如何重載 [][] 運算符？

一個類 A 有成員 double[][]，我希望直接能在對象上進行類似這樣的操作：

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

該怎麼實現？

需要一個 helper 類，大約是：

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); }
};

實際上還要考慮 const-ness 等問題。

operator[]返回一個double*或者不就夠了，使用時[][]裏的第二個[]是指針自帶。

不過存多維數組的話我還是推薦扁平化……

C++怎麼實現效率儘可能高地帶邊界檢查的多維數組模板類??

www.zhihu.com

與其重載[][]不如重載()。你所需要的操作它都能滿足，而且也不失優雅。

[][]這種表達，很容易聯想到一種數據類型，那就是矩陣：Matrix。

而關於Matrix該如何優雅的重載運算符。isocpp官方恰好有個demo，那就是重載 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]; }