2013/9/17 Xiangrong Fang <xrf...@gmail.com> > 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? >>> >> >> 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. >> > > 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. >
(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. -- Frederic Da Vitoria (davitof) Membre de l'April - « promouvoir et défendre le logiciel libre » - http://www.april.org
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