C++ : defining a two-dimensional array of a inner-class struct - c++

Question

I am trying to build a window controller for SDL with its own pseudo-pixels. I had the code working using a single big and ugly translation unit, but am now trying to refactor the same code into multiple units and classes.
In this class, I am attempting to define the controller for the window, and I am having trouble understanding how to work with a nested structure inside a class.
The errors I am getting are on the line Pixel* pixels[][];. The compiler complains that "incomplete type is not allowed". The struct seems to be fully defined in the .cpp.
The second error is that "an array may not have elements of this type"; I understand that the second dimension of an array has to be of a defined size, but how do I define this size at compile-time while keeping my code flexible for windows of different sizes?
//PxWindow.h
#include <SDL.h>
class PxWindow {
struct Pixel {
static int PARENT_PIXEL_SIZE;
// Data members
int x, y;
Color color;
// Methods
int screenX();
int screenY();
SDL_Rect getPixelRect();
};
// Private Data Member declarations
private:
int PIXEL_SIZE;
int NUM_PIXELS_X;
int NUM_PIXELS_Y;
// Public Data Member declarations
public:
Pixel* pixels[][];
SDL_Rect rects[][];
public:
PxWindow(int,int,int);
int getPixelSize();
int getNumX();
int getNumY();
};
// PxWindow.cpp
#include "PxWindow.h"
/*
* Pixel Structure method definitions
*/
SDL_Rect PxWindow::Pixel::getPixelRect() {
return SDL_Rect{ screenX(), screenY(), PARENT_PIXEL_SIZE, PARENT_PIXEL_SIZE };
};
int PxWindow::Pixel::screenX() { return x * PARENT_PIXEL_SIZE; }
int PxWindow::Pixel::screenY() { return y * PARENT_PIXEL_SIZE; }
/*
* PxWindow Class method definitions
*/
PxWindow::PxWindow(int px_size, int num_x, int num_y) :
PIXEL_SIZE(px_size),
NUM_PIXELS_X(num_x),
NUM_PIXELS_Y(num_y) {
Pixel::PARENT_PIXEL_SIZE = PIXEL_SIZE;
};
int PxWindow::getPixelSize() {
return PIXEL_SIZE;
};
int PxWindow::getNumX() {
return NUM_PIXELS_X;
};
int PxWindow::getNumY() {
return NUM_PIXELS_Y;
};

Answer 1

For the first error:
You should check your includes. Most likely one is missing to complete your struct "Pixel". You seem to use SDL_Rect, but I can't see the include for SDL in your header file.
For your second error:
To make the array a little more dynamic you would have to create a new array (arrayTwo) of array.size() + 1 and copy all data from array to arrayTwo yourself.
You could also use a (or two nested vectors for 2d), but it depends on your performance needs.