Constructor problems with class inside another class - c++

Question

I think the question title may be impossible to get right but, I have a class (lets call it A) which has a constructor that needs a pointer to another class (called B). This would be fine but when I define a class in a third class (called C), then I get this error after putting a variable into the constructor: error: expected identifier before ‘&’ token, then if I remove the & I get: error: ‘ClassA’ is not a type.
For those who can't understand what I just said (I don't blame you), here is an example:
class A
{
private:
int number;
//etc, etc...
};
class B
{
private:
A* ClassAPntr;
public:
B(A* objectPointer)
{
ClassAPntr = objectPointer;
}
};
class C
{
private:
A ClassA;
B ClassB(&ClassA);
};
int main()
{
C classC;
}

Answer 1

You didn't actually declare a constructor for class C. What you meant to do was this:
class C
{
public:
C()
: b(&a) // you have to provide the arguments to B's constructor here
{ }
private:
A a;
B b; // ... not here
};

Answer 2

Just for clarification, when you did composition by adding A classA private object to class C, the compiler accept this step because A has undefined constructor which means the compiler will make a default constructor for you and call it when make A classA.
The problem in class B because it has a non-default user defined constructor which means that compiler should force you to call it in class C through constructor initialization list or through curly braces in c++11, so it becomes your responsibility simply because compiler can't know which address for which class A Object you are going to pass for class B
constructor.
So why did you get this error message : class A is not a type or expected identifier before ‘&’ token?
the answer is that compiler considered B ClassB(&ClassA) a declaration for a private function with name ClassB and a return value of type B class.
and because you never use this function in your class the compiler will never give you any error related to definition. and you will also never use this member function in other functions like main because it is private
to produce the undefined reference error you can modify c class to be like this:
class C
{
private:
A classA;
B classB();
friend B func() ;
};
B func() {
C c;
return c.classB(); // undefined reference Error
}