如何使用'new'代替'malloc'动态分配2D数组？

Question

我想用二维指针制作矩阵.

当我使用'malloc'和'free'函数进行内存使用时没有问题(参见我的代码).但是,我无法使用'new'和'delete'编写相同的代码.

如您所知,1-D指针可以通过'new'声明.例如,

double *example = new double [10];
delete [] example;

那么,如何使用'new'声明二维指针？

    double **matrix;    // 2-D pointer which represents entries of a matrix
    int row, column;    // the number of rows and column of the matrix
    int i;

    // set the size of the matrix
    row = 3;
    column = 5;

    // allocate memories related to the number of rows
    matrix = (double **)malloc(sizeof(double *) * row);

    // allocate memories related to the number of columns of each row
    for(i = 0; i < row; i++)
    {
        matrix[i] = (double (*))malloc(sizeof(double) * column);
    }

    // example: use of matrix
    matrix[2][4] = 10.5;

    // return used memories
    free(matrix);

Answer 1

嗯,直接等价是这样的:

// allocate memories related to the number of rows
double** matrix = new double*[row];

// allocate memories related to the number of columns of each row
for(i = 0; i < row; i++)
{
    matrix[i] = new double[column];
}

// usage here

// de-allocate memories related to the number of columns of each row
// (YOU FORGOT THIS IN YOUR ORIGINAL CODE!!!)
for(i = 0; i < row; i++)
{
    delete matrix[i];
}

delete[] matrix;

但是,真的,你不想要这个.这是一个完整的混乱,并没有像没有记忆的地方.

更不用说手动内存管理完全容易出错,正如row double你原始代码中的s 泄漏所证明的那样.

这有什么问题:

struct Matrix
{
    Matrix(const unsigned int row, const unsigned int column)
       : row(row)
       , column(column)
       , data(row*column, 0)
    {}

    double& at(const unsigned int y, const unsigned int x)
    {
        return data[y + x*row];
    }

private:
    const unsigned int row, column;
    std::vector<double> data;
};

它用于vector避免任何讨厌的内存管理,并围绕实际上是单个数据缓冲区包装2D索引,这样就不会有n个指针间接.

您可以根据需要将布局调整为行主要或列主要.