override c++ virtual method

Question

I have a class template where some methods are defined as virtual to give the ability for the user of my class to give an implementation for them in his derived class. Note that in my template class there is some non-virtual methods that makes use of the virtual one (a virtual class that should return a value is called in a non-virtual class).

Can you give me a simple example of a correct code where the virtual method of the parent class should return a value (but it's implementation is provided in a child class) and the value returned by the virtual method in the parent class is used in other methods of that class. Because I saw somewhere (for example here: Safely override C++ virtual functions) that this can cause some problems and the user defined method will note override the virtual method of the parent class.

Note: I program with Code::Blocks using g++ compiler.

EDIT: as requested here a simple example of what I want:

template<typename T>
class parent {
public:
  // Public methods that user can call
  int getSomething(T t);
  void putSomething(T t, int x);

  // public method that user should implement in his code
  virtual float compute(T t) { }

  // protected or private methods and attributes used internally by putSomething ...
  float doComplexeThings(...); // this can call
};

The method compute() should be implemented by the user (the child class). However, this method compute() is called by putSomething() and doComplexeThings() for example.

Answer 1

If you can use C++11 features in your compiler then overrides can be tagged as so with the override special identifier:

 float compute() override;

The above line in a derived class will cause a compiler error as the function does not override a member function in the base (incorrect signature, missing argument). But note that this must be done in each derived class, it is not a solution that you can impose from the base class.

From the base class you can only force the override by making the function pure virtual, but that changes the semantics. It does not avoid problems while overriding, but rather forces overriding in all cases. I would avoid this approach, and if you are to follow it and there is a sensible implementation for the base type, make the function virtual and provide a definition so that your derived classes's implementation can just call the functions the base type (i.e. you force the implementation, but in the simplest cases it will just forward the call to the parent)

Answer 2

You just have to make sure that the methods have the same signature (including const/mutable modifiers and argument types). You can use a pure virtual definition to provoke compiler errors if you fail to override the function in a subclass.

class parent {
public:
  // pure virtual method must be provided in subclass
  virtual void handle_event(int something) = 0; 
};

class child : public parent {
public:
  virtual void handle_event(int something) {
    // new exciting code
  }
};

class incomplete_child : public parent {
public:
  virtual void handle_event(int something) const {
    // does not override the pure virtual method
  }
};

int main() {
  parent *p = new child();
  p->handle_event(1); // will call child::handle_event
  parent *p = new incomplete_child(); // will not compile because handle_event
                                      // was not correctly overridden
}