Assuming I'm correct and that Andrei's use cases don't make the case for static if - Is there a real case for static if.
Yes. Here's an example:
template <typename Iterator> void advance(Iterator it, size_t n) { static_if(iterator_category<it>::type == random_access_category_tag) { it += n; } else { for (size_t i = 0; i < n; ++i) ++it; } }
Were one to use a runtime if instead, and instantiate the function with an iterator that is not random access, one would get a compiler error of the form 'no match for operator+=(Iterator, size_t)'.
The idea is that with a runtime if, the compiler type-checks the unused branch even if it ends up eliminating it in later on in the optimization phase.
Well, this seems like a better example, but ...
the following wouldn't compile anyway as if can't compare types.
if(iterator_category<it>::type == random_access_category_tag)
You're right, of course. It should have been something like is_same<iterator_category<Iterator>::type, random_access_category_tag>::value
Anyway, one could right right now:
if(is_random_access_iterator<Iterator>::value){ it += n; } else{ for(size_ti = 0; i < n; ++i){ ++it; }
IF the compiler skipped the "false" branch. I would guess this behavior is undefined (but maybe not). So maybe everything would be just fine if behavior in this case were defined to skip branches known to be false at compile time.
I'm not sure whether compilers are required by the standard to type-check branches known not to be taken at compile time, but in practice I think they do because type-checking is done at an earlier stage of compilation than optimizations like constant propagation. Regards, Nate