C++ : how to define a Class just with static functions

Question

I want to creat a C++ class which is incorporated only with static functions that I can use no matter what. I have create a .h file with the declarations and a .cpp file with the definitions. However, when I am using it inside my code I am receiving some weird error messages which I do not know how to solve.

Here is the content of my Utils.h file:

#include <iostream>
#include <fstream>
#include <sstream>

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>

using namespace std;
using namespace cv;

#include <vector>
#include <opencv/cv.h>
#include <opencv/cxcore.h>

class Utils
{
public:
    static void drawPoint(Mat &img, int R, int G, int B, int x, int y);
};

Here is the content of my Utils.cpp file:

#include "Utils.h"

void Utils::drawPoint(Mat &img, int R, int G, int B, int x, int y)
{
img.at<Vec3b>(x, y)[0] = R;
img.at<Vec3b>(x, y)[1] = G;
img.at<Vec3b>(x, y)[2] = B;
}

And this is how I want to use it in my main function :

#include <iostream>
#include <fstream>
#include <sstream>

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>

using namespace std;
using namespace cv;

#include <vector>
#include <opencv/cv.h>
#include <opencv/cxcore.h>

#include "CThinPlateSpline.h"
#include "Utils.h"

int main()
{
Mat img = imread("D:\\image.png");
if (img.empty()) 
{
    cout << "Cannot load image!" << endl;
    system("PAUSE");
    return -1;
}
Utils.drawPoint(img, 0, 255, 0, 20, 20);
imshow("Original Image", img);
waitKey(0);
return 0;
}

And here are the errors that I am receiving.

Can someone point me out what I am doing wrong? What am I missing?

why not copy the errors too, dont have the headers to compile — Antti Haapala -- Слава Україні, Oct 13 '12 at 10:17

score 5 · Accepted Answer · answered Oct 13 '12 at 10:19

Utils::drawPoint(img, 0, 255, 0, 20, 20);
     ^^ (not period)

is how you would call a static function. the period is for member access (i.e. when you have an instance).

to illustrate this for completeness:

Utils utils; << create an instance
utils.drawPoint(img, 0, 255, 0, 20, 20);
     ^ OK here

score 1 · Answer 2 · answered Oct 13 '12 at 10:17

1

Think you're missing a semicolon after the class deceleration. Try,

class Utils
{
public:
    static void drawPoint(Mat &img, int R, int G, int B, int x, int y);
}; // <- Notice the added semicolon

answered Oct 13 '12 at 10:17

PherricOxide

15,493
3
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score 0 · Answer 3 · edited May 23 '17 at 12:18

This is not a direct answer to your question but using a namespace scoped functions might suit your needs better. What I mean is:

namespace Utils
{
    void drawPoint(Mat &img, int R, int G, int B, int x, int y);
}

The :: semantics are left the same but now:

You can't instantiate a Utils object, which is meaningless for "static class"
You could use using Utils to avoid the Utils:: prefix in chosen (and limited) scopes

For a deeper discussion of the pros and cons of static class member functions vs namespace scoped functions see: Namespace + functions versus static methods on a class

C++ : how to define a Class just with static functions

3 Answers3