Re: [boost] [hana] some notes, not a review

Subject: Re: [boost] [hana] some notes, not a review
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2015-06-19 09:28:54

Robert Ramey <ramey <at> rrsd.com> writes:

> 
> On 6/19/15 1:40 AM, Mathias Gaunard wrote:
> > What template meta-programming were you able to replace by constexpr?
> >
> > As far as I'm concerned, I've yet to see a use case where constexpr is
> > anything but an optimization hint.
> 
> Hmmm - what about all the mpl arithmetic functions built around mpl 
> integral constants?  All that gets replace by easy to understand compile 
> time expressions which can be used in other template expressions?  This 
> simplifies things immensely.

As far as my current experience goes, this is one of the only use cases where
constexpr can help (but it does help a lot in that case). You can replace 
complex arithmetic with equivalent constexpr code and it's going to be much
simpler to read and also more efficient at compile-time.

It's also possible to use constexpr to do some index calculations as an
implementation detail of an heterogeneous algorithm. For example, to compute
the cartesian product of tuples, I actually generate the proper sequence of 
indices using constexpr computations, and then index the sequences with these 
indices, getting back a tuple of tuples. But that's hardly useful outside of 
deep TMP algorithms.

Louis