Question

I need to create a large two dimensional array of objects. I’ve read some related questions on this site and others regarding multi_array, matrix, vector, etc, but haven’t been able to put it together. If you recommend using one of those, please go ahead and translate the code below.

Some considerations:

• The array is somewhat large (1300 x 1372).
• I might be working with more than one of these at a time.
• I’ll have to pass it to a function at some point.
• Speed is a large factor.

The two approaches that I thought of were:

Pixel pixelArray[1300][1372];
for(int i=0; i<1300; i++) {
    for(int j=0; j<1372; j++) {
        pixelArray[i][j].setOn(true);
        ...
    }
}

and

Pixel* pixelArray[1300][1372];
for(int i=0; i<1300; i++) {
    for(int j=0; j<1372; j++) {
        pixelArray[i][j] = new Pixel();
        pixelArray[i][j]->setOn(true);
        ...
    }
}

What’s the right approach/syntax here?

Edit:

Several answers have assumed Pixel is small – I left out details about Pixel for convenience, but it’s not small/trivial. It has ~20 data members and ~16 member functions.

Answer 1

Your first approach allocates everything on stack, which is otherwise fine, but leads to stack overflow when you try to allocate too much stack. The limit is usually around 8 megabytes on modern OSes, so that allocating arrays of 1300 * 1372 elements on stack is not an option.

Your second approach allocates 1300 * 1372 elements on heap, which is a tremendous load for the allocator, which holds multiple linked lists to chunks of allocted and free memory. Also a bad idea, especially since Pixel seems to be rather small.

What I would do is this:

Pixel* pixelArray = new Pixel[1300 * 1372];
for(int i=0; i<1300; i++) {
    for(int j=0; j<1372; j++) {
        pixelArray[i * 1372 + j].setOn(true);
        ...
    }
}

This way you allocate one large chunk of memory on heap. Stack is happy and so is the heap allocator.