Re: [boost] a (useful?) crtp trick

Subject: Re: [boost] a (useful?) crtp trick
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2012-08-27 04:28:16

On 24.08.2012 19:50, pavel wrote:
> hi all
> i post this in the hope someone finds this technique useful
>
> to make long story short, here is a snippet:
>
>     template<typename t>  //tag deducing util
>     struct base_tag
>     {
>       typedef decltype(get_tag((t*)42)) type;
>     };
>   
>     template<typename type,
>              typename tag = typename base_tag<type>::type>
>     struct Base  //crtp base class
>     {
>       friend tag get_tag(type*);  //helper declaration, never defined
>     };
>   
>     template<typename type>  //no mention of tag
>     void f(const Base<type>&)  //tag is deduced automatically
>     {}
>
I think this only happens to work by accident, because compilers tend to 
defer instantiation of function templates until the end of the 
translation unit, when a proper instantiation of Base is available. But 
without a compiler that doesn't I don't think I can come up with code 
that shows this.

In any case, this makes *my* head hurt, and that's saying something.

Sebastian