RE: [boost] Re: A couple of issues in submitting typelists to boo st

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2001-11-20 12:53:13

Andrei Alexandrescu wrote:
> You must be referring to the TYPELIST_nn macros.
> 
> It's very simple. I don't give a rat's ass on the preprocessor and 
> I'm fully aware of its vandalism. It's just that the set of 
> requirements that I had for typelist "constructors" left the 
> preprocessor as the only contender:
> 
> 1. easy to use
> 2. scaleable up to more parameters
> 
> (We can discuss separately whether or not these were good 
> criteria.)
> 
> So I put together the nasty TYPELIST_1 to TYPELIST_50 and 
> used them. I didn't notice name clashes etc., but I did notice an 
> important shortcoming:
> 
> 3. If you use a comma inside a macro, it won't work. Consider:
> 
> typedef TYPELIST_2(vector<int, myallocator>, vector<int, 
> yourallocator>)
>     TwoVectors;
> 
> that won't work because the preprocessor strongly believes 
> that you actually passed four arguments to TYPELIST_2. What 
> a nuisance. You can fix that by putting in place typedefs for 
> those types.
> 
> So it's a tradeoff issue. Any solution that fixes point 3 
> fails on point 1, point 2, or (most often) both 1 and 2.

The below is copy & paste from "boost/mpl/test.cpp":

    typedef mpl::type_list<int,char,long,short,char,long,double,long> types;
    typedef mpl::find<types, short> types_find;
    BOOST_MPL_ASSERT_IS_SAME(types_find::iterator::type, short);

You can have as much as 128 parameters in a type_list :). Of course, I
cheated and used the Vesa Karvonen's PREPROCESSOR library internally :), but
before that I had my own stuff that did pretty much the same thing, so yes,
it's possible to satisfy all three criteria.

--
Aleksey