Hi,
As some might know, I'm currently working on a reimplementation of the MPL
for C++11. In doing so, I came across a technique that I did not know of and
I thought I would share my discovery with the community. If this is already
well known, please excuse my ignorance.
The goal is to implement fast compile-time conjunction and disjunction.
It goes like this:
template void allow_expansion(T&&...);
template
struct or_ {
static constexpr bool value = !noexcept(
allow_expansion((T::value ? throw : 0)...)
);
};
That's it! Note that disjunction is easily obtained in a similar way. The
benefit of this technique over traditional implementations is that it's
several times faster according to my benchmarks.
However, you might have noticed that short-circuit evaluation is not
respected so the above technique is only suitable when short-circuit
evaluation is not required.
If you find this interesting, see [1] for a collection of similar techniques.
The one I posted above seems to be the fastest with Clang and GCC.
Regards,
Louis Dionne
[1] https://gist.github.com/ldionne/7522393
