Re: [Boost-users] [MPL] Runtime traversal of a type sequence untilacondition is true

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-05-23 02:33:53

Scott Meyers ha escrito:

> 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?

AFAIK you must roll your own. Please find attached a possible implementation
using mpl::fold.

HTH,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo