英文:
Giving two template class as arguments, choose the return template class using a hierarchy order
问题
I have a class Matrix defined as template<class T, int m_rows, int m_cols = m_rows> class Matrix and an overloaded operator of the type template <class V, int m_cols2> Matrix operator * (const Matrix<V,m_cols,m_cols2>& other). The idea is that, for example, if T is complex and V is double, the returning matrix should be of the type V (complex). How do I accomplish that in the code?
As of now I have
template <class V, int m_cols2>Matrix operator * (const Matrix<V,m_cols,m_cols2>& other) const {Matrix<T,m_rows,other.m_cols> res;for (int i = 0; i < m_rows; ++i) {for (int k = 0; k < other.m_rows; ++k) {res[i][k] = 0;for (int j = 0; j < m_cols; ++j) {res[i][k] += m_matrix[i][j]*other[j][k];}}}return res;}
But this automatically sets the matrix "res" to the type T.
How can I tell that it should decide the type of res depending on the type hierarchy? For example
complex > double > integer?
Thanks for your answers 
I have no idea what to try
英文:
I have a class Matrix defined as template<class T, int m_rows, int m_cols = m_rows> class Matrix and an overloaded operator of the type template <class V, int m_cols2> Matrix operator * (const Matrix<V,m_cols,m_cols2>& other). The idea is that, for example, if T is a complex and V is a double, the returning matrix should be of the type V (complex). How do I accomplish that in the code?
As of now I have
template <class V, int m_cols2>Matrix operator * (const Matrix<V,m_cols,m_cols2>& other) const {Matrix<T,m_rows,other.m_cols> res;for (int i = 0; i < m_rows; ++i) {for (int k = 0; k < other.m_rows; ++k) {res[i][k] = 0;for (int j = 0; j < m_cols; ++j) {res[i][k] += m_matrix[i][j]*other[j][k];}}}return res;}
But this automatically sets the matrix "res" to the type T.
How can I tell that it should decide the type of res depending on the type hierarchy? For example
complex > double > integer?
Thanks for your answers
I have no idea what to try:)
答案1
得分: 3
你可以生成一个"common type"的矩阵：
// 注意：最好不要将模板参数命名为"m_something"，因为这会与类的数据成员造成混淆。template <class OtherT, int OtherCols>auto operator * (const Matrix<OtherT, Cols, OtherCols>& other) const-> Matrix<std::common_type_t<T, OtherT>, Rows, OtherCols>> // trailing return type{// 使用 decltype 避免复制和粘贴返回类型。// 我们还可以使用推导返回类型（只是 auto，没有 trailing -> ...），// 但这不会对 SFINAE 友好，即：即使没有共同的类型，operator* 也会被实例化。decltype(*this * other) result;for // ...return result;}
如果你的 complex 类是 std::complex，你可以像上面的示例一样使用 std::common_type。否则，你可能需要为你的类型专门化 std:common_type<Complex, ...> 以使其工作。
这是因为 double 可以隐式转换为 std::complex<double>，但反之则不行。它也适用于 int 和 double，即：
static_assert(std::is_same_v<double, std::common_type_t<int, double>>); // 通过
英文:
You could produce a matrix of a "common type":
// note: it's best not to call template parameters "m_something", because// it creates confusion with data members of the class.template <class OtherT, int OtherCols>auto operator * (const Matrix<OtherT, Cols, OtherCols>& other) const-> Matrix<std::common_type_t<T, OtherT>, Rows, OtherCols>> // trailing return type{// Avoid copying and pasing the return type with decltype.// We could also use a deduced return type (just auto, no trailing -> ...),// however, this wouldn't be SFINAE-friendly, i.e.: operator* gets// instantiated even if there is no common type.decltype(*this * other) result;for // ...return result;}
If your complex class is std::complex, you can just use std::common_type like in the above example. Otherwise you may have to specialize std:common_type<Complex, ...> for your type to make it work.
This works because a double is implicitly convertible to a std::complex<double>, but not the other way around. It also works for int and double, i.e.
static_assert(std::is_same_v<double, std::common_type_t<int, double>>); // passes
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