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

19 Mar 2006


      "Paul Mensonides" wrote:
...
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.)
Thanks for the idea; the program compiles and works fine; it is easy to 
maintain since I have only the preprocessor data structure to keep updated 
when I add T classes. I am getting some type checking for instance if I add 
to the TYPES preprocessor data structure a type that does not exist. MPL and 
preprocesor library are great tools!
JCR

#define MY_TYPES (T0) (T1) (T2) (T3) (T4)
typedef boost::mpl::vector<BOOST_PP_SEQ_ENUM(MY_TYPES)> TTypes;
struct Functor
{
  void operator()(const int& i)
  {
    switch(i)
    {
      #define CASES(z, n, text) 
\
        case n: 
\
          vec.push_back(new boost::mpl::at<TTypes, boost::mpl::int_<n>
...
::type);  \
          break;
      BOOST_PP_REPEAT(BOOST_PP_SEQ_SIZE(MY_TYPES), CASES, ~);
    }
  }
};
"Paul Mensonides" <pmenso57@comcast.net> wrote in message 
news:20060319010923.420C61055A1@wowbagger.osl.iu.edu...
...
...
-----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<TTypes,
boost::mpl::int_<n>  >::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