Hi All,

Could anyone please explain this:

5.4 Constructors and destructors
<http://www.freepascal.org/docs-html/ref/ref.html#QQ2-64-77> As can be seen in the syntax diagram for an object declaration, Free Pascal supports constructors and destructors. The programmer is responsible for calling the constructor and the destructor explicitly when using objects.

My question is, since the programmer is responsible for explicitly call object constructors, then why do we need constructors at all? In another word, what's the difference between an object constructor and an object method?

Thanks,
Xiangrong

Using the constructor tells the compiler that it should allocate memory on the heap for a new instance, and return a method to this new memory. There is then an implicit return value for the constructor, and this is stored in the variable to which you assign the result of the constructor.

Similarly, the compiler knows that the memory allocated by the constructor must be freed again in the destructor.

Michael.

Well, could you please explain what's the difference between p1 and p2 in the program below:

  1 program test;
  2 type
  3   TObj = object
  4   public
  5     Value: Integer;
  6     procedure SetValue(AValue: Integer);
  7     constructor Init(AValue: Integer);
  8   end;
  9
 10 procedure TObj.SetValue(AValue: Integer);
 11 begin
 12   Value := AValue;
 13 end;
 14
 15 constructor TObj.Init(AValue: Integer);
 16 begin
 17   Value := AValue;
 18 end;
 19
 20 var
 21   p1, p2 : ^TObj;
 22 begin
 23   New(p1);
 24   p1^.SetValue(1);
 25   WriteLn(p1^.Value);
 26   New(p2, Init(2));
 27   WriteLn(p2^.Value);
 28 end.

Ah.

You are using TP-style objects. Forget what I said then. What I wrote applies to classes.

My apologies for the misundestanding.

Michael.

2013/9/17 Xiangrong Fang

(Thanks to Michael for spotting that you are using TP objects)

A little bit below the citation in your first post, you'll find this: "A constructor/destructor pair is required if the object uses virtual methods. The reason is that for an object with virtual methods, some internal housekeeping must be done: this housekeeping is done by the constructor" :-) In Turbo Pascal, it seems that some VMT housekeeping was also done in the destructor, I don't know if this is still true in fpc.

You cannot use fail outside of a constructor. So there is no proper error handling method available which does no create memory leaks.