Re: boost::bind and the preprocessor

From: vesa_karvonen (vesa_karvonen_at_[hidden])
Date: 2002-03-08 10:11:50

--- In boost_at_y..., "David Abrahams" <david.abrahams_at_r...> wrote:
> From: "vesa_karvonen" <vesa_karvonen_at_h...>
> > hmm... I've tried to use the same one character names for every
> > macro parameter used for the same purpose in the library, but
> > there are still many exceptions to the rules. I'll try to fix
> > those exceptions and add a separate page that explicitly explains
> > the naming of parameters.
> 
> I'm sorry; I don't see how that's relevant.

Don't worry about it :). I'm just trying to find ways to minimize the 
amount of documentation needed while making the documentation 
accessible.

> My point was that the user needs to choose something to be passed
> for R when she invokes BOOST_PP_LIST_FOR_EACH_I(). In general,
> almost anything will do.

BOOST_PP_LIST_FOR_EACH_I(F,P,L) takes 3 parameters none of which is 
named R. This is why I explained the difference between the generated 
R and D parameters and the user specified P parameter.

The root cause for all this confusion is really the design of the C 
preprocessor which does not allow recursion. The technique of passing 
around explicit recursion depths (R and D parameters) is a technique 
for overcoming this limitation of the C preprocessor.

> > > You could look at boost/python/converter/from_python_data.hpp
> > > for what I ended up with.
[...]
> > I think that it should be possible to achieve the same effect
> > using only MPL, which might be preferable.
> 
> Trust me, the PP library was a last resort. It was too slow with
> various EDG compilers when I just used template metaprogramming.

That is quite interesting. Are you really saying that iteration using 
BOOST_PP_LIST_FOR_EACH_I() in this case is faster than when using 
boost::mpl::accumulate<>? The template metaprogramming version 
potentially does the iteration many more times than the pp 
metaprogramming version, but still it sounds quite odd. I suppose 
that with precompiled headers, the pp metaprogramming version should 
clearly be faster, because the iteration is then "unrolled".