## Evaluation of Expressions in Class Definitions

### Why Use Expressions

An expression used in a class definition can be any valid MATLAB^{®} statement that evaluates to a single array. Use expressions to define property default values and in attribute specifications. Expressions are useful to derive values in terms of other values. For example, suppose that you want to define a constant property with the full precision value of `2`

π. You can assign the property the value returned by the expression `2*pi`

. MATLAB evaluates the function when first loading the class.

For information on assigning property default values and attribute values, see Initialize Property Values and Property Attributes.

### Where to Use Expressions in Class Definitions

Here are some examples of expressions used in a class definition:

classdef MyClass % Some attributes are set to logical values properties (Constant = true) CnstProp = 2*pi end properties % Static method of this class Prop1 = MyClass.setupAccount % Constant property from this class Prop2 = MyClass.CnstProp % Function that returns a value Prop3 = datestr(now) % A class constructor Prop4 = AccountManager end methods (Static) function accNum = setupAccount accNum = randi(9,[1,12]); end end end

MATLAB does not call property set methods when assigning the result of default value expressions to properties. (See Property Get and Set Methods for information about these special methods.)

Enumerations that derived from MATLAB types can use expressions to assign a value:

classdef FlowRate < int32 enumeration Low (10) Medium (FlowRate.Low*5) High (FlowRate.Low*10) end end

MATLAB evaluates these expressions only once when enumeration members are first accessed.

#### Expressions in Attribute Specifications

For attributes values that are logical `true`

or `false`

, class definitions can specify attribute values using expressions. For example, this assignment makes `MyClass`

sealed (cannot be subclassed) for versions of MATLAB before R2014b (`verLessThan`

)

classdef (Sealed = verLessThan('matlab','8.4')) MyClass

The expression on the right side of the equal sign (`=`

) must evaluate to `true`

or `false`

. You cannot use any definitions from the class file in this expression, including any constant properties, static methods, and local functions.

While you can use conditional expression to set attribute values, doing so can cause the class definition to change based on external conditions. Ensure that this behavior is consistent with your class design.

**Note**

The `AllowedSubclasses`

and the `InferiorClasses`

attributes require an explicit specification of a cell array of `meta.class`

objects as their values. You cannot use expressions to return these values.

#### Expressions That Specify Default Property Values

Property definitions allow you to specify default values for properties using any expression that has no reference to variables. For example, `VectorAngle`

defines a constant property (`Rad2Deg`

) and uses it in an expression that defines the default value of another property (`Angle`

). The default value expression also uses a static method (`getAngle`

) defined by the class:

classdef VectorAngle properties (Constant) Rad2Deg = 180/pi end properties Angle = VectorAngle.Rad2Deg*VectorAngle.getAngle([1 0],[0 1]) end methods function obj = VectorAngle(vx,vy) obj.Angle = VectorAngle.getAngle(vx,vy); end end methods (Static) function r = getAngle(vx,vy) % Calculate angle between 2D vectors cr = vx(1)*vy(1) + vx(2)*vy(2)/sqrt(vx(1)^2 + vx(2)^2) * ... sqrt(vy(1)^2 + vy(2)^2); r = acos(cr); end end end

You cannot use the input variables to the constructor to define the default value of the `Angle`

property. For example, this definition for the `Angle`

property is not valid:

properties Angle = VectorAngle.Rad2Deg*VectorAngle.getAngle(vx,vy) end

Attempting to create an instance causes an error:

a = VectorAngle([1,0],[0,1])

Error using VectorAngle Unable to update the class 'VectorAngle' because the new definition contains an error: Undefined function or variable 'vx'.

#### Expressions in Class Methods

Expression in class methods execute like expressions in any function. MATLAB evaluates an expression within the function workspace when the method executes. Therefore, expressions used in class methods are not considered part of the class definition and are not discussed in this section.

### How MATLAB Evaluates Expressions

MATLAB evaluates the expressions used in the class definition without any workspace. Therefore, these expressions cannot reference variables of any kind.

MATLAB evaluates expressions in the context of the class file, so these expressions can access any functions, static methods, and constant properties of other classes that are on your path at the time MATLAB initializes the class. Expressions defining property default values can access constant properties defined in their own class.

### When MATLAB Evaluates Expressions

MATLAB evaluates the expressions in class definitions only when initializing the class. Initialization occurs before the first use of the class.

After initialization, the values returned by these expressions are part of the class definition and are constant for all instances of the class. Each instance of the class uses the results of the initial evaluation of the expressions without re-evaluation.

If you clear a class, then MATLAB reinitializes the class by reevaluating the expressions that are part of the class definition. (see Automatic Updates for Modified Classes)

### Expression Evaluation in Handle and Value Classes

The following example shows how value and handle object behave when assigned to properties as default values. Suppose that you have the following classes.

#### Expressions in Value Classes

The `ClassExp`

class has a property that contains a `ContClass`

object:

classdef ContClass properties % Assign current date and time TimeProp = datestr(now) end end

classdef ClassExp properties ObjProp = ContClass end end

When you first use the `ClassExp`

class, MATLAB creates an instance of the `ContClass`

class. MATLAB initializes both classes at this time. All instances of `ClassExp`

include a copy of this same instance of `ContClass`

.

a = ClassExp; a.ObjProp.TimeProp

ans = 08-Oct-2003 17:16:08

The `TimeProp`

property of the `ContClass`

object contains the date and time when MATLAB initialized the class. Creating additional instances of the `ClassExp`

class shows that the date string has not changed:

b = ClassExp; b.ObjProp.TimeProp

ans = 08-Oct-2003 17:16:08

Because this example uses a value class for the contained object, each instance of the `ClassExp`

has its own copy of the object. For example, suppose that you change the value of the `TimeProp`

property on the object contained by `ClassExp`

object`b`

:

b.ObjProp.TimeProp = datestr(now)

ans = 08-Oct-2003 17:22:49

The copy of the object contained by object `a`

is unchanged:

a.ObjProp.TimeProp

ans = 08-Oct-2003 17:16:08

#### Expressions in Handle Classes

Now consider the behavior if the contained object is a handle object:

classdef ContClass < handle properties TimeProp = datestr(now) end end

Creating two instances of the `ClassExp`

class shows that MATLAB created an object when it initialized the `ContClass`

. MATLAB used a copy of the object’s *handle* for each instance of the `ClassExp`

class. Therefore, there is one `ContClass`

object and the `ObjProp`

property of each `ClassExp`

object contains a copy of its handle.

Create an instance of the `ClassExp`

class and note the time of creation:

a = ClassExp; a.ObjProp.TimeProp

ans = 08-Oct-2003 17:46:01

Create a second instance of the `ClassExp`

class. The `ObjProp`

contains the handle of the same object:

b = ClassExp; b.ObjProp.TimeProp

ans = 08-Oct-2003 17:46:01

Reassign the value of the contained object `TimeProp`

property:

b.ObjProp.TimeProp = datestr(now); b.ObjProp.TimeProp

ans = 08-Oct-2003 17:47:34

The `ObjProp`

property of object `b`

contains a handle to the same object as the `ObjProp`

property of object `a`

. The value of the `TimeProp`

property has changed on this object as well:

a.ObjProp.TimeProp

ans = 08-Oct-2003 17:47:34