Boost logo

Boost :

From: Eric Niebler (eric_at_[hidden])
Date: 2007-05-05 11:11:04

Maurizio, I didn't see this message the first time because for some
reason, all your messages appear out of order on GMane. Is the clock on
your computer wrong? Is it something you can fix?

Maurizio Vitale wrote:
> I don't know if it is a bug or not, but I was surprised by the effect
> of a seemingly minor code change.
> So I said to myself: let's get rid of all those operators and replace them with
> a single templatized version, like:
> template <typename Expr>
> struct my_expression : proto::extends<Expr, my_expression<Expr>, my_domain> {
> typedef proto::extends<Expr, my_expression<Expr>, my_domain> base_type;
> my_expression (Expr const& expr = Expr()) : base_type (expr) {};
> using base_type::operator =;
> template<typename T>
> operator T () const {
> return static_cast<T>(proto::eval(*this, my_context<Expr> ()));
> }
> };

OK, your expression wrapper has an implicit conversion to any type T.
That is, IMO, a bad idea, but nevertheless, proto shouldn't stop you
from doing that. I think I have fixed the problem in CVS.

Incidentally, you were running into an ambiguous conversion deep in the
guts of callable_context, which uses an implicit conversion to a hidden
type to detect whether your context has an overload of operator() that
accepts the current expression. The code now uses a less-preferred
conversion, avoiding the ambiguity with yours.

Eric Niebler
Boost Consulting

Boost list run by bdawes at, gregod at, cpdaniel at, john at