Hello,
I wrote a little simulation prog to try and understand better the semantics of
var parameters (see below code and output).
It seems they behave like if passed via pointers, but also locally _operated_
via pointers. Meaning there is in ChangeVar no real local variable n (on the
stack). But instead Pascal silently behaves like if using p^ for both getting
and setting the variable ('s value) "on-place". This is illustrated by the
difference of result between the simulating funcs changePtrVar &
changePtrDirect.
Is this interpretation more or less correct?
How are var parameters actually implemented?
Maybe there is still a local var, but the behaviour rather looks like the
following code:
procedure change(p:IntPointer);
var
n : Integer;
begin
n := p^; // <=> var parameter passing
p^ := n+1; // <=> n := n+1 but:
// setting silently reuses p
end;
======= code =======
var
n,m : Integer;
p : IntPointer;
procedure changeVar(var n:Integer);
begin
n := n+1;
end;
procedure changePtrVar(p:IntPointer);
var
n : Integer;
begin
n := p^;
n := n+1;
end;
procedure changePtrDirect(p:IntPointer);
begin
p^ := p^+1;
end;
begin
// change var
n := 3;
changeVar(n);
writeln(n);
// change ptr var
n := 3;
p := @n;
changePtrVar(p);
writeln(n);
// change ptr direct
n := 3;
p := @n;
changePtrDirect(p);
writeln(n);
end.
======================
======= output =======
4
3
4
======================
Denis
PS: Is there a reason why I don't receive my own posts? (this only happens to
me on fpc and lazarus mailing lists).
________________________________
vit esse estrany ☣
spir.wikidot.com
_______________________________________________
fpc-pascal maillist - fpc-pascal@lists.freepascal.org
http://lists.freepascal.org/mailman/listinfo/fpc-pascal
