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

2 Oct 2007


      Nat Goodspeed wrote:
...
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().doit();
      break;
    case circle << 8 | square:
      A().doit();
      break;
    case square << 8 | circle:
      A().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?
I think that was "Modern C++ Design" by Alexandrescu.

Jeff Flinn