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