On 12/09/2010 21:04, Daniel Walker wrote:
I'm not sure if this could work. The C++0x draft is pretty specific about the context in which substitution failure is allowed, namely, it's only allowed in the immediate context of the function and not as a side effect of instantiating a template (such as a Proto class). Here's the relevant language from 14.8.2.
SFINAE does apply in that kind of context: template<typename T, typename U> decltype(make<T>() + make<U>()) add(T t, U u) { return t + u; } The overload is not considered if the expression isn't valid. Of course this works with sizeof too, and in C++03 as well (if the compilers supports it). This is the kind of thing I want Proto's default transform to do.
However, I just checked, and I was pleased to find that gcc 4.5 is now accepting some SFINAE for expressions.
It's been there since 4.4 I think, but has only been working well since 4.5. I've put a macro for it in Boost.Config a while ago.