c++ - C++ 自动类型转换：容器类的错误行为

Answer 1

Use std::common_type:

template <std::size_t s, typename L, typename R>
MyMathVector<typename std::common_type<L, R>::type, s> operator+(MyMathVector<L, s> const& l, MyMathVector<R, s> const& r)
{
    // do addition
}

Ot in case of the member function (in the class body, where T and s are visible):

template <typename TRHS>
MyMathVector<typename std::common_type<T, TRHS>::type, s> operator+(MyMathVector<TRHS, s> const& rhs) const
{
    // do addition
}

Answer 2

Use the std::common_type trait to figure out the correct result type for a mixed operation.

The linked page even has an example that's very similar to your case.

Answer 3

Absolutely; use decltype:

template<typename Other>
auto operator+(const MyMathVector<Other, size> &other)
    -> MyMathVector<decltype(std::declval<T>() + std::declval<Other>()), size>;

As a non-member operator, it might be better to say what you mean by actually referencing a vector member:

template<typename size, typename L, typename R>
auto operator+(const MyMathVector<L, size> &l, const MyMathVector<R, size> &r)
    -> MyMathVector<decltype(l[0] + r[0]), size>;