Re: [boost] [preprocessor] Warning: Incoming

Subject: Re: [boost] [preprocessor] Warning: Incoming
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-07-01 16:51:38

On 07/01/2011 08:09 AM, Paul Mensonides wrote:

> For better variadic support, what's needed most are a few low-level
> macros such as IS_VARIADIC (small change) and a variadic/placemarker
> sequence implementation (large, though possibly not difficult, change).
> This notion of a, b, c being a good way to store elements (which are
> possibly empty sequences of preprocessing tokens and whitespace
> separations) needs to die.  I was just watching some of the Boostcon
> videos and in one of them (something like Haskell = C++ TMP) there are
> examples that utilize variadic templates to pass around lists of types.
> However, in the example, they are "open".  I.e. not bounded by something
> that collects them as a singular entity.  I don't have it in front of me,
> but something like:
>
> template<class, class... T>  struct count {
>      enum { value = 1 + count<T...>::value };
> };
>
> template<>  struct count<>  {
>      enum { value = 0 };
> };
>
> ...but this is terrible.  It doesn't take away from the point of the
> talk, but it should be something like count<typelist<A, B, C>>, not
> count<A, B, C>.

Yes, if I remember well Dave made a comment like that during the talk 
and I think they agreed it would probably be a better mapping but that 
he wanted to keep things simple for the presentation.

When you try to map functions to meta-functions, it's best if a single 
"list" argument stays a single "typelist" argument as well.

Otherwise you have obvious problems when you want to pass multiple lists 
to a function.
There are a couple of languages, though, where calling a function with a 
list of N arguments is the same as calling it with N arguments, and 
there is no way to pass two distinct lists within using a delimiter 
argument. CMake is an example of this.