Object Precedence in Methods

Establishing an object precedence enables the MATLAB® runtime to determine which of possibly many versions of an operator or function to call in a given situation.

For example, consider the expression

objectA + objectB

Ordinarily, objects have equal precedence and the method associated with the left-most object is called. However, there are two exceptions:

  • User-defined classes have precedence over MATLAB fundamental classes (see Fundamental MATLAB Classes) and certain built-in classes.

  • User-defined classes can specify their relative precedence with respect to other user-defined classes using the InferiorClasses attribute.

In Class for Polynomials, the polynom class defines a plus method that enables the addition of DocPolynom objects. Given the object p:

p = DocPolynom([1 0 -2 -5])
p =

the expression:

1 + p
ans =

calls the DocPolynom plus method (which converts the double, 1, to a DocPolynom object and then implements the addition of two polynomials). The user-defined DocPolynom class has precedence over the built-in double class.

Specifying Precedence of User-Defined Classes

You can specify the relative precedence of user-defined classes by listing inferior classes using a class attribute. The InferiorClasses property places a class below other classes in the precedence hierarchy. Define the InferiorClasses property in the classdef statement:

classdef (InferiorClasses = {?class1,?class2}) myClass

This attribute establishes a relative priority of the class being defined with the order of the classes listed.

Location in the Hierarchy

If objectA is above objectB in the precedence hierarchy, then the expression

objectA + objectB

calls @classA/plus.m. Conversely, if objectB is above objectA in the precedence hierarchy, then the MATLAB runtime calls @classB/plus.m.

See Rules for Naming to Avoid Conflicts for related information.

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