How virtual/abstract methods are displayed in class diagrams?

Question

First, I am coming from the Java community, and still a learner in C++.

Please have a look at the following classes

The second picture shows a sub class of the class "GameObject". It also has a Display() method. There are 5 sub classes of the class GameObject, and they all have this Display() method. So, is this display method in GameObject is a virtual one?

I think it is not abstract virtual (100% virtual) because the Display() in GameObject displays some texts in GameObject.cpp

Anyway, I am not quite sure. Please help!

Answer 1

In C++, virtual functions are virtual if and only if the have been explicitly declared virtual in a base class or the class itself. Since GameObject seems to have no base class, GameObject::Display() hast to be declared virtual, if displaying a GameObject reference (or pointer) should behave according to actual (i.e. runtime) type of the object behind that reference. Having a function body in GameObject.cpp does not mean the function is not pure virtual (the C++ equivalent for java's abstract). You can make the function pure virtual by adding a = 0 to its declaration. Nevertheless you can provide a implementation for pure virtual methods, e.g. to have a default implementation that you explicitly call in derived classes implementing the method:

class GameObject { 
  //...
public:
  virtual void Display() = 0;
};

//GameObject.cpp
void GameObject::Display() { 
  /* do something */ 
}

//DerivedGO.h
class DerivedGO : public GameObject {
public:
  // virtual can be left out here, since it's declared virtual in the base class
  virtual void Display() {
    prepDisplay(); // do some preparation
    GameObject::Display(); //call the original/default implementation
  }
};

However be aware that making a function pure virtual means making the containing class abstract and every derived class as well that does not override all pure virtual functions from its base classes.