Question

I have found a similar question here, but my intent is little different.

class B is the embedding class while class A is the embedded class. I want to let B::A have access to member functions of class B. I have seen compilation errors through g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2. The detailed errors are as follows:

~/Documents/C++ $ g++ embed.cpp 
embed.cpp:5:7: error: ‘B’ has not been declared
embed.cpp: In constructor ‘B::B()’:
embed.cpp:10:27: error: invalid use of incomplete type ‘struct B::A’
embed.cpp:14:9: error: forward declaration of ‘struct B::A’ 

Is there a way that I can make it work?
Thank you

#include <iostream>
#include <string>
using namespace std;

class B
{
public:
  B() : impl(new B::A(this)) {}
  ~B(){}

private:
  class A; // want to hide the implementation of A
  A* impl;
};

class B::A
{
public:
  A(B* _parent) : parent(_parent) {} // let embedded class A has access to this parent class
  ~A() { parent = NULL; }

  B* parent;
};

int main(void)
{
  return 0;
}

Answer 1

This problem is solved easily if you follow the convention of having the header in one file and the implementation in another.

In file b.h:

class B
{
public:
  B();
  ~B();

private:
  class A; // want to hide the implementation of A
  A* impl;
};

class B::A
{
public:
  A(B* _parent);
  ~A();

  B* parent;
};

In file b.cpp:

B::B(void)
    :impl(new A(this))
{

}

//other methods and such

My compiler was giving me a different error: in the constructor for B, you were default-constructing an object that had no default constructor (because it’s an incomplete class). The solution is to implement the B constructor after class A has been fully defined, and the header/implementation separation is a natural way to achieve that.