On Fri, 7 May 2010, spir ☣ wrote:
On Fri, 7 May 2010 06:10:30 +0200
cobines <cobi...@gmail.com> wrote:
2010/5/6 spir ☣ <denis.s...@gmail.com>:
(By the way, started playing with TFPList already, and could not find how to
get data back! I mean the symtric of add(). Even tried indexing (who knows,
with the syntactic magic of modern language? ;-).)
It is indexing.
var
l: TFPList;
p: Pointer;
index: Integer;
begin
l := TFPList.Create;
index := l.Add(p);
p := l[index];
end;
[...]
Thank you, I must have written a typo or messed up indices, since now it works
fine. Still, remains a mystery about untyped pointers, illustrated by the code
below:
var
l : TFPList;
i : Integer;
p : ^Integer;
begin
l := TFPList.Create;
i := 1 ; new(p) ; p^ := i; l.Add(p);
p := l[0] ; writeln(p^);
writeln(l[0]^); // error
end.
I cannot directly "fish" and use the pointer's target. Seems I always need to
pass _via_ the pointer, then all is fine. It's not really annoying. But I don't
understand the difference: in code, both versions are indeed strictly equivalent. I guess
there's an implicit (pointer?) conversion somewhere?
Are there cases where the pointer's mediation is not needed?
L[0] is an untyped pointer. Dereferencing it gives you a memory address,
but not a type. So the compiler doesn't know what to do with it.
p is a pointer to an integer. When dereferencing it, the compiler knows that
at the address an integer is waiting, and it knows how to deal with
integers.
Adding a typed pointer (p) to an untyped pointer list does not store
the type information in the list, any type information available is
lost: the compiler or the list do not remember that you stored a pointer
to an integer. They just remember the address, not the type.
Michael.
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