Lesson 5 · 5 of 20

Abstract classes · Pascal

Declare abstract methods in Object Pascal to define a common outline that each child class fills in, with payment methods and their fees.

Pascal · Abstract classes in DMarlon Studio: the example's code and its output in the terminal

An `abstract` method is declared but not implemented: TPayment.Fee says every payment has a fee, but not which one. It must also be virtual, because its only purpose is to be completed by child classes with override.

TPayment is an abstract class: a template, not something to create objects from. If you wrote TPayment.Create, Free Pascal would compile it with a warning, and calling Fee would raise EAbstractError. The real objects are TCard (1.5 % of the amount) and TTransfer (a flat 0.35 EUR).

The interesting part is Charge: a regular method of the abstract class that uses the abstract one. Every payment is charged with the same steps and only the fee changes. This is the *template method* pattern: the parent sets the outline, the children supply the detail.

Copy(ClassName, 2, 9) drops the leading T from the class name to print Card or Transfer. The loop creates each payment method, charges it 20 and 250 EUR and frees it with Free.

Code · main.pas

program main;type  TPayment = class    function Fee(amount: Double): Double; virtual; abstract;  // no body here    procedure Charge(amount: Double);  end;  TCard = class(TPayment)    function Fee(amount: Double): Double; override;  end;  TTransfer = class(TPayment)    function Fee(amount: Double): Double; override;  end;procedure TPayment.Charge(amount: Double);begin  // the same steps for every method; only Fee changes  WriteLn(Copy(ClassName, 2, 9):8, amount:8:2, ' + fee', Fee(amount):5:2, ' =', amount + Fee(amount):7:2);end;function TCard.Fee(amount: Double): Double;begin  Result := amount * 0.015;  // 1.5 % of the amountend;function TTransfer.Fee(amount: Double): Double;begin  Result := 0.35;  // flat feeend;var method: TPayment;begin  for method in [TCard.Create, TTransfer.Create] do  begin    method.Charge(20);    method.Charge(250);    method.Free;  end;end.

Output · fpc main.pas && ./main

    Card   20.00 + fee 0.30 =  20.30
    Card  250.00 + fee 3.75 = 253.75
Transfer   20.00 + fee 0.35 =  20.35
Transfer  250.00 + fee 0.35 = 250.35

Key points

virtual; abstract; declares a method with no body.
An abstract class is a template; you use its children.
Calling an abstract method raises EAbstractError.
Regular methods in the parent can call the abstract ones.
Template method: shared steps, details in each child.

Exercise

Add TBizum = class(TPayment) with no fee for amounts up to 50 EUR and 0.20 EUR above that, and charge it the same two amounts.