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What are static class methods in Pascal?

Khizar Hayat Saani

When using Pascal, we can define static methods within a class.

Before we move on to how to create and use a static class method in Pascal, we must first understand what a static method is.

Static methods

Static methods of a class are just like any other method. However, when implementing a static class method, the Self identifier is not available. This means that static methods do not have access to any class variables or methods. Static methods cannot be virtual, nor can they be assigned to regular procedural variables.

Static methods are associated with the namespace of the class rather than the unitobject variable. Hence, they are limited to the class in which they are defined.

Code

Now that we have familiarized ourselves with the concept of static methods, it’s time to see them in action.

Let’s define our classes:

Type  
  ClassA = Class  
    Class procedure foo; virtual;  
    Class Procedure bar; static;  
  end;  
 
  ClassB = Class(ClassA)  
    Class procedure foo; override;  
  end;  

We define two classes, ClassA and ClassB. ClassB is derived from ClassA.

ClassA contains a virtual method (foo) and a static method (bar).

ClassB is derived from ClassA and overrides the foo method.

Class procedure ClassA.foo;  
 
begin  
  Writeln('ClassA.foo says: ', Self.ClassName);  
end;  
 
Class procedure ClassA.bar;  
 
begin  
  foo;  
  Writeln('ClassA.bar says : ', ClassName);  
end;  
 
Class procedure ClassB.foo;  
 
begin  
  Inherited;  
  Writeln('ClassB.foo says: ', Self.ClassName);  
end;  

The foo method in ClassA is a simple Writeln statement that outputs the class name.

bar is a static method and does not have a Self identifier. Therefore, we use the ClassName identifier to refer to the class name in the function defined in ClassA.

The bar method starts by calling the foo function of its respective class followed by a simple Writeln statement.

The overridden foo method in ClassB first calls the foo method for its parent class (classA), followed by a simple Writeln statement.

Let’s writeup a main:

begin  
  Writeln('Through static method:');  
  ClassB.bar;  
  Writeln('Through class method:');  
  ClassB.foo;  
end. 

Output

Through static method:  
ClassA.foo says : ClassA  
ClassA.bar says: ClassA  
Through class method:  
ClassA.foo says : ClassB  
ClassB.foo says : ClassB 

When the static method is called through ClassB, the ClassName identifier returns ClassA. This is because the static method is only associated with ClassA, as that was the class in which it was defined.

When we call the non-static method, the Self identifier is correctly associated with ClassB, and we see the right class names.

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Khizar Hayat Saani
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