Re: [boost] [metaparse] performance comparisons?

Subject: Re: [boost] [metaparse] performance comparisons?
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2015-06-02 08:54:15

Peter Dimov <lists <at> pdimov.com> writes:

> 
> Louis Dionne wrote:
> > `constexpr` char arrays and character packs are not equivalent. The 
> > problem is that constexpr-ness is stripped away by argument passing. For 
> > illustration, consider the following example:
> >
> >     template <typename CompileTimeString>
> >     void f(CompileTimeString s) {
> >         static_assert(s == "abc", "");
> >     }
> >
> >     constexpr std::string_literal<n> n4121 = "abc";
> >     constexpr auto n4236 = "abc"_s;
> >
> >     f(n4121); // error: s is not constexpr within the body of f
> >     f(n4236); // ok: the value of s is contained in its type
> 
> I assume that you mean for 'f' to be constexpr? 

No, it does not change a thing. It was on purpose that I did _not_ make it 
constexpr, in order not to mix concerns. What we're interested in is the 
constexpr-ness of `s` within the body of `f`, whether `f` is constexpr or 
not does not change anything. The above scenario is equivalent to something 
I presented in my C++Now talk [1]. The same concept is also explained in 
Hana's tutorial, in the "Advanced constexpr" section [2].

It seems that anyone trying to design something constexpr for the standard 
should have a solid grasp of those concepts, or we may very well end up 
missing the target of offering a useful compile-time string facility in 
C++17.

Regards,
Louis

[1]: http://ldionne.com/hana-cppnow-2015/#/19/2
[2]: http://ldionne.com/hana/#tutorial-appendix-constexpr