Eric Lemings wrote:
-----Original Message-----
From: Martin Sebor [mailto:[EMAIL PROTECTED] On Behalf Of Martin Sebor
Sent: Thursday, June 26, 2008 11:31 AM
To: [email protected]
Subject: Re: __rw_and (Was RE: Some internal aliases for
__rw_integral_constant?)
...
Maybe I misunderstood the purpose of the __rw_and template. I wasn't
asking to see a solution w/o variadic templates, but one without
__rw_and. If I understand your response correctly, you're proposing
to add __rw_and to hide the dual implementation of ANDing variable
numbers of constraints, one with variadic templates and the other
without. I.e., __rw_and would be more than just syntactic sugar.
Correct?
Yes, simplifying compile-time expressions by using it in either context
would be one reason, e.g.:
template <class T, int I>
struct S :
__rw_and<std::is_class<T>, 0 != I> {}; // explicit/finite
arguments
I can't say I understand this use case. What's S meant to represent?
It looks like some sort of a type trait. If so, why isn't plain &&
sufficient?
template <class T, int I>
struct S:
__rw_conditional<__rw_is_class<T>::value && 0 != I, T, ???>
{ };
(Or enable_if instead of conditional, depending on what you want
to do with S).
template <class... T> struct O {
template <class... U> struct I
: __rw_and<std::is_convertible<T, U>::value...> {}; //
variable arguments
Same here.
};
The other reason is portability. Case in point. Travis recently ran
into a problem where the compiler rejected a simple constant expression
like `sizeof (T) < sizeof (U)' but worked with the metafunction
equivalent of `__rw_less_than<sizeof (T), sizeof (U)>::value'.
This sounds like an argument for __rw_less_than, not for __rw_and.
Btw., I'm not opposed to __rw_and in principle. I just want to see
some of its uses and the rationale for it (in case there's a better
or simpler way of doing the same thing).
Martin
