Re: [boost] Compile Time String in C++14

Subject: Re: [boost] Compile Time String in C++14
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2014-08-25 12:01:30

TONGARI J <tongari95 <at> gmail.com> writes:

> 
> Hi all,
> 
> I found a way to make CT strings in C++14, with the help of generic
> lambda.
> 
> FYI, Ábel Sinkovics has presented a c++11 CT string in his
> mpllibs::metaparse[1] lib before, but it requires some PP trick which has
> a
> predefined limit MPLLIBS_LIMIT_STRING_SIZE.
> 
> The new trick doesn't suffer from the limitation, but it assumes a
> null-terminated string.
> It's fairly simple, see below:
> 
> ```c++
>     template<char...>
>     struct str {};
> 
>     template<char c>
>     struct ch {};
> 
>     template<int n>
>     using idx = std::integral_constant<int, n>;
> 
>     template<class S, char c, char... cs>
>     auto accum(S s, ch<c>, str<cs...>)
>     {
>         return accum(s, s(idx<sizeof...(cs) + 1>()), str<cs..., c>());
>     }
> 
>     template<class S, char... cs>
>     auto accum(S, ch<0>, str<cs...> ret)
>     {
>         return ret;
>     }
> 
>     template<class S>
>     auto make_str(S s)
>     {
>         return accum(s, s(idx<0>()), str<>());
>     }
> 
>     #define LIT(s) make_str([](auto idx) { return ch<s[idx.value]>(); })
> ```
> 
> Then you can use `doSomething(LIT("abc"))` in your code.
> What do you think?

This is genius, thanks a lot for posting. I've been searching for a
way to do this to implement compile-time strings in Boost.Hana[1].

Louis

[1]: https://github.com/ldionne/hana

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