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ZBasic Language Reference
110
ZX Microcontroller Family
Class A
Public:
Sub DoSomething()
End Sub
Protected:
Private:
End Class
Class B Includes A
Public:
Sub Identify()
Call DoSomething()
End Sub
End Class
Dim MyB as B
Sub Main()
Call MyB.DoSomething()
End Sub
One restriction on using mixins is that the public and protected methods and data members of each mixin
must be unique among all of the mixins included and must not duplicate any method or data member
names of the including class. This restriction is necessary because all public and protected methods and
data members need to reside in the namespace of the including class and therefore must be unique. It is
possible, however, to have methods of the same name in multiple mixins and/or the including class as
long as they each have unique signatures, i.e. they must have different number and/or types of
parameters. Note, however, that the including class may contain methods that overload methods of mixin
classes. It is important to be aware that a mixin class method that is marked as Abstract must be
overloaded by the including class.
Finally, if a class is defined using both inheritance and mixins, the Extends <class name>
specification must appear before the Includes <class name list> specification, e.g.
Class D Extends B Includes A, C
Public:
Sub DoSomething()
End Sub
End Class
4.15 Using the Const Attribute for Methods
Earlier in this chapter, it was stated that an object may be passed to a subroutine or function ByRef or
ByVal. If it is passed ByVal, the object instance is considered to be “read-only” within the receiving
procedure. As such, the procedure has read access to the public data members of the object but it is not
allowed to modify those data members. Also, the receiving procedure may not pass the object ByRef to
any other procedure or method. Additionally, in order to invoke a method of the object, that method must
be declared to be “constant”, meaning that it isn’t allowed to modify the object or pass the object by
reference to another method or procedure.
To define a class method as constant, add the keyword Const following the closing parenthesis of the
parameter list. Optionally, for methods that are functions, you may instead place the Const keyword
after the function’s return type. An example is shown below using a previously defined method.
Sub Identify() Const
Call MyObject::Identify()
Debug.Print "I'm an A, size="; m_size
End Sub
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