Why does SFINAE not apply to this?
- by Simon Buchan
I'm writing some simple point code while trying out Visual Studio 10 (Beta 2), and I've hit this code where I would expect SFINAE to kick in, but it seems not to:
template<typename T>
struct point {
T x, y;
point(T x, T y) : x(x), y(y) {}
};
template<typename T, typename U>
struct op_div {
typedef decltype(T() / U()) type;
};
template<typename T, typename U>
point<typename op_div<T, U>::type>
operator/(point<T> const& l, point<U> const& r) {
return point<typename op_div<T, U>::type>(l.x / r.x, l.y / r.y);
}
template<typename T, typename U>
point<typename op_div<T, U>::type>
operator/(point<T> const& l, U const& r) {
return point<typename op_div<T, U>::type>(l.x / r, l.y / r);
}
int main() {
point<int>(0, 1) / point<float>(2, 3);
}
This gives error C2512: 'point<T>::point' : no appropriate default constructor available
Given that it is a beta, I did a quick sanity check with the online comeau compiler, and it agrees with an identical error, so it seems this behavior is correct, but I can't see why.
In this case some workarounds are to simply inline the decltype(T() / U()), to give the point class a default constructor, or to use decltype on the full result expression, but I got this error while trying to simplify an error I was getting with a version of op_div that did not require a default constructor*, so I would rather fix my understanding of C++ rather than to just do what works.
Thanks!
*: the original:
template<typename T, typename U>
struct op_div {
static T t(); static U u();
typedef decltype(t() / u()) type;
};
Which gives error C2784: 'point<op_div<T,U>::type> operator /(const point<T> &,const U &)' : could not deduce template argument for 'const point<T> &' from 'int', and also for the point<T> / point<U> overload.
