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(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
