Multiple Inheritance

Multiple inheritance (several extends statements) is useful when a class wishes to include several orthogonal kinds of behavior and data, e.g. combining geometry and color. By writing several extends statements, multiple inheritance is achieved.

record ColorData parameter Real red=4; parameter Real blue=3; Real green; end ColorData;
class Color extends ColorData; equation red + blue + green=1; end Color;
class Point public Real x; Real y, z; end Point;
class ColoredPoint extends Point; extends Color; end ColoredPoint;

Show the flattened class with inheritance expanded:

instantiateModel(ColoredPoint)

1 Diamond Inheritance

A special kind of multiple inheritance is diamond inheritance, which is a problem that can arise when using multiple inheritance.

class Point2 parameter Real x = 2; parameter Real y = 3; end Point2;
class VerticalLine extends Point2; parameter Real vlength = 5; end VerticalLine;
class HorizontalLine extends Point2; parameter Real hlength = 7; end HorizontalLine;

The potential problem is that we have diamond inheritance since the coordinate position defined by the fields x and y is inherited twice. In Modelica this is not a problem, since there is a rule stating that if several identical declarations or equations are inherited, only one of them is kept.

class Rectangle extends VerticalLine; extends HorizontalLine; Point p1(x = x - vlength, y = y - hlength); Point p2(x = x + vlength, y = y + hlength); end Rectangle;

2 Flattened Version of Rectangle with Diamond Inheritance

The Point2 definitions x, y, etc. are only present once despite being inherited twice, via VerticalLine and HorizontalLine.

instantiateModel(Rectangle)