[boost] Re: Preprocessor Library

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-04-10 17:01:48

David Abrahams wrote:
> 1. I need to pre-generate a large number of repetitions for most EDG
> compilers, which it seems basically can't handle any PP lib constructs,
> making me wonder how much the PP lib is really buying me.

It still frees you from going through N (consequently more complex, but
essentially the same) copies of code when its interface/implementation
changes.

> 2. It moves the generated examples far from the PP code which they help
> to document.

It's true. However, I find that after you become familiar with the library,
you don't really need to see the expanded version in oder to understand what
is being produced by generator macros. You often _do_ need to see the result
of the expansion in order to fix typos etc., and here the expanded headers
are a big help, but again, personally, when doing that, I don't find it a
problem that the generator and the expanded code I am debugging lie in
separate files.

> 3. I still don't have a good way to quickly clean up the output. I think
> it would be fairly trivial to implement something, but that's an extra
> step I haven't taken yet.

That's the part I am still struggling with as well. After looking at all C++
code beautifiers I could find, I am inclined to just sit and write a
100-lines python program that would do what I want :).

> 4. Things aren't so simple when there conditional compilation to work
> around compiler bugs is involved.

Yep. I don't have real experience of dealing with this issue, but I was
thinking about something along these lines:

#if !defined(BOOST_MY_LIBRARY_GENERATOR_MODE)

#   define BOOST_MY_LIBRARY_HEADER \
    BOOST_PP_STRINGIZE(
        BOOST_PP_CAT( \
          BOOST_PP_CAT(
              my_library/preprocessed/my_header \
            , BOOST_MY_LIBRARY_COMPILER \
            ) \
        , .hpp) \
        ) \
/**/

#   include BOOST_MY_LIBRARY_HEADER
#   define BOOST_MY_LIBRARY_REPETITION_START 5
#   define BOOST_MY_LIBRARY_REPETITION_END 15
#else
#   include
#   define BOOST_MY_LIBRARY_REPETITION_START 0
#   define BOOST_MY_LIBRARY_REPETITION_END 5
#endif

#if ( defined(BOOST_MY_LIBRARY_COMPILER) \
    && BOOST_MY_LIBRARY_COMPILER == BOOST_MY_LIBRARY_COMPILER_MSVC65 ) \
    || ( defined(BOOST_MSVC) && BOOST_MSVC < 1300 )

#   define AUX_MY_CONSTRUCT_GEN(i) \
template< BOOST_MPL_PP_PARAMS(0,i, typename T) > \
struct my_construct { /* more stuff */ }; \
/**/

#else
#   define AUX_MY_CONSTRUCT_GEN(i) \
...
/**/
#endif

Here, to produce a version of expanded headers for, let's say, MSVC, you
invoke the compiler in preprocessing-only mode with two macros defined -
BOOST_MY_LIBRARY_GENERATOR_MODE and BOOST_MY_LIBRARY_COMPILER ==
BOOST_MY_LIBRARY_COMPILER_MSVC65. The compiler you use for generation
doesn't have to be the same as the one you produce the headers for.

Aleksey