bnz
asked on
Pointer typecast
Can anybody tell me why the pointer typecast below is necessary.
AllocMem returns a pointer so the typecast is pointless the way I see it, but if it's omitted it crashes.
type
TMyRec = record
ip: string[15];
port: integer;
end;
var
MyAry: array of TMyRec;
begin
// ||
// ||
// \/
MyAry := Pointer( AllocMem(SizeOf(TMyRec)*2)
MyAry[0].ip := '0.0.0.0';
MyAry[1].ip := '255.255.255.255';
showmessage(MyAry[0].ip);
showmessage(MyAry[1].ip);
end;
AllocMem returns a pointer so the typecast is pointless the way I see it, but if it's omitted it crashes.
type
TMyRec = record
ip: string[15];
port: integer;
end;
var
MyAry: array of TMyRec;
begin
// ||
// ||
// \/
MyAry := Pointer( AllocMem(SizeOf(TMyRec)*2)
MyAry[0].ip := '0.0.0.0';
MyAry[1].ip := '255.255.255.255';
showmessage(MyAry[0].ip);
showmessage(MyAry[1].ip);
end;
Why don't you use SetLength?
Uah! The pointer typecast is not necessary, in fact it's plain and simply *wrong*!!!
You can't assign an allocated memory block to a dynamic array. Instead you HAVE to use SetLength (as Robert already said).
Regards, Madshi.
You can't assign an allocated memory block to a dynamic array. Instead you HAVE to use SetLength (as Robert already said).
Regards, Madshi.
The reason is that dynamic arrays are more than just a simply memory buffer. They have a reference count and such stuff. Just like dynamic strings. You wouldn't do the following, either, or would you?
var str : string;
begin
str := pointer(AllocMem(100)); // crash code
var str : string;
begin
str := pointer(AllocMem(100)); // crash code
ASKER
>You wouldn't do the following, either, or would you?
>
>var str : string;
>begin
> str := pointer(AllocMem(100)); // crash code
Heh, I think I would ;)
isn't a string just a simple nullterminated pointer ?
var
s: string;
begin
s := 'delphi';
showmessage(s);
end;
if I open the cpu window and look at what it points to it is delphi#0
If the array is wrong, why does it then works ?
>
>var str : string;
>begin
> str := pointer(AllocMem(100)); // crash code
Heh, I think I would ;)
isn't a string just a simple nullterminated pointer ?
var
s: string;
begin
s := 'delphi';
showmessage(s);
end;
if I open the cpu window and look at what it points to it is delphi#0
If the array is wrong, why does it then works ?
ASKER CERTIFIED SOLUTION
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To add to Madshi: A string in Delphi is much more a class in the C++ sense than anything else.
Declaring a string variable implies calling the constructor.
For a Delphi string this is simply assigning nil. nil serves as '' string (empty string).
Any string variable is placed inside a try finally block.
In the finally part the reference counter is decremented and the memory is freed if the counter reaches zero.
Declaring a string variable implies calling the constructor.
For a Delphi string this is simply assigning nil. nil serves as '' string (empty string).
Any string variable is placed inside a try finally block.
In the finally part the reference counter is decremented and the memory is freed if the counter reaches zero.
ASKER
Thank you both
I have put a follow up question here
https://www.experts-exchange.com/jsp/qManageQuestion.jsp?ta=delphi&qid=20274342
I have put a follow up question here
https://www.experts-exchange.com/jsp/qManageQuestion.jsp?ta=delphi&qid=20274342