How does a class implement an interface in Pascal

Typically, interfaces achieve abstraction. They serve as blueprints or skeletons for designing classes.

An interface defines a common set of methods that must be defined by an implementing class. It includes only the declaration of these methods, that defines the operations that can be performed or the behavior of the class implementing it.
Different classes can implement the same interface according to their requirements.

In order to implement an interface in Pascal, it is important to note the following:

In Pascal, there are two commonly used modes of operation for interfaces—DELPHIDelphi compatibility mode and OBJFPCThe original Free Pascal compiler mode including classes, interfaces and exceptions.
All interface members are public.
An interface instance cannot be created directly. Instead, a class implementing the interface must be created. There is no constructor or destructor.
An interface cannot be implemented in part; it must be implemented in full.
Within a class, interface methods can be aliased.
An interface that inherits from another interface should contain both the methods from the parent interface as well as the methods explicitly listed in the interface definition. In such case, It is mandatory for a class implementing an interface to implement all members of the interface as well as the methods of the parent interface(s).

Following are some examples showing how interfaces work:

Code example 1

This example illustrates what is a Pascal Interface and how to implement it:

program hello;
{$mode objfpc}
type   
  vehiculeInterface = Interface  
    Function changeSpeed  : Integer;  
    Function startEngine  : Integer;
  end;  
  
  carClass = Class(TInterfacedObject,vehiculeInterface)
    Function changeSpeed  : Integer;  
    Function startEngine  : Integer;  
    Function changeColor  : Integer;  
  end;
  Function carClass.changeSpeed : Integer;  
  begin  
    Result := 5;
  end; 
  Function carClass.startEngine : Integer;  
  begin  
    Result := 10;
  end; 
  Function carClass.changeColor : Integer;  
  begin  
    Result := 20;
  end;   
var  
  myCar: carClass;
begin
  writeln('Start...');
  myCar:=carClass.create;
  writeln('myCar.changeSpeed  = ',myCar.changeSpeed());
  writeln('myCar.startEngine  = ',myCar.startEngine());
  writeln('myCar.changeColor  = ',myCar.changeColor());
  writeln('End...');
end.

Let's go through the code widget above:

Line 1: It refers to the program header. It's used for backward compatibility, and it is ignored by the compiler.
Line 2: Here, we direct the compiler to allow defining classes.
Lines 5–8: We use the Interface keyword to define an interface, vehiculeInterface, comprising two member functions—changeSpeed and startEngine.
Lines 10–14: We define a carClass class implementing the interface previously declared. To avoid errors in compilation, we should inherit as well from a base object, TInterfacedObject. This class includes the two functions already defined in the interface in addition to a local member function, changeColor.
Lines 16–27: We define the functions included within the class.
Lines 28–29: We declare a variable myCar having as type the class already defined.
Line 32: We create an instance of the class carClass and store it in the myCar variable previously declared.
Lines 33-35: We invoke the functions included within the class and display their respective results.

Code example 2

This example aims to highlight the error returned by the compiler once it matches the methods of the class, missing one method startEngine, to those defined in the interface.

program hello;
{$mode objfpc}
type   
  vehiculeInterface = Interface  
    Function changeSpeed  : Integer;  
    Function startEngine  : Integer;  
  end;  
  
  carClass = Class(TInterfacedObject,vehiculeInterface)
    Function changeSpeed  : Integer;  
    Function vehiculeInterface.startEngine = changeSpeed;  
  end;
  Function carClass.changeSpeed : Integer;  
  begin  
    Result := 5;
  end; 
var  
  myCar : carClass;
begin
  writeln('Start...');
  myCar:=carClass.create;
  writeln('myCar.changeSpeed = ',myCar.changeSpeed());
  writeln('End...');
end.

How does a class implement an interface in Pascal

Code example 1

Code example 2

Code example 3