On Friday, 17 August 2012 at 04:17:05 UTC, Jonathan M Davis wrote:
On Friday, August 17, 2012 05:11:49 Mehrdad wrote:
On Friday, 17 August 2012 at 02:49:45 UTC, Jonathan M Davis
wrote:
> But take this code for example:
>
> auto i = new int;
> *i = 5;
> const c = i;
> writeln(c);
> func(c); //obviously takes const or it wouldn't compile
> writeln(c);
>
> The compiler _knows_ that c is the same before and after the
> call to func, because it knows that no other references to
> that
> data can exist.
Is there any reason why your example didn't just say
> const(int*) c = null;
> writeln(c);
> func(c);
> writeln(c);
i.e. What was the point of 'i' there?
And why can't a C++ compiler do the same thing?
'c' is a const object, so if C++ code was to modify it, it
would
be undefined behavior, just like in D.
Sorry, I'm a little confused at what you were illustrating
here.
1. Because it wasn't creating as const, C++ could legally
mutate the object in
func by casting away const (meaning that it can't assume that c
is unchanged
after the call to func), which is not the case in D. But if it
were created as
const, then it would undefined behavior in both languages.
2. If you want to assign an actual value to c rather than null,
you either
need to use a helper function or create a mutable one first,
because there's no
do something like
const int* c = new int(5);
and have c point to an int with value 5.
So, with my example, the D code can guarantee that func doesn't
modify either
c or what's pointed to by c, whereas C++ provides no such
guarantee. So, any
optimizations which could be done based on the fact that func
didn't change
C's value can be done in D but not C++.
- Jonathan M Davis
Oh, so you're talking about the value of 'i' /after/ all that
code has executed... I kinda got lost in there because you didn't
seem to use 'i' afterwards.
Interesting, that's a great example I think. It might be worth
putting on the website somewhere, especially because it clearly
shows that the compiler doesn't need the source of func to infer
that 'i' won't be changed.
Cool, thanks a bunch, as always! :)
