Re: [Boost-users] Concept check vs enable_if

From: Bruno Lalande (bruno.lalande_at_[hidden])
Date: 2008-08-10 16:23:32

Hello,

> how can I build a concept
> which requires a given type X to either have an associated type named foo
> or an associated type named bar (silly names for example purpose).
> Checking for both is easy but i can't find how to check for OR another.

Maybe using SFINAE?

template <class T>
struct foo_or_bar
{
        template <class U>
        static typename U::bar f();

        template <class U>
        static typename U::foo f();

        typedef BOOST_TYPEOF(f<T>()) type;
};

foo_or_bar<something>::type will give you the type of the "foo" or
"bar" nested in "something"... But it's an exclusive OR: if both are
present, you have an ambiguity.

Anyway I'm pretty sure someone else will find a better way... :-)

Bruno