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

From: François Duranleau (duranlef_at_[hidden])
Date: 2006-03-17 00:37:48

On Thu, 16 Mar 2006, John Christopher wrote:

>  void operator()(const int& i)
>  {
>    switch(i)
>    {
>      case 0:
>        vec.push_back(new T0);
>        break;
>      case 1:
>        vec.push_back(new T1);
>        break;
>    }
>    // I'd like to simplify the above swtich statement by writing something
> like:
>    vec.push_back(new boost::mpl::at<s,i>::type);
>    // but it does not compile and MinGW returns:
>    // error: i cannot appear in a constant expression
>    // error: template argument 2 is invalid.
>  }

Of course it doesn't compile, because non-type template arguments must be 
constants known at compile time. However here, the parameter i isn't, and 
thus cannot be used as a template argument. It would seem like you are 
stuck with a switch, unless you create something like an array of 
generating function, e.g.:

template < typename T >
Tbase* genT() { return new T ; }

typedef Tbase* (* genT_type)() ;
genT_type generators[] = { & genT< T0 > , & genT< T1 > } ;

//...
     void operator () ( const int i )
     {
         vec.push_back( generators[ i ]() ) ;
     }

It won't be as efficient as the switch though because of the function call 
through a function pointer.

-- 
François Duranleau
LIGUM, Université de Montréal
"A person's truth is so simple that most ignore it to concentrate on what
  they think are deeper truths."
                                           - from _Neon Genesis Evangelion_