RE: [boost] Meta function variants of mpl::apply_if neccesary?

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-09-19 16:45:57

Jaap Suter wrote:
> However, consider the following case:
> 
> apply_if< some_condition,
>                some_type_1,
>                plus< some_type_2::type, some_number >
>             >::type
> 
> In this case, I get the impression that my compiler (Intel) 
> evaluates the template parameters of plus, and thus of some_type_2::type, 
> even when it is not used. 

It does indeed.

[...]

> Thus, I provide the following indirection:
> 
> template< ... >
> struct recurse
> {
>     typedef plus< some_type_2::type, some_number > type
> };
> 
> apply_if< some_condition,
>                some_type_1,
>                recurse
>             >::type
> 
> And now it works, because the compiler no longer evaluates the
> some_type_2::type, if it is not needed.
> 
> Now, I have the following questions:
> 
> 1. Can you acknowledge this problem? The fact that the 
> compiler evaluates template parameters for non-used types, 
> and consequently that the mpl::apply_if rationale applies here 
> as well?

Yes.

> 2. Is this the best solution to solve this problem?

I've been inclining towards something like this:

    typedef apply_if<
          some_condition
        , some_type_1
        , lazy< plus< apply0<some_type_2>, some_number > >
        >::type f;

The parameter of lazy<...> template is basically a lambda expression, but
without any placeholders; I haven't got to implementing it yet, though.

> 
> 3. Maybe we can introduce the indirection in plus and other 
> meta-functions that act on integral constants directly. 

I am afraid it's not such a good idea as it might seem at the first sight -
please see my reply to Terje.

> Anyway, just some questions, comments and ideas....

Thanks for raising the issue - it definitely needs to be solved.

Aleksey