"David Abrahams" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> [...]
> Borland is particularly strange about layout.
Suprisingly enough, BCB 5 has maintained the optimal size throughout
all the changes.
> [...]
> Whaaa? A temporary that never gets created?
Well, I'll show you the code, and you tell me if it gets optimized away
or not:
// Inherit first
template <class T1, class T2>
class optimal_parents1 : public T1
{
// ...
template <class U1, class U2>
optimal_parents1(optimal_parents1<U1, U2> const& rhs)
: T1(static_cast<U1 const&>(rhs))
{ U2 u2; T2 t2(u2); } // Here
// ...
};
If T2(U2 const&) is empty, I would think that it wouldn't bother to
call it in the first place. Am I missing something? Maybe I shouldn't
have called t2 a temporary, since it is an lvalue. I just meant that
it's not really used for anything other than making sure that U2 can
be converted to T2.
> [...]
> Maybe you should illustrate what you mean, 'cause I'm lost.
assert_check(no_check const&)
{ }
This allows you to create a pointer with stricter checking from a
pointer with no checking. But no_check has no such conversion
c'tors, because presumably, one does not want to *reduce* the
level of checking. Since the checking policies generally don't have
any state, the c'tors are all trivial, and hopefully, will get optimized
out of existence. They merely serve as a compile-time check of
compatibility.
> [...]
> I don't think I can do any better than you. I have the same tools at
> my disposal. I'd do a bunch of experiments and look at the
> resulting class layouts by checking the size and the relative addresses
> of members.
I was afraid it was going to come down to that.
> [...]
> You could probably figure out the VC++ algorithm by
> experimentation, but that still wouldn't help a user trying to write
> portable, efficient code, unless you find a workaround for every
> other compiler.
Yeah, that's the problem. :( Even more disturbingly, it's not clear if
this is just an MI problem, or a problem with inheritance in general
with VC++. I mean, when I tried the chained policy implementation,
it did provide the right size, but that doesn't mean it won't change if
I update the implementation (and that version would require a lot of
changes to get it up to speed with the MI version).
Dave
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