This information is part of the Modelica Standard Library maintained by the Modelica Association.

Inheritance from base class

Examples

class A
  parameter Real a, b;
end A;

class B
  extends A(b=2);
end B;

class C
  extends B(a=1);
end C;

From the example above we get the following instantiated class:

class Cinstance
  parameter Real a=1;
  parameter Real b=2;
end Cinstance;

The ordering of the merging rules ensures that, given classes A and B defined above,

class C2
  B bcomp(b=3);
end C2;

yields an instance with bcomp.b=3, which overrides b=2.

Syntax

See section on extends_clause in the Modelica Grammar.

Description

The name of the base class is looked up in the partially instantiated parent of the extends clause. The found base class is instantiated with a new environment and the partially instantiated parent of the extends clause. The new environment is the result of merging

1. arguments of all parent environments that match names in the instantiated base class
2. the optional class modification of the extends clause

in that order.

The elements of the instantiated base class become elements of the instantiated parent class.

The declaration elements of the instantiated base class shall either

• not already exist in the partially instantiated parent class [i.e., have different names].
• be exactly identical to any element of the instantiated parent class with the same name and the same level of protection (public or protected) and same contents. In this case, one of the elements is ignored (since they are identical it does not matter which one).

Otherwise the model is incorrect.

Equations of the instantiated base class that are syntactically equivalent to equations in the instantiated parent class are discarded. [Note: equations that are mathematically equivalent but not syntactically equivalent are not discarded, hence yield an overdetermined system of equations.]

Since specialized classes of different kinds have different properties, only specialized classes that are "in some sense compatible" to each other can be derived from each other via inheritance. The following table shows which kind of specialized class can be used in an extends clause of another kind of specialized class (the "grey" cells mark the few exceptional cases, where a specialized class can be derived from a specialized class of another kind):

	Base Class
Derived Class	package	operator	function	operator function	type	record	operator record	expandable connector	connector	block	model	class
package	yes											yes
operator		yes										yes
function			yes									yes
operator function			yes	yes								yes
type					yes							yes
record						yes						yes
operator record							yes					yes
expandable connector								yes				yes
connector					yes	yes	yes	yes	yes			yes
block						yes				yes		yes
model						yes				yes	yes	yes
class												yes

The specialized classes package, operator, function, type, record, operator record and expandable connector can only be derived from their own kind [(e.g., a package can only be base class for packages. All other kinds of classes can use the import statement to use the contents of a package)] and from class.