C ++内存分配。矩阵 [英] C++ Memory allocation. Matrix

问题描述

我研究了两种不同的方法来为矩阵的元素分配内存。

I looked into two different method to allocate memory for the elements of a matrix

方法n.1

int** matrix = new int*[rows];
for (int i = 0; i < rows; ++i)
    matrix[i] = new int[cols];

方法n.2

int** matrix = new int*[rows];
if (rows)
{
    matrix[0] = new int[rows * cols];
    for (int i = 1; i < rows; ++i)
        matrix[i] = matrix[0] + i * cols;
}

我可以找出什么方法n.1，但我不能找出在方法n.2中应该做什么if子句（我将实现它没有，它不工作，与if子句，它...）

I can figure out what Method n.1 does, but I can't figure out what exactly is supposed to do the if clause in Method n.2 (I would implement it without and it doesn't work, with the if clause, it does...)

编辑：这是一个代码显示我的问题。为什么加载时间过长（约30秒）？

Here is a code showing my problem. Why does it take so long to load (~30seconds)?

http ：//codepad.org/uKvI8Tk3

键盘拒绝显示输出（超时），所以如果你想运行它，只需自己编译。

Codepad refuses to show the output (timeout) so if you want to run it, just compile it on your own.

此外，为什么cout<

Also, why cout << statements are not executed once the program starts?

推荐答案

方法n.3：编写自己的Matrix类，内部使用单个 std :: vector< int> 并且聪明地关于通过（row，col）索引访问。

Method n.3: write your own Matrix class, internally using a single std::vector<int> and being clever about access by (row,col) indices.

struct Matrix
{
  explicit Matrix(unsigned int rows, unsigned int cols) : data_(rows*cols), cols_(cols) {}
  const int& operator()(unsigned int row, unsigned int col) const
  {
    return data_[row*cols_ + col];
  }
 private:
  std::vector<int> data_;
  unsigned int cols_;
};

编辑： iff 向量的内存开销是最后一个例子，你可以考虑使用单个动态分配的长度 rows * cols 的数组，并确保调用 delete [] 在析构函数中。

iff the memory overhead of a vector is an issue in the last example, you can consider using a single dynamically allocated array of length rows*cols, and make sure to call delete [] on it in the destructor.

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