c++ - 为不适用于输出流的模板矩阵重载 "+"

Question

我正在尝试重载 + 以将两个矩阵相加，然后立即输出。例如:

matrix<int> a, b;
...
cout << a + b << endl;      //doesn't work
matrix<int> c = a + b;      //works
cout << a << endl;          //works

error: cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'|
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ostream|602|error:   initializing argument 1 of 'std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = matrix<int>]'|

我已经重载了 << 但我不确定如何让它协同工作。到目前为止，这是我所拥有的:(<< 适用于单个矩阵)

template <typename Comparable>
class matrix
{
    private:
        size_t num_cols_;
        size_t num_rows_;
        Comparable **array_;

    public:
        friend ostream& operator<< (ostream& o, matrix<Comparable> & rhs){
            size_t c = rhs.NumRows();
            size_t d = rhs.NumCols();
            for (int i = 0; i < c; i++){
                for (int j = 0; j < d; j++){
                    o << rhs.array_[i][j] << " ";
                }
                o << endl;
            }
            return o;
        }


        matrix<Comparable> operator+ (matrix<Comparable> & rhs){
            matrix<Comparable> temp(num_rows_,num_cols_);
            for (int i = 0; i < num_rows_; i++){
                for (int j = 0; j < num_cols_; j++){
                    temp.array_[i][j] = array_[i][j] + rhs.array_[i][j];
                }
            }
            return temp;
        }
    }

Answer 1

a + b表达式根据为 matrix<T>::operator+ 声明的返回类型产生纯右值:

matrix<Comparable> operator+ (matrix<Comparable> & rhs);
~~~~~~~~~~~~~~~~~^

反过来，operator<<期望一个可修改的左值:

friend ostream& operator<< (ostream& o, matrix<Comparable> & rhs);
                                        ~~~~~~~~~~~~~~~~~~~^

自 operator<<不应该修改它的参数，你可以安全地将它变成一个 const左值引用(如果 NumRows() 和 NumCols() 是 const 合格的成员函数，这应该可以工作):

friend ostream& operator<< (ostream& o, const matrix<Comparable> & rhs);
                                        ~~~~^

旁注:operator+还应将其操作数视为 const引用资料，本身应该是const合格(如果它保留为成员函数):

matrix<Comparable> operator+ (const matrix<Comparable> & rhs) const;
                              ~~~~^                           ~~~~^