Subject: Re: [boost] "Simple C++11 metaprogramming"
From: Peter Dimov (lists_at_[hidden])
Date: 2015-06-01 19:55:24
Bruno Dutra wrote:
> The need for so called metafunction-classes arises most naturally as soon
> as one decides to write a metafunction that returns another metafunction.
> As pointed out by Vincente Escriba, template aliases can't allow for an
> indistinguishable handling of metafunctions and variables (that is types),
> for the obvious reason they can't match template type parameters.
I actually know all that. That's kind of the point.
You can't just say "we need higher-order metaprogramming to return
metafunctions from metafunctions" - this is a tautology. This is what
"higher-order metaprogramming" means. You're basically saying that we need
higher-order metaprogramming to do higher-order metaprogramming. True but
trivial.
There obviously do exist occasions that call for higher-order
metaprogramming. The question is can we get by in 97% of the cases without
it. Not whether it's useful, but whether it's indispensable. Whether there's
a room for a "simple" metaprogramming library that doesn't provide
higher-order constructs and is therefore based on template aliases and not
on metafunction classes, and whether such a library can be adequately useful
for real world tasks. (I'm open to the possibility that the answer is "no",
but I'd like it to be "yes".)