Why does SFINAE not apply to this?

问题:

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.

回答1:

Not 100% sure. It appears that the compiler needs to instantiate both overloads to determine which is better, but while trying to instantiate the other op_div with T = int and U = point<float>, this leads to an error that is not covered by SFINAE (the error is not that op_div doesn't have type in this case, but that type cannot be determined).

You could try to disable the second overload if the second type is a point (boost::disable_if).

Also, what seems to work is postponed return type declaration (doing away with the op_div struct, but depending on which C++0x features are supported by your compiler):

template<typename T, typename U>
auto
operator/(point<T> const& l, point<U> const& r) -> point<decltype(l.x / r.x)> {
    return {l.x / r.x, l.y / r.y};
}

template<typename T, typename U>
auto
operator/(point<T> const& l, U const& r) -> point<decltype(l.x / r)> {
    return {l.x / r, l.y / r};
}

回答2:

I would say your error is in here:

template<typename T>
struct point {
    T x, y;
    point(T x, T y) : x(x), y(y) {}
};

Change your struct definition to this:

template<typename T>
struct point<T> {
    T x, y;
    point(T x, T y) : x(x), y(y) {}
};

If you want to use a generic type T, you need to specify it in the definition.