C++ Memory allocation. Matrix

问题

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

Method n.1

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

Method 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;
}

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...)

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

http://codepad.org/uKvI8Tk3

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

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

回答1:

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_;
};

Edit: 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.

回答2:

Method n.2 is allocating a unique block to contain the sequence of all rows. Hence the first row is a pointer to the whole block. If rows==0 you have not space to hold the pointer to the (empty) space, so you cannot make the allocation.

I would steer toward method 4 suggested in the other answer:

class Matrix {
   Matrix(int rows, int cols): rows_(rows), cols_(cols) {
      data_ = new int[rows*cols];
   }

   ~Matrix() {
       delete[] data_;
   }

   int &operator()(int i,int j) {return data_[cols_*i+j];}

   int operator()(int i,int j) const {return data_[cols_*i+j];}

 private:
   int rows_,cols_;
   int *data_;
};

来源：https://stackoverflow.com/questions/14926763/c-memory-allocation-matrix

标签

c++

memory

dynamic

matrix

allocation