C++ Programming/Classes/Abstract Classes

Abstract Classes

An abstract class is, conceptually, a class that cannot be instantiated and is usually implemented as a class that has one or more pure virtual (abstract) functions.

A pure virtual function is one which must be overridden by any concrete (i.e., non-abstract) derived class. This is indicated in the declaration with the syntax " = 0" in the member function's declaration.

Example

class AbstractClass {
public:
  virtual void AbstractMemberFunction() = 0; // Pure virtual function makes
                                             // this class Abstract class.
  virtual void NonAbstractMemberFunction1(); // Virtual function.

  void NonAbstractMemberFunction2();
};

In general an abstract class is used to define an implementation and is intended to be inherited from by concrete classes. It's a way of forcing a contract between the class designer and the users of that class. If we wish to create a concrete class (a class that can be instantiated) from an abstract class we must declare and define a matching member function for each abstract member function of the base class. Otherwise, if any member function of the base class is left undefined, we will create a new abstract class (this could be useful sometimes).

Sometimes we use the phrase "pure abstract class," meaning a class that exclusively has pure virtual functions (and no data). The concept of interface is mapped to pure abstract classes in C++, as there is no "interface" construct in C++ the same way that there is in Java.

Example

 class Vehicle {
 public:
     explicit
     Vehicle( int topSpeed )
     : m_topSpeed( topSpeed )
     {}
     int TopSpeed() const {
        return m_topSpeed;
     }

     virtual void Save( std::ostream& ) const = 0;
 
 private:
     int m_topSpeed;
 };

 class WheeledLandVehicle : public Vehicle {
 public:
     WheeledLandVehicle( int topSpeed, int numberOfWheels )
     : Vehicle( topSpeed ), m_numberOfWheels( numberOfWheels )
     {}
     int NumberOfWheels() const {
       return m_numberOfWheels;
     }

     void Save( std::ostream& ) const; // is implicitly virtual
 
 private:
     int m_numberOfWheels;
 };

 class TrackedLandVehicle : public Vehicle {
 public:
    TrackedLandVehicle ( int topSpeed, int numberOfTracks )
    : Vehicle( topSpeed ), m_numberOfTracks ( numberOfTracks )
    {}
    int NumberOfTracks() const {
       return m_numberOfTracks;
    }
    void Save( std::ostream& ) const; // is implicitly virtual
 
  private:
    int m_numberOfTracks;
  };

In this example the Vehicle is an abstract base class as it has an abstract member function.The class WheeledLandVehicle is derived from the base class. It also holds data which is common to all wheeled land vehicles, namely the number of wheels. The class TrackedLandVehicle is another variation of the Vehicle class.

This is something of a contrived example but it does show how that you can share implementation details among a hierarchy of classes. Each class further refines a concept. This is not always the best way to implement an interface but in some cases it works very well. As a guideline, for ease of maintenance and understanding you should try to limit the inheritance to no more than 3 levels. Often the best set of classes to use is a pure virtual abstract base class to define a common interface. Then use an abstract class to further refine an implementation for a set of concrete classes and lastly define the set of concrete classes.

An abstract class is a class that is designed to be specifically used as a base class. An abstract class contains at least one pure virtual function. You declare a pure virtual function by using a pure specifier (= 0) in the declaration of a virtual member function in the class declaration.

The following is an example of an abstract class:

class AB {
public:
  virtual void f() = 0;
};

Function AB::f is a pure virtual function. A function declaration cannot have both a pure specifier and a definition.

Abstract class cannot be used as a parameter type, a function return type, or the type of an explicit conversion, and not to declare an object of an abstract class. It can be used to declare pointers and references to an abstract class.

Pure Abstract Classes

An abstract class is one in which there is a declaration but no definition for a member function. The way this concept is expressed in C++ is to have the member function declaration assigned to zero.

Example

class PureAbstractClass 
{
public:
  virtual void AbstractMemberFunction() = 0;
};

A pure Abstract class has only abstract member functions and no data or concrete member functions. In general, a pure abstract class is used to define an interface and is intended to be inherited by concrete classes. It's a way of forcing a contract between the class designer and the users of that class. The users of this class must declare a matching member function for the class to compile.

Example of usage for a pure Abstract Class

 class DrawableObject 
 {
  public:
    virtual void Draw(GraphicalDrawingBoard&) const = 0; //draw to GraphicalDrawingBoard
 };

 class Triangle : public DrawableObject
 {
 public:
   void Draw(GraphicalDrawingBoard&) const; //draw a triangle
 };

 class Rectangle : public DrawableObject
 {
 public:
   void Draw(GraphicalDrawingBoard&) const; //draw a rectangle
 };

 class Circle : public DrawableObject
 {
 public:
   void Draw(GraphicalDrawingBoard&) const; //draw a circle
 };

 typedef std::list<DrawableObject*> DrawableList;

 DrawableList drawableList;
 GraphicalDrawingBoard drawingBoard;

 drawableList.pushback(new Triangle());
 drawableList.pushback(new Rectangle());
 drawableList.pushback(new Circle());

 for(DrawableList::const_iterator iter = drawableList.begin(), 
    endIter = drawableList.end();
    iter != endIter;
    ++iter)
 { 
   DrawableObject *object = *iter;
   object->Draw(drawingBoard);
 }

Note that this is a bit of a contrived example and that the drawable objects are not fully defined (no constructors or data) but it should give you the general idea of the power of defining an interface. Once the objects are constructed, the code that calls the interface does not know any of the implementation details of the called objects, only that of the interface. The object GraphicalDrawingBoard is a placeholder meant to represent the thing onto which the object will be drawn, i.e. the video memory, drawing buffer, printer.

Note that there is a great temptation to add concrete member functions and data to pure abstract base classes. This must be resisted, in general it is a sign that the interface is not well factored. Data and concrete member functions tend to imply a particular implementation and as such can inherit from the interface but should not be that interface. Instead if there is some commonality between concrete classes, creation of abstract class which inherits its interface from the pure abstract class and defines the common data and member functions of the concrete classes works well. Some care should be taken to decide whether inheritance or aggregation should be used. Too many layers of inheritance can make the maintenance and usage of a class difficult. Generally, the maximum accepted layers of inheritance is about 3, above that and refactoring of the classes is generally called for. A general test is the "is a" vs "has a", as in a Square is a Rectangle, but a Square has a set of sides.