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gajo
Guest
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 Posted: Sun Dec 21, 2003 12:47 pm Post subject: Error with pointers |
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Hello, I'm new to pointers, and I have a problem that I think is easy to
solve. I get an EAccessViolation when trying to initialize my object with
the Init procedure. The carret points at "elem", or in other words, I think
I'm trying to access M.elem without M being initialized. But M IS
initialized, at least I think so!
So what's wrong?
type
Main = ^MainType;
MainType = class
private
connect: array[1..maxNum] of Main;
elem: integer;
public
procedure Init;
procedure KillObj;
end;
....
procedure MainType.Init;
var
i: integer;
begin
elem := 0;
for i := 1 to maxNum do
connect[i] := nil
end;
....
procedure Button1Click
var
M: Main;
begin
GetMem(M, SizeOf(M));
G.Init;
G.KillObj
end;
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Tom de Neef
Guest
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| Posted: Sun Dec 21, 2003 3:11 pm Post subject: Re: Error with pointers |
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"gajo" <gajo (AT) eunet (DOT) yu> wrote
| Quote: | Hello, I'm new to pointers, and I have a problem that I think is easy to
solve. I get an EAccessViolation when trying to initialize my object with
the Init procedure. The carret points at "elem", or in other words, I
think
I'm trying to access M.elem without M being initialized. But M IS
initialized, at least I think so!
So what's wrong?
type
Main = ^MainType
MainType = class
private
connect: array[1..maxNum] of Main;
elem: integer;
public
procedure Init;
procedure KillObj;
end;
...
procedure MainType.Init;
var
i: integer;
begin
elem := 0;
for i := 1 to maxNum do
connect[i] := nil
end;
...
procedure Button1Click
var
M: Main;
begin
GetMem(M, SizeOf(M));
G.Init;
G.KillObj
end;
|
If you want to try out pointers, let them point to records, not classes. A
class reference is itself a pointer. So SizeOf(M) will be the size of a
pointer. But in a record you can't have private parts or methods.
Anyway, what is the G you reference in Button1Click?
So, maybe you don't want pointers, but just classes. In that case check how
I modified your code below.
Note that I've named the type with a starting T as is customary in Delphi.
Tom
type
TMainType = class
private
connect: array[1..maxNum] of TMainType;
elem: integer;
public
constructor create;
procedure free;
end;
...
constructor TMainType.create;
var
i: integer;
begin
inherited;
elem := 0;
for i := 1 to maxNum do
connect[i] := nil
end;
procedure TMainType.free;
var i : integer;
begin
for i:=1 to maxNum do FreeAndNil(connect[i]);
inherited
end;
procedure Button1Click
var
M: TMainType;
begin
M:=TMainType.create;
M.Init;
M.free;
end;
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Duncan McNiven
Guest
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| Posted: Sun Dec 21, 2003 3:46 pm Post subject: Re: Error with pointers |
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On Sun, 21 Dec 2003 16:11:40 +0100, "Tom de Neef" <tdeneef (AT) qolor (DOT) nl> wrote:
| Quote: | constructor TMainType.create;
var
i: integer;
begin
inherited;
elem := 0;
for i := 1 to maxNum do
connect[i] := nil
end;
|
Why explicitly initialize the connect array? The Delphi help says "Because a constructor
sets the fields of a new object to zero or empty values before performing other actions,
class-type and pointer-type fields in a partially constructed object are always nil". Do
you have any reason to believe this is not so for arrays of class types? Or is this purely
for clarity / example?
| Quote: | procedure TMainType.free;
var i : integer;
begin
for i:=1 to maxNum do FreeAndNil(connect[i]);
inherited
end;
|
Why implement Free instead of Destroy?
I think there is a bug here. Some programmers might rely on the documented behaviour of
Free,ie "Free checks for a nil reference before calling Destroy". Your code will give an
access violation if called on a nil object. Better to always put your clean-up code in an
over-ridden Destroy method rather than Free.
--
Duncan
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