Main Content

Method Invocation

MATLAB® classes support both dot and function syntaxes for invoking methods. This topic demonstrates both syntaxes and describes how MATLAB determines what method to invoke.

Dot and Function Syntaxes

To invoke a nonstatic method with one argument arg, where obj is an object of the class that defines the method, use dot syntax or function syntax.

obj.methodName(arg)
methodName(obj,arg)

For example, dataSetSummary stores a set of numeric data along with the mean, median, and range of that data. The class defines two methods: showDataSet displays the current data stored in the data property, and newDataSet replaces the current value of data and calculates the mean, median, and range of that data.

classdef dataSetSummary
    properties (SetAccess=private)
        data {mustBeNumeric}
        dataMean
        dataMedian
        dataRange
    end

    methods
        function showDataSet(obj)
            disp(obj.data)
        end
        function obj = newDataSet(obj,inputData)
            obj.data = inputData;
            obj.dataMean = mean(inputData);
            obj.dataMedian = median(inputData);
            obj.dataRange = range(inputData);
        end
    end
end

Create an instance of dataSetSummary and invoke newDataSet to add data to the object. Use dot syntax to call newDataSet. Because dataSetSummary is a value class, assign the result back to the original variable to preserve the change.

a = dataSetSummary;
a = a.newDataSet([1 2 3 4])

a = 

  dataSetSummary with properties:

          data: [1 2 3 4]
      dataMean: 2.5000
    dataMedian: 2.5000
     dataRange: 3

Invoke the showDataSet method, but use function syntax for this call.

showDataSet(a)
     1     2     3     4

Referencing Method Names with Expressions

You can invoke a class method dynamically by enclosing an expression in parentheses.

obj.(expression)

The expression must evaluate to a character vector or string that is the name of a method. For example, these two statements are equivalent for an object a of class dataSetSummary.

a.showDataSet
a.("showDataSet")

This technique does not work when used with function syntax.

Indexing into the Result of Method Call

You can dot index into the result of any method that returns a value for which dot indexing is defined, such as an object property or structure field name. For example, add a new method returnSummary to the dataSetSummary class that returns all of the stored data in a struct.

function outStruct = returnSummary(obj)
   outStruct = struct("Data",obj.data,...
                      "Mean",obj.dataMean,...
                      "Median",obj.dataMedian,...
                      "Range",obj.dataRange);
end

Call returnSummary and use dot indexing to return the median of the data set.

a.returnSummary.Median
ans =

    2.5000

For more information on indexing into the result of function calls, see Indexing into Function Call Results.

Determining Which Method Is Invoked

When dot syntax is used to invoke a method, MATLAB calls the method defined by the class of the object to the left of the dot. For example, if classA and classB both define a method called plus, then this code always invokes the plus method defined by classA.

A = classA;
B = classB;
A.plus(B)

No other arguments are considered. Methods of other arguments are never called, nor are functions.

In other syntaxes, MATLAB must determine which of possibly many versions of an operator or function to call in a given situation. The default behavior is to call the method associated with the leftmost argument. In both of these statements, the plus method defined by classA is called.

objA + objB
plus(objA,objB)

However, this default behavior can be changed when one object has precedence over another.

Object Precedence

Depending on how classes are defined, the objects of those classes can take precedence over other objects when it comes to method dispatching:

  • Classes defined with the classdef syntax have precedence over these MATLAB classes:

    double, single, int64, uint64, int32, uint32, int16, uint16, int8, uint8, char, string, logical, cell, struct, and function_handle.

  • Classes defined with the classdef syntax can specify their relative precedence to other classes using the InferiorClasses attribute.

In Representing Polynomials with Classes, the DocPolynom class defines a plus method that enables the addition of DocPolynom objects. Construct a DocPolynom instance.

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

p =
    x^3 - 2*x - 5

This statement adds a double to the DocPolynom instance. The DocPolynom class is dominant over the built-in double class, even though the double is the leftmost argument in 1 + p. This code invokes the DocPolynom plus method to add the polynomials. 

1 + p

ans =
    x^3 - 2*x - 4

You can also specify the relative precedence of classes defined with the classdef syntax by listing inferior classes in a class attribute. The InferiorClasses attribute gives the class higher precedence than the classes listed as arguments for the attribute. Define the InferiorClasses attribute 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. For more information, see Class Precedence.

Related Topics