What is interface-interface inheritance in Pascal?

In Pascal programming language, an interface acts like an abstract class, including a group of related methods but with empty bodies.

In Pascal, it is possible for interfaces to inherit from one another, just like classes do. As a result, an interface can be descended from another interface.
Hence, when a class implements an interface, it has to implement all the child interface's functions as well as all the base/parent interface's functions.

Generally, interfaces are used to achieve security and conceal certain details, and they specify what a class must do, not how it should do it.

The following figure exhibits the inheritance hierarchy:

program hello;
{$mode objfpc}
type   
   myParentInterface = Interface  
    Function myPFunc  : Integer;  
  end;  
  myChildInterface = Interface (myParentInterface)  
    Function myCFunc : Integer;  
  end;  
  
  myClass = Class(TInterfacedObject,myChildInterface)
    Function myPFunc  : Integer;  
    Function myCFunc  : Integer;  
  end;
  Function myClass.myPFunc : Integer;  
  begin  
    Result := 5;
  end; 
  Function myClass.myCFunc : Integer;  
  begin  
    Result := 10;
  end; 
var  
  cl1 : myClass;
begin
  writeln('Start...');
  cl1:=myClass.create;
  writeln('cl1.myPFunc  = ',cl1.myPFunc());
  writeln('cl1.myCFunc  = ',cl1.myCFunc());
  writeln('End...');
end.

Code explanation

Let's go through the code widget above:

Line 1: We refer to the program header. It is used for backward compatibility, and the compiler ignores it.
Line 2: This is a directive to the compiler to allow defining classes.
Lines 4–7: We use the Interface keyword, and define a parent interface called myParentInterface including one member function named myPFunc.
Lines 9–11: We use the Interface keyword, and define a child interface called myChildInterface extending the base interface myParentInterface and including one member function named myCFunc.
Lines 13–26: We define a class myClass implementing the child interface previously declared. To avoid errors in compilation, we should also inherit from a base object TInterfacedObject. This class must include the definition of the myPFunc function already declared in the parent interface as well as the definition of the function myCFunc declared earlier in the child interface.
Lines 28–29: We declare a variable cl1 having as type the class already defined.
Line 33: We create an instance of the myClass class and store it in the variable cl1 previously declared.
Lines 34–35: We invoke the functions included within the class and display their respective results.

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What is interface-interface inheritance in Pascal?

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Code explanation