Programming Using Objects

Objects - Basics

Of the two OOP implementations FPC provides, the one used less often seems to be what is referred to as "Objects" which probably gets its name from the type definition syntax. The syntax diagram for an object type declaration can be found in the FPC language reference - Chapter 5. Basically, an object type looks like a record type with additional fields including procedure fields and also optional keywords which indicate the scope of the fields. In fact, an oversimplified (but valid) simple object is hard to distinguish from a record structure as can be seen below.

<delphi> Type

  MyObject = Object
     f_Integer : integer;
     f_String : ansiString;
     f_Array : array [1.3] of char;
  end;

</delphi>

The only difference in the above example is that the Pascal keyword record has been replaced with the keyword object. Things start to be different when methods are added to the object. Object methods are declared in FPC using the keywords procedure or function. These object methods (procedures or functions) are declared the same way as normal Pascal procedures and functions; just that they are declared within the object declaration itself. Lets create a different object; this time possibly as part of an oversimplified graphics drawing program.

<delphi> Type

  DrawingObject = Object
     x, y : single;
     height, width : single;
     procedure Draw;
  end;

Var

 Rectangle : DrawingObject;

</delphi>

Besides the simple datatype fields providing some application specific location and size attributes, the object shown above declares an additional parameter; a procedure called Draw. The type declaration is followed by a variable identifier called Rectangle of the type DrawingObject. Next, the Draw procedural code itself needs to be written as well as well as code for accessing and manipulating the data fields, as well as how to invoke the Draw procedure. The following simple program shows how all this works. It should compile and run on any system with FPC 2.2.2 and above. Note: For Mac OS X, the -macpas compiler directive must be turned off.

<delphi> Program TestObjects;

Type

  DrawingObject = Object
     x, y : single;
     height, width : single;
     procedure Draw;  //  procedure declared in here
  end;

 procedure DrawingObject .Draw;
 begin
      writeln('Drawing an Object');
      writeln(' x = ', x, ' y = ', y);  // object fields
      writeln(' width = ', width);
      writeln(' height = ', height);
      writeln;

// moveto (x, y); // probably would need to include a platform dependent drawing unit to do actual drawing // ... more code to actually draw a shape on the screen using the other parameters

 end;

Var

 Rectangle : DrawingObject;

begin

 Rectangle.x:= 50;  //  the fields specific to the variable "Rectangle"
 Rectangle.y:= 100;
 Rectangle.width:= 60;
 Rectangle.height:= 40;

 writeln('x = ', Rectangle.x);

 Rectangle.Draw;  //  Calling the method (procedure)

 with Rectangle do   //  With works the same way even with the method (procedure) field
  begin
      x:= 75;
      Draw;
  end;

end. </delphi>

As can be seen in the above program, the body of the Draw method (procedure) is declared after the object type declaration by concatenating the type identifier with the procedure name. In a more realistic situation, the object would likely be declared in the interface section of a separate unit while the procedure body would be written in the implementation section of the same external unit. In this example, only standard vanilla Pascal is used, but if actual graphic primitives are available, they can be used if desired. The second thing to notice is that inside the Draw method (procedure), the object's data fields are referenced as if they were regular local variables. The only difference to regular local variables is that the values of these fields will persist between calls to the Draw procedure.

In the main program, the fields are assigned values and accessed just like record fields are. Similarly, the Draw procedure is invoked using the same dot notation as the fields. And like records, the with keyword works the same for accessing object fields and invoking methods. Finally, notice that the second time the Draw method is called, all the fields persisted between calls; the only field different is the x attribute which was explicitly changed.

The above example is meant to show the basic mechanics of simple Objects. There are a couple of problems, however. The first problem is that the Draw method only draws one thing and that is a rectangle (although even that is questionable.) Additional methods could be declared in the DrawingObject object such as DrawRectangle, DrawCircle, DrawTriangle, etc..., but this would not be too much different than declaring separate regular procedures and having to use case statements to select the appropriate procedure. So doing it this way with objects would require more work (and is not how it is accomplished.) The other problem is, the object's fields are able to be accessed and modified anywhere in the program very similar to using global variables which makes it harder to write well encapsulated and robust programs. These problems will be addressed in the next section.