如何使用"new"而不是"malloc"动态分配2D数组? [英] How to use 'new' instead of 'malloc' to allocate a 2D-array, dynamically?

问题描述

我想使用2D指针制作矩阵.

I'd like to make matrix using 2-D pointer.

当我将'malloc'和'free'函数用于内存使用时，没有问题(请参阅我的代码). 但是，我无法使用'new'和'delete'编写相同的代码.

There was no problem when I used 'malloc' and 'free' functions for memory use (see my code). However, I couldn't write the same code using 'new' and 'delete'.

您知道，一维指针可以由'new'声明.例如，

As you know, 1-D pointer can be declared by 'new'. For example,

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

然后，如何使用'new'声明二维指针?

Then, how can I declare 2-D pointer using 'new'?

    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);

推荐答案

嗯，直接等效项是这样的:

Well, the direct equivalent is like this:

// 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;

确实，您不想要这个.这完全是一团糟，并且没有本地内存.

Really, though, you don't want this. It's a complete mess and has like no memory locality.

更不用说手动存储管理完全容易出错了，这可以从原始代码中泄漏row double s的事实得到证明.

Not to mention all that manual memory management is totally error-prone, as evidenced by the fact that you have a leak of row doubles in your original code.

这是怎么回事:

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;
};

它使用vector来避免任何那种讨厌的内存管理，并且将2D索引访问包装在实际上是单个数据缓冲区的周围，从而使您没有 n 指针间接.

It uses vector to avoid any of that nasty memory management, and wraps 2D index access around what's actually a single data buffer so that you don't have n pointer indirections.

您可以根据需要将布局调整为以行为主或以列为主.

You can adjust the layout to be row-major or column-major depending on your needs.

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