Class

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• Class 20 Abstract Classes

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ABCs

• Abstract Classes vs. Concrete Classes What if we
  wanted a class that never needs to instantiate an
  object?  It is possible that we would never need
  to instantiate a generic object such as a Shape. 
  Even though all Circles, Triangles, Rectangles,
  etc. are, in the most generic sense, Shapes,
  there is no need to define a Shape when defining
  these derived class objects.  In this case a
  Shape class would be an abstract class.

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Simple Abstract Class

• class Shape
• public    virtual void Draw() const 0 //
  pure virtual function private

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Abstract classes

• The abstract class Shape has no data members, and
  provides no functional operations that aren't
  handled at the derived class level. 
• However, if we wanted to declare a pointer to a
  concrete class object circle, triangle, or
  rectangle, we might not know in advance what the
  derived class of the object will be. 
• Therefore, the Shape class allows us to define
  one type of pointer, a Shape pointer, and use it
  to draw whichever type of Shape is created.

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Properties of ABCs

• Abstract classes provide a base class from which
  classes may inherit a common interface and/or
  implementation.
• They are too generic to define any real objects.
• Identified by having one or more purely virtual
  member functions.
• NOTE If a derived class (2-D Shape) has no
  definition for a pure virtual member function,
  that function remains a pure virtual function,
  and so the derived class is also an abstract
  class.
• Attempts to define an object from abstract class
  is a syntax error.
• Concrete classes provide the specifics to define
  each object.