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

From: Jeff Flinn (TriumphSprint2000_at_[hidden])
Date: 2007-10-02 13:43:56

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<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?

I think that was "Modern C++ Design" by Alexandrescu.

Jeff Flinn