the compiler has not seen B
};
class B
{
public:
B() {}
};
// At this point both classes have been seen
A::A()
{
::B foo; // So now you can use B
// Note I am using ::B here
// As the error message suggests that you have some class B defined
// Within A which is confusing it. the prefix :: means take the class
// B from the global scope rather than a closer scope.
}

Answer 2

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;
}