Daniel Wallin ha escrito:

Joaquín Mª López Muñoz wrote:

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Scott Meyers ha escrito:

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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.

This is unnecessarily inefficient. It's pretty easy to just use iterators, something like:

[...] So nice! Yep, this is much clearer, I guess I've got a case of fold-obsession :) Best, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo

Scott Meyers wrote: <...>

This is passing a runtime parameter to f, but what I want to pass to f is a compile-time type -- the type to dynamically_cast to. The pointer I'm casting is fixed, so I can bind it into f when I create f. Am I just missing something?

Here is a modified version of Daniel's code to suit the case from your original post (seems it wasn't intended to be a complete solution but rather a tutorial on how to implement the recursively inlined iteration with the MPL). Regards, Tobias #include <iostream> #include #include #include #include namespace mpl = boost::mpl; // a toy model namespace ast { class node { public: virtual ~node() { }; }; class expression_node : public node { }; class literal : public expression_node { }; class operator_node : public node { }; class unary_operator : public operator_node { }; class binary_operator : public operator_node { }; class statement_node : public node { }; class for_loop : public statement_node { }; } // a toy visitor struct ast_visitor { typedef mpl::vector accepted_types; void operator()(ast::expression_node &) { std::cout << "got an expression_node" << std::endl; } void operator()(ast::operator_node &) { std::cout << "got an operator_node" << std::endl; } }; // visit facility namespace detail { template bool visit(Last, Last, ast::node *, Visitor &) { throw "bummer"; } template bool visit(First, Last, ast::node * node, Visitor & v) { typedef typename mpl::deref<First>::type type; type * typed_node = dynamic_cast(node); if (! typed_node) detail::visit(typename mpl::next<First>::type(), Last(), node, v); else v(*typed_node); } } template<class Visitor> bool visit(ast::node * node, Visitor & with_visitor) { typedef typename Visitor::accepted_types seq; return detail::visit( typename mpl::begin<seq>::type() , typename mpl::end<seq>::type() , node, with_visitor ); } int main() { ast_visitor v; { ast::literal test; visit(& test, v); } { ast::unary_operator test; visit(& test, v); } { ast::binary_operator test; visit(& test, v); } { try { ast::for_loop test; visit(& test, v); } catch(...) { std::cout << "error handling works" << std::endl; } } return 0; }

Scott Meyers wrote:

Daniel Wallin wrote:

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This is unnecessarily inefficient. It's pretty easy to just use iterators, something like:

template bool do_until_impl(First, Last, F f) { boost::value_initialized x;

Does this not require that mpl::deref<First>::type be default-constructable? Presumably this is the first type in my type sequence, and since I'm dynamic_casting to all types in the sequence, it's not unreasonable to assume that the type I'm casting to may be abstract. Will this work if my sequence consists of abstract types? And even if the type is concrete, I can't, in general, know anything about constructing it, i.e., I can't know if it can be value-constructed.

I'm certainly not an MPL expert, but as far as I understand, the same problem exists in mpl::for_each. The unary functor passed to mpl::for_each looks like: struct F { template <typename T> void operator()(T) { .... } }; mpl::for_each<seq>(F()); so T is deduced by the argument being passed. T is constructed (probably default constructed) and then sent to the functor. Since we usually don't want to instantiate the types in the MPL sequence, we use a transform meta-function. Something like: struct F { template <typename T> void operator()(mpl::identity<T>) { .... } }; mpl::for_each >(F()); and then the T's being instantiated are not the types in the sequence, but mpl::identity<> of the types in the sequence, which is much better. The solution proposed here could probably be extended to support transform meta-functions just like mpl::for_each does, and the problem is solved. I sure hope this (or similar) mpl::for_each_until will be added to MPL. I also find it useful. As I said, I'm not an expert, so I hope I'm not misleading anyone here...