Question

Background: I am working on a framework that generates C++ code based on an existing Java class model. For this reason I cannot change the circular dependency mentioned below.

Given:

• A Parent-Child class relationship
• Parent contains a list of Children
• Users must be able to look up the list element type at run-time

I’ve modeled this in the following testcase:

Main.cpp

#include "Parent.h"

#include <iostream>
using std::cout;
using std::endl;

int main(int argc, char* argv[])
{
    Parent parent;
    cout << Parent::getType() << endl;
    cout << parent.getChildren().getType() << endl;
    return 0;
}

Parent.h

#ifndef PARENT_H
#define PARENT_H

#include <string>

#include "Array.h"
class Child;

class Parent
{
public:
    Array<Child> getChildren()
    {
        return Array<Child>();
    }

    static std::string getType()
    {
        return "parent";
    }
};

#endif

Child.h

#ifndef CHILD_H
#define CHILD_H

#include "Parent.h"

class Child: public Parent
{
};

#endif

Array.h

template <typename ElementType>
class Array
{
public:
    static std::string getType()
    {
        return ElementType::getType();
    }
};

1. When I compile the above code I get:
   error C2027: use of undefined type 'Child' at return ElementType::getType();

2. If I try #include "Child.h" instead of the forward declaration I get:
   error C2504: 'Parent' : base class undefined at class Child: public Parent

3. If I try Array<Child*> instead of Array<Child> I get:
   error C2825: 'ElementType': must be a class or namespace when followed by '::' at return ElementType::getType();

The circular dependency comes about because:

1. Child.h needs to know about class Parent
2. Parent.h needs to know about class Array
3. Array.h needs to know about class Child

Any ideas?

Answer 1

The error is due to the Child class not being present when the template is instantiated.

Add the following either to Main or at the end of Parent.h:

#include "Child.h"

This compiles fine with both g++ 4 and VS 2010.