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In the following code 1)I have defined two classes one to store an image in it and another class which gets that image from first class for further manipulation. 2)Also I have fetched some image properties in main,passed them to class store and used them in class grayimage. So,any changes in main wont directly affect the images or data in classes.To make changes in them you need to call them(classes and its objects) personally.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <cv.h>
#include <highgui.h>
#include <iostream>
#include<stdio.h>
using namespace cv;
using namespace std;

class store              
{
    public:             //Keep it public so that it can be used in other classes
        Mat image;
        int blue;
        int green;
        int red;



    public: 
    void getimage(Mat src) //storing image into a class variable named image
    {               

        if(! src.data )                              
        {
            cout <<  "src not filled" << endl ;
        }

        else
        {
            image = src.clone();
            cout << "got image" <<endl;
        }
    }

    void getdata(int bb,int gg,int rr)//storing data into class variables
    {
        blue =bb;
        green= gg;
        red=rr;
    }

    void showimage()//Display stored image using a class function
    {       
        if(! image.data )                              
        {
            cout <<  "Image not passed into this function" << std::endl ;

        }

        else
        {

            namedWindow( "Display window", WINDOW_AUTOSIZE );
            imshow( "Display window", image );
            cout << "image ready to get displayed" << endl;
        }   
    }

};  //first class ends here;

class grayimage   //creating a class where i need to manipulate images and data obtained from another class
{

    private:
        Mat gray_image;
        int btrans,gtrans,rtrans;

    public:
        void setgray(Mat src) //converting the image to grayscale
        {
            if(! src.data )                              
            {
                cout <<  "src in second class not filled" << endl ;
            }

            else
            {
                cvtColor(src, gray_image, CV_BGR2GRAY );
                cout << "image conversion done" <<endl;
            }

        }

        void transferdata(int b,int g,int r)  //creating a transfer function
        {
            btrans = b;
            gtrans = g;
            rtrans = r;
        }       


        void showdata()
        {
            cout << "values of b,g,r before gray scale coversion are" << btrans << "," << gtrans << "," << rtrans << "," <<endl; 
        }

        void displaygray()
        {
            if(! gray_image.data )                              
            {
                cout <<  "Image not passed into this function" << std::endl ;

            }

            else
            {

                namedWindow( "gray image", WINDOW_AUTOSIZE );
                imshow( "gray image", gray_image );
                cout << "gray image ready to get displayed" << endl;
            }   
        }

};  

//main starts here

int main( int argc, char** argv )
{
    if( argc != 2)
    {
     cout <<" Usage: display_image ImageToLoadAndDisplay" << endl;
     return -1;
    }

    Mat img;
    img = imread(argv[1], CV_LOAD_IMAGE_COLOR);    

    if(! img.data )                              
    {
        cout <<  "Could not open or find the image" << std::endl ;
        return -1;
    }

    unsigned char *input = (unsigned char*)(img.data);
    int i,j,r,g,b;
    for(int i = 0;i < img.rows;i++)
    {
        for(int j = 0;j < img.cols;j++)
        {
                b = input[img.step * j + i ] ;
                g = input[img.step * j + i + 1];
                r = input[img.step * j + i + 2];
            }
    }

    store image1; //object to store image created
    image1.getimage(img);//image from main stored in object
    image1.getdata(b,g,r);//data from main stored in object
    image1.showimage();//displaying the stored image 

    grayimage image2;   //creating a manipulator object
    image2.setgray(image1.image); //fetches image from the first object that was stored and converts it into grayscale 
    image2.transferdata(image1.blue,image1.green,image1.red);//fetches data from the first object that was stored 

    //This data transferred to the new object can be manupulated anytime later

    image2.showdata();
    image2.displaygray();          

    waitKey(0);                                         
    return 0;
}

Hope this helps you :)