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

From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2006-03-18 20:12:20

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] 
> [mailto:boost-users-bounces_at_[hidden]] On Behalf Of 
> John Christopher
> Sent: Saturday, March 18, 2006 8:26 AM
> To: boost-users_at_[hidden]
> 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