An recent coding error revealed that it is possible to pass a bounded array
to a dynamic array. Here is how.
type
TIntArray = array of integer;
PIntArray = ^TIntArray;
(* helper, it displays the ref counter of a dynamic array *)
procedure DisplayRefCount(A: TIntArray);
var
P : Pointer;
begin
P := A;
P := Pointer(Integer(P) - 4);
ShowMessage(IntToStr(PInteger(P)^));
end;
(* the first pair of the test: uses a valid syntax *)
procedure PassArray(pA: PIntArray);
var
B : TIntArray;
begin
B := pA^;
ShowMessage(IntToStr(B[1]));
DisplayRefCount(B);
end;
procedure Test;
var
A : TIntArray;
begin
SetLength(A, 2);
A[0] := 22;
A[1] := 33;
PassArray(@A);
end;
(* the second pair of the test: assigns the address of a dynamic array
to a dynamic array variable. I believe that souldn't work.
But it works. Merely. The returned ref count is wrong, of cource,
because while pA is a pointer to a dynamic array it is passed the
address of a typed array.
It probably works because a dynamic array is implemented as a pointer to a
typed array
and because it is valid to pass an un-typed pointer expression to a dynamic
array
(in fact it is valid to pass an un-typed pointer to any typed pointer)...
*)
procedure PassArray2(pA: PIntArray);
var
B : TIntArray;
begin
B := @(pA^);
ShowMessage(IntToStr(B[1]));
DisplayRefCount(B);
end;
(* I have the type address ($TYPEDADDRESS) compiler directive as {$T-}, the
default
setting, wich makes the @ operator to return an un-typed pointer.
The fact that it is valid to pass an un-typed pointer to a typed pointer,
is the source of the error
var
pI : PInteger;
P : Pointer;
begin
PI := P;
end;
*)
procedure Test2;
var
A : array[0..1] of integer; // now, try a bounded array
begin
A[0] := 22;
A[1] := 33;
PassArray2(@A); // here lies the root of the problem
end;
(* and the best of all: putting the code of the second test
in a single function, makes it to raise an AV *)
procedure Test3;
var
A : array[0..1] of integer;
B : TIntArray;
pA : PIntArray;
begin
A[0] := 22;
A[1] := 33;
pA := @A;
B := @(pA^);
ShowMessage(IntToStr(B[1]));
DisplayRefCount(B);
end;
--
Regards
Theo
------------------------
Theo Bebekis
Thessaloniki, Greece
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