12.4 Inheritance

Inheritance is one of the most fundamental ideas behind object-oriented programming. The basic idea is that a class inherits, or copies, all the fields from some class and then possibly expands the number of fields in the new data type. For example, suppose you created a data type point that describes a point in the planar (two-dimensional) space. The class for this point might look like the following:

type
     point: class

          var
               x:int32;
               y:int32;

          method distance;

     endclass;

Suppose you want to create a point in 3D space rather than 2D space. You can easily build such a data type as follows:

type
     point3D: class inherits( point )

          var
               z:int32;

     endclass;

The inherits option on the class declaration tells HLA to insert the fields of point at the beginning of the class. In this case, point3D inherits the fields of point. HLA always places the inherited fields at the beginning of a class object. The reason for this will become clear a little later. If you have an instance of point3D, which you call P3, then the following 80x86 instructions are all legal:

mov( P3.x, eax );
     add( P3.y, eax );
     mov( eax, P3.z );
     P3.distance();

Note that the p3.distance method invocation in this example calls the point.distance method. You do not have to write a separate distance method for the point3D class unless you really want to do so (see the next section for details). Just like the x and y fields, point3D objects inherit point's methods.