RE: [boost] PP lambda?

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2003-04-11 15:56:44

Paul Mensonides wrote:
> > Not bad at all. Can we somehow get rid of the extra commas and
> > have the "canonical" placeholder names, e.g.
> >
> > CHAOS_PP_EXPR(CHAOS_PP_REPEAT(
> >       5
> >     , ( CHAOS_PP_CAT_(_1, _0) = _0 )
> >     , int x
> >     ))
> 
> Not without making it worse.  Two fundamental problems exist here.  
> First, the "canonical" placeholders.  The only way that I could tell
> what they are is with concatenation.  That would disallow direct 
> input of operators, since token-pasting that produces multiple tokens
> is undefined behavior. 

OK, understand.

> (I'm currently working on a paper for the committee to explicitly 
> make this well-defined behavior, because it causes all sorts of 
> problems.  I can use the credentials of as many people as possible
> in this endeavour, because, after all, we're dealing with "extending"
> the preprocessor here.) I suppose I could define _1, _2, etc., but 
> that is not a good idea for obvious reasons.  The second problem with
> the above is that I can do absolutely nothing with the text between
> two operators (e.g. operators that are not commas or parentheses).  
> Such as the following:
> 
> + text +
> 
> I cannot even get at "text" at all in order to deal with it.

That's too bad. But suppose I am willing to spell all non-alphabetic 
tokens as macros, e.g. 

    BOOST_PP_PLUS text BOOST_PP_PLUS 

for your above example - would it change anything?

[snip]

> There are many things that are different.  For instance, the entire 
> library is implemented from the ground up to support variadics/
> placemarkers if using C99 (or C++ with those features).  This alters 
> the signatures a quite a few library primitives that use a data or 
> state parameter.  REPEAT is an example that doesn't have to change.
> 
> REPEAT(count, macro, data)
>                      ^^^^
> 
> In C99, this is:
> 
> REPEAT(count, macro, ...)
> 
> And the variadic parameters are passed straight through the mechanism 
> and dealt with directly on the other end--which eliminates the need 
> for excessive TUPLE_ELEM types of calls:
> 
> #define DECL(s, n, id, init, ...) \
>     __VA_ARGS__ id ## n = init; \
>     /**/
> 
> CHAOS_PP_EXPR(CHAOS_PP_REPEAT(
>     5, DECL, x, "Aleksey Gurtovoy", std::string<char, 
> my_traits<char> >
> ))
> 
> std::string<char, my_traits<char> > x0 = "Aleksey Gurtovoy";
> std::string<char, my_traits<char> > x1 = "Aleksey Gurtovoy";
> std::string<char, my_traits<char> > x2 = "Aleksey Gurtovoy";
> std::string<char, my_traits<char> > x3 = "Aleksey Gurtovoy";
> std::string<char, my_traits<char> > x4 = "Aleksey Gurtovoy";
> 
> ...as an example.

Very interesting!

Thanks,
Aleksey