Generic Classes

1 General Description

There are essentially two cases of generic class parameterization in Modelica: class parameters (formal parameters to the class) can either be instance parameters, i.e. have instances as values, or be type parameters, i.e. have types as values.

Formal class parameters are replaceable variable or type declarations within the class (usually) marked with the prefix replaceable. The special case of replaceable local functions is roughly equivalent to virtual methods in some object-oriented programming languages.
Actual arguments to classes are modifiers, which when containing whole variable declarations or types are preceded by the prefix redeclare.

2 Class Parameters being Component Declarations

2.1 GreenClass and YellowClass

type GreenClass = Real(unit = "green");
type YellowClass = Real(unit = "yellow");

2.2 The Class C

Here we handle the case when class parameters are component being instances or objects. The class C has three class parameters, marked by the keyword replaceable.

class C replaceable GreenClass obj1(p1=5); replaceable YellowClass obj2; replaceable GreenClass obj3; end C;

Now a class C2 is defined by providing two declarations of obj1 and obj2 as actual arguments to class C, being red and green respectively, instead of the defaults green and yellow. The keyword redeclare must precede an actual argument to a class formal parameter in order to avoid accidentally changing the type of an object through a standard modifier. The type of a class component cannot be changed if it is not declared as replaceable and a redeclaration is provided.

class C2 = C(redeclare RedClass obj1, redeclare GreenClass obj2);

The definition of C2 is equivalent to C2b.

class C2b RedClass obj1(p1=5); //type GreenClass obj2; GreenClass obj3; end C2b;

3 Class Parameters being Types

3.1 BlueClass

type BlueClass = Real(unit = "blue");

3.2 The Class C

A class parameter can also be a type, for example in the class C below it is easy to change the color of all objects of the type ColoredClass.

class C replaceable class ColoredClass = GreenClass; //type parameter ColoredClass obj1(p1 = 5); replaceable YellowClass obj2; ColoredClass obj3; end C;

In class C2 the type parameter of ColoredClass are redeclared as BlueClass.

class C2 = C(redeclare class ColoredClass = BlueClass);

The definition of C2 is equivalent to C2b.

class C2b BlueClass obj1(p1=5); YellowClass obj2; BlueClass obj3; end C2b;