[Boost-users] [MPL] Runtime traversal of a type sequence until a condition is true

From: Scott Meyers (usenet_at_[hidden])
Date: 2007-05-23 00:52:30

I'm interested in taking Andrei Alexandrescu's Ad Hoc Visitor technique 
(http://tinyurl.com/2k8h2h) and implementing it using the MPL.  I need to 
implement a runtime function that iterates over an MPL type sequence until a 
dynamic_cast succeeds.  Here's some pseudocode, where I use "typelist" to mean 
an MPL type sequence:

class UnknownType { ... };                  // for exceptions

template <typename TL>                      // TL = "typelist"
void visit(ASTNode *pn)
{
   for (each type T in TL) {                 // try dynamic_
     if (T *pT = dynamic_cast<T*>(pn)) {     // casting to all
       v.process(pT);                        // the types in TL
       return;
   }

   throw UnknownType(typeid(*pn));           // throw an ex. if
}                                           // no type matches

I can make this work if I replace the for loop with a use of mpl::for_each, but 
for_each iterates over all elements of the type sequence, and I want to stop 
when a dynamic_cast succeeds.  I can hack the function object I pass to for_each 
to have it be a no-op once a successful dynamic_cast has been encountered, but 
the operative word there is "hack."

What I really want is something more like 
do_until<Sequence>(unaryFunctionObject) that will loop over all the entries in 
Sequence until the unaryFunctionObject returns true.  Is there something like 
this in the MPL, must I roll my own, or am I approaching this the wrong way 
entirely?

Thanks,

Scott