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Specifying Properties

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What You Can Define How to Initialize Property Values Defining Default Values Assigning Property Values from Within the Constructor Initializing Properties to Unique Values Property Attributes Property Access Methods Referencing Object Properties Using Variables

What You Can Define

You can control aspects of property definitions in the following ways:

• Specifying a default value for each property individually

• Assigning attribute values on a per block basis

• Defining methods that execute when the property is set or queried

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How to Initialize Property Values

There are two basic approaches to initializing property values:

• In the property definition — MATLAB evaluates the expression only once and assigns the same value to the property of every instance. See Defining Default Values.

• In the class constructor — MATLAB evaluates the assignment expression for each instance, which ensures that each instance has a unique value. See Assigning Property Values from Within the Constructor.

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Defining Default Values

Within a properties block, you can control an individual property's default value. Default values can be constant values or MATLAB expressions. Expressions cannot reference variables. For example:

classdef class_name
   properties
      PropertyName % No default value assigned
      PropertyName = 'some text'; 
      PropertyName = sin(pi/12); % Expression returns default value
   end
end

Keep in mind that the evaluation of default values occurs only once before MATLAB first instantiates the class.

MATLAB sets property values not specified in the class definition to empty ([]).

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Assigning Property Values from Within the Constructor

To assign values to a property from within the class constructor, reference the object that the constructor returns (the output variable obj):

classdef MyClass
   properties
      PropertyOne 
   end
   methods
      function obj = MyClass(intval)
         obj.PropertyOne = intval;
      end
   end
end

When you assign an object property from the class constructor, MATLAB evaluates the assignment statement for each instance created. Assign property values in the constructor if you want each object to contain a unique instance of a handle object.

See Referencing the Object in a Constructor for more information on constructor methods.

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Initializing Properties to Unique Values

MATLAB assigns properties to the specified default values only once when MATLAB loads the class definition. Therefore, if you initialize a property value with a handle-class constructor, calls this constructor only once and every instance references the same handle object. If you want a property value to be initialized to a new instance of a handle object each time you create an object, assign the property value in the constructor.

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Property Attributes

All properties have attributes that modify certain aspects of the property's behavior. Specified attributes apply to all properties in a particular properties block. For example:

classdef class_name
   properties
      PropertyName % No default value assigned
      PropertyName = sin(pi/12); % Expression returns default value
   end
   properties (SetAccess = private, GetAccess = private)
      Stress
      Strain
   end
end

In this case, only methods in the same class definition can modify and query the Stress and Strain properties. This restriction exists because the class defines these properties in a properties block with SetAccess and GetAccess attributes set to private.

Table of Property Attributes provides a description of property attributes.

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Property Access Methods

You can define methods that MATLAB calls whenever setting or querying a property value. Define property set access or get access methods in methods blocks that specify no attributes and have the following syntax:

methods
   function value = get.PropertyName(object)
      ...
   end
   function obj = set.PropertyName(obj,value)
      ...
   end
end

If a handle class defines the property, the set access method does not need to return the modified object.

Property Access Methods for more information on these methods.

Defining Properties for information on properties.

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Referencing Object Properties Using Variables

MATLAB can resolve a property name from a char variable using an expression of the form:

object.(PropertyNameVar)

where PropertyNameVar is a variable containing the name of a valid object property. Use this syntax when passing property names as arguments:

PN = 'KeyType';
function o = getPropValue(obj,PN)
   ...
   o = obj.(PropName); % Returns value of KeyType property
   ...
end

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The Classdef Block

Specifying Methods and Functions

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