如何重载 [][] 运算符？

一个类 A 有成员 double[][]，我希望直接能在对象上进行类似这样的操作：

A a;
double x = a[0][0];

该怎么实现？

需要一个 helper 类，大约是：

class ARowHelper { friend class A; A a_; size_t r_; ARowHelper(A a, size_t r) : a_(a), r_(r) {} public: double operator[](size_t c) { return a_.data_[r_][c]; } };

class A {
double** data_;
public:
ARowHelper operator[](size_t r) { return ARowHelper(*this, r); }
};

实际上还要考虑 const-ness 等问题。

operator[]返回一个double*或者不就够了，使用时[][]里的第二个[]是指针自带。

不过存多维数组的话我还是推荐扁平化……

C++怎么实现效率尽可能高地带边界检查的多维数组模板类??

www.zhihu.com

与其重载[][]不如重载()。你所需要的操作它都能满足，而且也不失优雅。

[][]这种表达，很容易联想到一种数据类型，那就是矩阵：Matrix。

而关于Matrix该如何优雅的重载运算符。isocpp官方恰好有个demo，那就是重载 operator () 而不是[]：

class Matrix { public: Matrix(unsigned rows, unsigned cols); double operator() (unsigned row, unsigned col); // Subscript operators often come in pairs double operator() (unsigned row, unsigned col) const; // Subscript operators often come in pairs // ... ~Matrix(); // Destructor Matrix(const Matrix m); // Copy constructor Matrix operator= (const Matrix m); // Assignment operator // ... private: unsigned rows_, cols_; double* data_; }; inline Matrix::Matrix(unsigned rows, unsigned cols) : rows_ (rows) , cols_ (cols) //, data_ ← initialized below after the if...throw statement { if (rows == 0 || cols == 0) throw BadIndex("Matrix constructor has 0 size"); data_ = new double[rows * cols]; } inline Matrix::~Matrix() { delete[] data_; } inline double Matrix::operator() (unsigned row, unsigned col) { if (row &>= rows_ || col &>= cols_) throw BadIndex("Matrix subscript out of bounds"); return data_[cols_*row + col]; } inline double Matrix::operator() (unsigned row, unsigned col) const { if (row &>= rows_ || col &>= cols_) throw BadIndex("const Matrix subscript out of bounds"); return data_[cols_*row + col]; }