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

From: David Abrahams (dave_at_[hidden])
Date: 2006-03-17 01:05:00

François Duranleau <duranlef_at_[hidden]> writes:

> 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.

Chapter 11 of "C++ Template Metaprogramming" shows how to generate the
equivalent of a switch statement using the MPL.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com