Re: [boost] "Simple C++11 metaprogramming"

Subject: Re: [boost] "Simple C++11 metaprogramming"
From: Peter Dimov (lists_at_[hidden])
Date: 2015-05-31 08:00:20

Vicente J. Botet Escriba wrote:
> Your rational to use a prefix mp_ let me perplex. Does it mean that 
> namespaces have not reached its goal?

Namespaces work well most of the time, but metaprogramming is a bit of a 
special case, because many of the identifiers we'd like to use in a 
metaprogramming library are either keywords (if, int, false) or already 
exist in namespace std (list, fill). The former forces us to add the suffix 
_ at times, the latter has been known to create problems via 
argument-dependent lookup.

It's of course possible to use a namespace, meta:: as in Eric's library, or 
perhaps mp::, but I don't think that it buys us much.

> IMO, both mp_append_impl and mp_append are useful. Do you think that we 
> don't need any more the functionality of the mp_append_impl in c++1/c++14? 
> If you considered it still useful, maybe you can follow the standard 
> naming convention mp_append and mp_append_t.

The point I am making by not using the standard _t convention is that in 
C++11 aliases should be our primary primitive. The origin of the _t 
convention is that the ordinary non-_t traits were there first because there 
was no other way, and then the _t convenience aliases were added as an 
afterthought. This is a historical artifact. A C++11 design that does not 
have its origins in C++03 could and should choose the non-_t name for the 
alias.

> The use of mp_list<> in mp_append_impl when there are 0 arguments is 
> weird. Just wondering if adding a template <class ...> class R argument 
> would make everything clearer.

The "idiomatic" way of making mp_append use a specific R is mp_append<R<>, 
...>, which is not that much harder to use than the hypothetical 
mp_append<R, ...>.

As a general rule, I prefer to keep functions homogeneous if possible.