C++ 算术提升标头的使用

I've been playing around with a set of templates for determining the correct promotion type given two primitive types in C++. The idea is that if you define a custom numeric template, you could use these to determine the return type of, say, the operator+ function based on the class passed to the templates. For example:

// Custom numeric class
template <class T>
struct Complex {
    Complex(T real, T imag) : r(real), i(imag) {}
    T r, i;
// Other implementation stuff
};

// Generic arithmetic promotion template
template <class T, class U>
struct ArithmeticPromotion {
    typedef typename X type;  // I realize this is incorrect, but the point is it would
                              // figure out what X would be via trait testing, etc
};

// Specialization of arithmetic promotion template
template <>
class ArithmeticPromotion<long long, unsigned long> {
    typedef typename unsigned long long type;
}

// Arithmetic promotion template actually being used
template <class T, class U>
Complex<typename ArithmeticPromotion<T, U>::type>
operator+ (Complex<T>& lhs, Complex<U>& rhs) {
    return Complex<typename ArithmeticPromotion<T, U>::type>(lhs.r + rhs.r, lhs.i + rhs.i);
}

如果您使用这些促销模板，您可以或多或少地将您的用户定义类型视为对其应用了相同促销规则的原语.所以，我想我的问题是这会是有用的东西吗?如果是这样，为了便于使用，您希望模板化哪些类型的常见任务?我正在假设仅拥有促销模板不足以实际采用.

template<typename Atype, typename Btype>
type_promote<Atype, Btype>::type operator+(Atype A, Btype B);