Dynamically allocating multidimensional arrays?

[DanglingPointer]

How can I dynamically allocate a 2d array with parameters passed to a constructor?

I want to do this:

Thing::Thing(int num1, int num2)
{
ptr = new float[num1][num2];
}

But this isn't valid syntax. How would I go about this? Overloading the new operator?

 

[Akarui]

The easiest, and less efficient way, would be to do this

Thing::Thing(int row, int col)
{
    ptr = new float*[row];
    
    for (int i = 0; i < row; i++)
        ptr[i] = new float[col];
}

However, this requires row + 1 calls to new. A better way that only calls new twice would be

Thing::Thing(int row, int col)
{
    data = new float[row * col];
    ptr = new float*[row];
    
    for (int i = 0; i < row; i++)
    {
        ptr[i] = data;
        data += col;
    }
}

It's not as easy to follow, but you allocate one large chunk and then place pointers at set intervals to simulate a matrix. In the code I've given no error checking is done, be sure to do that in your program.

 

[mingis]

In multidimensional allocating is possible dynamically change only very first index, the rest indexes must have constant size.

#define num2 10

Thing::Thing(int num1)
{
float (*ptr)[num2];
ptr = new float[num1][num2];

}

If you want more dynamical indexes, follow Akarui's advice.