Question

I have an inclusion pattern as follows:

/*
 * Class1.h
 */

#ifndef CLASS1_H_
#define CLASS1_H_

#include "Class2.h"

namespace Class1_namespace
{

class Class1
{
  Class2* Class2_ptr;
  void Class1_member()
  {
      (*Class2_ptr).Class2_method();
  }
};

}

#endif /* CLASS1_H_ */

/*
 * Class2.h
 */

#ifndef CLASS2_H_
#define CLASS2_H_

#include "Class1.h"

class Class2
{
    Class1_namespace::Class1 Class2_data;

public:
    void Class2_method(){};
};

#endif /* CLASS2_H_ */

/*
 * main.cpp
 */

#include "Class1.h"

int main()
{
    return 0;
}

However, this leads to the error “’Class1_namespace’ does not name a type.”

Is this error caused by the ordering of my inclusions?

What are some possible solutions? I’m dubious about forward declarations solving my problem.

Answer 1

Class1 doesn’t need to include Class2.

When you have mutual dependency (which you don’t — you could just not include 2 in 1), you can usually solve it by using forward declarations instead of inclusions.

For example, let’s say that Class1 looked like this

#include "Class2.h"

namespace Class1_namespace
{

    class Class1
    {
        Class2* class2;
    };

}

Where you think you need the include, you could instead do this:

class Class2;

namespace Class1_namespace
{

    class Class1
    {
        Class2* class2;
    };

}

to break the mutual inclusion.