C ++使用Const int定义数组

Question

I'm just learning about C++. I'm doing practice with initialize list, so I made a class like this

class Matrix
{
    public:
        const int x_size;
        const int y_size;
        int *data;

        Matrix(int _x_size, int _y_size) : x_size(_x_size), y_size(_y_size)
        {
            data = new int[y_size][x_size];
        }

        ~Matrix()
        {
            delete[][] data;
        }
};

int main(void)
{
    Matrix A = Matrix(10, 10);
    return 0;
}

And compiler said as: array size in operator new must be constant. So I searched and someone said, these are not 'compiler time constant'.

But it is obvious that I can't use that size as macros in here... Then. How should I get proper-sized array with Constructor?

Answer 1

If you are just learning c++ then the best tip is stay away from memory management. Use the stl types if you can. Use a std::vector to replace that array:

std::vector<std::vector<int>> data;

create it like this:

data(y_size, std::vector<int>(x_size, 0));

and access it like this:

data[i][j];

As jaunchopanza said, you can also use a 1D array which might be better. You would create and edit it in similar ways:

std::vector<int> data;
data(y_size * x_size, 0);
data[y_size*i + j];

The advantage is that it is faster for accessing, especially if x_size and y_size are going to be large. There is also an advantage in that the vector of vectors may be stored all over the place, as in each row(or column) will be in different places in memory. If you intend to get data that overlaps more than one row (or column) then it would be better to use the 1D array for speed.

You can Find more info here: http://en.cppreference.com/w/cpp/container/vector

If you want to turn this into a matrix and do math etc. Than I would highly recomend Eigen it is by far the best matrix library: http://eigen.tuxfamily.org/index.php?title=Main_Page

Answer 2

you should write sth like this ( although it is not exception safe)

class Matrix
{
    public:
        const int x_size;
        const int y_size;
        int ** data;

        Matrix(int _x_size, int _y_size) : x_size(_x_size), y_size(_y_size)
        {
            data = new int* [y_size];
            for(int i =0 ; i < y_size ; i++)
            {
                data[i] = new int[x_size] ;
            }
        }

        ~Matrix()
        {
            for(int i= 0 ; i < y_size ; i++)
            {
                delete [] data[i];
            }
            delete[] data;
        }
};