Re: [Boost-users] MPL to get rid of cumbersome switch

19 Mar 2006


      
...
-----Original Message-----
From: boost-users-bounces@lists.boost.org 
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of 
John Christopher
Sent: Saturday, March 18, 2006 8:26 AM
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] MPL to get rid of cumbersome switch
...
Given its prepetitive nature you could use boost 
preprocessor library 
to generate this switch...
That's a good idea.
I could define
#define NUMBER_CASES 4
and the switch would become something like that:
    switch(i)
    {
      #define CASES(z, n, text)
\
        case n: 
\
          vec.push_back(new boost::mpl::at::type);  \
          break;
      BOOST_PP_REPEAT(NUMBER_CASES, CASES, ~);
    }
this compiles and works fine, but, since the number of cases 
in the switch must equal the size of the vector of type, I'd 
rather write something like:
#define NUMBER_CASES boost::mpl::size<TTypes>::value
but this does not compile and I am getting the error
Yeah, that won't work because the preprocessor cannot evaluate
'boost::mpl::size<TTypes>::value'.  If you're going to use the preprocessor for
this, you need to store the types in a preprocessor data structure, not an
mpl::vector.  (Note, BTW, than an MPL vector can be trivial created from a
preprocessor data structure.)

Regards,
Paul Mensonides