在没有 setter 的情况下修改基类中的私有数据，c++ visual studio 2013

我正在为我的计算机科学课做一个项目，在那里我们创建了一个源图像的照片马赛克。我们得到了一个基本图像类。与标准构造函数一起，此类包含用于返回私有像素数组数据和图像高度和宽度的 getter 函数。我们被告知为所有填充图像创建一个派生类，这些图像需要存储在链表中。我已经完成了大部分程序，但我不知道如何用填充图像中的像素数据替换源图像的块。

    #include "stdafx.h"  //needed for the Windows display
    #include <cstdlib>
    #include <cmath>
    #include <string>
    #include <iostream>
    using namespace std;
    #include "globals.h"  //some global variables are included here
    #include "pixel.h"  //includes the pixel class for storing individual pixels
    #include "image.h"  //includes the image class we will be using.
    #include "fillerImage.h" //includes the fillerImage
    #include "fillNode.h"
    #include <sstream>

image* mosaic = displayed;  //displayed is the current image in the gui, mosaic is the copy that will be the photomosaic
pixel** mospix = mosaic->getPixels();  
pixel** headpix = head->data->pix;   //I created a public member pixel** in the filler image class with the hope that I could use it to access the private pixel data of the base image class.
int mosaicWidth = displayed->getWidth() / blockWidth * FILLER_IMAGE_WIDTH;
int mosaicHeight = displayed->getHeight() / blockHeight * FILLER_IMAGE_HEIGHT;
//mosaic->createNewImage(mosaicWidth, mosaicHeight);
for (int i = 0; i < mosaicHeight; i = i + blockHeight)
{
    for (int j = 0; j < mosaicWidth; j = j + blockWidth)
    {
        for (int blockrow = 0; blockrow < blockHeight; blockrow++)
        {
            for (int blockcol = 0; blockcol < blockWidth; blockcol++)
            {
                mospix = headpix;
            }
        }
    }
    displayed = mosaic;
}
displayed = mosaic;

我一直遇到尝试写入受保护数据的问题;如何修改马赛克的像素数据？我不允许编辑基本图像类，只能编辑填充图像类。如果我的帖子格式不正确，我深表歉意，这是我第一次寻求帮助。提前感谢！

编辑：这是图像类标题。我不允许以任何方式修改它，它不包含像素数据的设置器。

    #include "globals.h"
    #include "pixel.h"
    using namespace std;
    class image { 
public:
    image();            //the image constructor (initializes everything)
    image(string filename);  //a image constructor that directly loads an image from disk
    ~image();           //the image destructor  (deletes the dynamically created pixel array)
    void createNewImage(int width, int height); //this function deletes any current image data and creates a new blank image
                                                //with the specified width/height and allocates the needed number of pixels
                                                //dynamically.
    bool loadImage(string filename);        //load an image from the specified file path.  Return true if it works, false if it is not a valid image.
                                            //Note that we only accept images of the RGB 8bit colorspace!
    void saveImage(string filename);       //Save an image to the specified path
    pixel** getPixels();                    //return the 2-dimensional pixels array
    int getWidth();                     //return the width of the image
    int getHeight();                    //return the height of the image
    void viewImage(CImage* myImage);  //This function is called by the windows GUI.  It returns the image in format the GUI understands.

private:
    void pixelsToCImage(CImage* myImage);  //this function is called internally by the image class.
                                            //it converts our pixel object array to a standard BGR uchar array with word spacing.
                                            //(Don't worry about what this does)
    pixel** pixels;             // pixel data array for image 
    int width, height;      // stores the image dimensions 


    };
    #endif