ZBasic Language Reference
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ZBasic Microcontrollers
It is important to understand that when a class is derived from another class, the compiler generates code
to ensure that the base class object is fully initialized before the constructor for the derived class
executes. Similarly, for assignment constructors the base class assignment constructor is executed
before that of the derived class. This order guarantees that the derived class object may safely access
the methods and members of the base class in its constructors.
The situation with destructors is similar but the order is reversed. In the case of destructors the derived
class destructor executes before the base class destructor. This order guarantees that the derived class
may rely on the integrity of the base class data members if necessary.
One aspect of inheritance that must be carefully considered is that since the base class constructor
executes before a derived class constructor, base class constructors must avoid invoking methods that
would result in the derived class method executing before derived class’ constructor has executed.
4.12 Abstract Classes, Abstract Methods
When designing an object-oriented solution, it is often useful to define a class that embodies some
essential characteristics and functions that can then be inherited by several other classes. In some
situations, the base class is “incomplete” in the sense that its only purpose is to serve as the base class
for other classes. You can prevent such an incomplete class from being instantiated (i.e. defining an
object of that type) by defining the base class to be abstract. This is done by adding the Abstract
attribute to the class definition.
Abstract Class MyObject
Public:
Protected:
Dim m_size as Integer
Dim m_weight as Integer
Private:
End Class
If you try to create an object of the class MyObject, the compiler will produce an error message
indicating that you cannot instantiate an abstract class. This is to remind you that the purpose of the
class is as a building block for other classes.
You can also indicate that a class is abstract by defining the class with at least one abstract method as in
the class definition below.
Class MyObject
Public:
Abstract Sub Identify()
End Sub
Protected:
Dim m_size as Integer
Dim m_weight as Integer
Private:
End Class
The result of both of the preceding class definitions is an abstract class. The difference is that when you
define abstract methods, in addition to making the class abstract it also establishes a requirement that all
derived classes must define a method that has the same signature, i.e. the same name and same
number and types of parameters.
If you wish, you may include code in the body of the abstract method. One reason for doing so would be
to define some essential functionality that many or most derived classes could use. The derived class
method can invoke the abstract base class method using the class namespace prefix or the parent class
prefix. For example, class A could be derived from MyObject and the Identify() methods of the
MyObject and A classes could be rewritten as shown below, respectively. You should compile and run
such and example to observe the result.
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