Re: [Boost-users] Using boost (maybe mpl?) to generalize a switch statement to call different template functions

From: Nat Goodspeed (nat_at_[hidden])
Date: 2007-10-02 10:59:35

Peng Yu wrote:

> I have the following program, which uses a switch statement to call
> different template functions. The example is simple, but as the number
> of shapes increase, the switch statement might be very hard to
> maintain.
> 
> I'm looking for a more general solution (maybe using mpl, not sure),
> such that the switch statement can be automatically generated,
> therefore, more maintainable.
> 
> void f(shape s1, shape s2) {
> // each defined shape-combination corresponds to one case
> // I want to generalize the code by using boost::mpl
> // (or some other packages if applicable)
>   switch(s1 << 8 | s2) {
>     case circle << 8 | circle:
>       A<circle, circle>().doit();
>       break;
>     case circle << 8 | square:
>       A<circle, square>().doit();
>       break;
>     case square << 8 | circle:
>       A<square, circle>().doit();
>       break;
>     default:
>       std::cout << "not defined" << std::endl;
>   }
> }
> 
> int main() {
>   f(circle, circle);
>   f(circle, square);
>   f(square, circle);
>   f(square, square);
> }

I believe this is essentially a double-dispatch problem, though you're 
using enum values rather than distinct subclass types. Scott Meyers 
talks about double dispatch (in More Effective C++? I don't have it with 
me). At that point he was advocating a map of target functions, keyed by 
type pairs.

Does MPL give us a more robust way to address multiple dispatch?