c++ - C++ include files confusion

Question

I'm trying to include files in my c++ program but I keep encountering the error:

ShapeVisitor.h:9:28: error: ‘Circle’ has not been declared

I think the problem is that the way the classes are structured, it results in a circular dependence. How do I solve this?

The class headers are below...

 //Circle.h
 #ifndef CIRCLE_H
 #define CIRCLE_H
 // headers, ...
 #include "Shape.h"
class Circle: public Shape { 
//class declaration 
}
#endif

//Shape.h
#ifndef SHAPE_H
#define SHAPE_H
// headers, ...
#include <iostream>

class Shape {  
//a certain method in the class declaration looks like this
virtual void accept(ShapeVisitor& v) = 0; 
//rest of class
}
#endif

//ShapeVisitor.h
#ifndef SHAPEVISITOR_H
#define SHAPEVISITOR_H
#include "Circle.h"
class ShapeVisitor {
//a method in the class looks like this:
virtual void visitCircle(Circle *s) = 0;
//rest of class
}
#endif

As you can see, circle includes shape, which includes shapevisitor, which again, includes circle.

Any ideas?

Answer 1

ShapeVisitor.h does not need to include Circle.h, a forward declaration class Circle; will do. Function declarations do not require the full definitions of their argument and return types (not even if the return/arguments are by value!). Only the function's implementation file (in your case: ShapeVisitor.cpp) would need to include Circle.h.

This ancient (but still very true!) column by Herb Sutter is a nice reference.