Re: [boost] [static_if] Is there interest in a `static if` emulation library?

18 Sep 2014

      Le 18/09/14 00:28, lcaminiti a écrit :
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On 08 Sep 2014, at 6:50, Lorenzo Caminiti <
lorcaminiti@
> wrote:
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1. Why is not possible to use enabled_if with template variables?
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enable_if only works in conjunction with SFINAE.
Look at that line of code of yours. First, think about what this line is:
n<1>;
It’s not an explicit template instantiation, even though it may look like
that. It’s an expression statement, containing a single declaration
reference expression, referencing the variable template instantiation
n<1>.
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I hope issue #2 has already been sufficiently answered by the others.
I understand. Thanks to you and everyone else for the explanations. It seems
Sebastian Redl wrote
the best (and possibly only) way to disable data member declarations based
on compile-time constants is via inheritance after all.
Given that:
1. Disabling data members with a static-if library seems not possible;
2. And, based on all critiques in N3613, we will likely never have language
support for static-if.
I will reiterate my original question:
Is there interest in a Boost.StaticIf library to do something like the
following?
template< typename Iter, typename Dist >
void myadvance ( Iter& i, Dist n ) {
     Iter* p = &i;
     static_if::value>(
         std::bind([ ] ( auto p, auto n ) {
             *p += n;
         }, p, n)
     ).template elif::value>(
         std::bind([ ] ( auto p, auto n ) {
             if(n >= 0) while(n--) ++*p;
             else while(n++) --*p;
         }, p, n)
     ).template elif::value>(
         std::bind([ ] ( auto p, auto n ) {
             while(n--) ++*p;
         }, p, n)
     ).else_(
         std::bind([ ] ( auto false_ ) {
             static_assert(false_, "requires InputIterator<Iter>");
         }, std::false_type())
     );
}
Yes, I think this is an interesting addition to Boost.

Lorenzo, as the scope of your static_if emulation is restricted to void 
expressions would you mind to rename it, e.g.  call_if?

Thanks for you work,
Vicente

Re: [boost] [static_if] Is there interest in a `static if` emulation library?

Vicente J. Botet Escriba