Article #16668: Typecasting

 Question and Answer Database
FAQ1668D.txt Typecasting
Category :Object Pascal
Platform :All
Product :All 32 bit
Question:
Many Windows functions claim to want PChar parameters in the
documentation, but they are defined as requiring LongInts. Is
this a bug?
Answer:
No, this is where "typecasting" is used. Typecasting allows you
to fool the compiler into thinking that one type of variable is
of another type for the ultimate in flexibility. The last
parameter of the Windows API function SendMessage() is a good
example. It is documented as requiring a long integer, but
commonly requires a PChar for certain messages
(i.e. WM_WININICHANGE). Generally, the variable you are
typecasting from must be the same size as the variable type you
are casting it to. In the SendMessage example, you could
typecast a PChar as a longint, since both occupy 4 bytes
of memory:
Example:
var
s : array[0..64] of char;
begin
StrCopy(S, 'windows');
SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, LongInt(@S));
end;
Pointers are the easiest to typecast, and are the most flexible
since you can pass anything to the called procedure. They are
also the most dangerous for the same reason. You can also use
untyped variable parameters for convenience, although var
parameters are really pointers behind the scenes.
Example:
type
PBigArray = ^TBigArray;
TBigArray = Array[0..65000] of char;
procedure ZeroOutPointerVariable(P : pointer; size : integer);
var
i : integer;
begin
for i := 0 to size — 1 do
PBigArray(p)^[i] := #0;
end;
procedure ZeroOutUntypedVariable(var v, size : integer);
var
i : integer;
begin
for i := 0 to size — 1 do
TBigArray(v)[i] := #0;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
s : string[255];
i : integer;
l : longint;
begin
s := 'Hello';
i := 10;
l := 2000;
ZeroOutPointerVariable(@s, sizeof(s));
ZeroOutPointerVariable(@i, sizeof(i));
ZeroOutPointerVariable(@l, sizeof(l));
s := 'Hello';
i := 10;
l := 2000;
ZeroOutUntypedVariable(s, sizeof(s));
ZeroOutUntypedVariable(i, sizeof(i));
ZeroOutUntypedVariable(l, sizeof(l));
end;
7/16/98 4:31:28 PM

Last Modified: 01-SEP-99