c++ - 迷失在指针的世界里

Question

请帮助我，我无法理解这段代码是如何工作的。 我似乎无法在脑海中想象它。详尽的解释可以避免我将来成为失败者的麻烦。谢谢。

int **data = new int*[row];

    for(i=0;i<row;i++)
        data[i] = new int[col];

    for(i=0;i<row;i++)
    {
        for(j=0;j<col;j++)
        {
            data[i][j] = rand() % u;
        }
    }

Answer 1

#include <cstddef>  // std::size_t
#include <cstdlib>  // std::srand(), std::rand()
#include <ctime>    // std::time()

int main()
{
    // Initialize the random number generator with the current time.
    // If you don't, std::rand() behaves as if std::srand(1) had been called.
    std::srand(static_cast<unsigned>(time(nullptr)));

    std::size_t rows =  5;
    std::size_t cols = 10;
    int u = 100;

    int **data =  // data is a pointer to a pointer to int
        new int*[rows];  // allocates an array of type int* with size of row
                         // and return a pointer to it

    for (std::size_t i = 0; i < rows; i++)
        data[i] =  // every pointer in that array is assigned
            new int[cols];  // a pointer to an array of type int with size col

    for (std::size_t i = 0; i < rows; i++)  // walk through the (int**) in data
    {
        for (std::size_t j = 0; j < cols; j++)  // walk through the (int*) in data[i]
        {
            data[i][j] = std::rand() % u;  // access the j th int in data[i]
        }
    }

    // free the memory, when you are done using it *):
    for (std::size_t i = 0; i < rows; i++)
        delete[] data[i];
    delete[] data;
}

*) 更好:使用智能指针，例如 std::unique_ptr 和 std::shared_ptr，这样您就不必关心资源管理:

#include <memory>

// ...

auto data = std::make_unique<std::unique_ptr<int[]>[]>(rows);

for (std::size_t i = 0; i < rows; i++)
    data[i] = std::make_unique<int[]>(cols);

甚至更好:使用像 std::vector 或 std::deque 这样的容器。