undefined reference to `main' - but main function included [closed] - c++

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I am getting the following error when trying to compile my code:
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
I am using the following command:
g++ detectTemplatePoints.cpp -o SURF_TemplatePoints `pkg-config --cflags --libs opencv`
From what I can find online this seems to happen when you do not have a main entry point included but I do have that, my code below:
#include <stdio.h>
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/nonfree/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/nonfree/nonfree.hpp"
using namespace cv;
void readme();
int main (int argc, char* argv[]) {
if( argc != 2 ) {
readme(); return -1;
}
Mat img_1 = imread( argv[1], CV_LOAD_IMAGE_GRAYSCALE );
if( !img_1.data ) {
std::cout<< " --(!) Error reading images " << std::endl; return -1;
}
int minHessian = 400;
SurfFeatureDetector detector( minHessian );
std::vector<KeyPoint> keypoints_1;
detector.detect( img_1, keypoints_1 );
Mat img_keypoints_1;
drawKeypoints( img_1, keypoints_1, img_keypoints_1, Scalar::all(-1), DrawMatchesFlags::DEFAULT );
imshow("Keypoints 1", img_keypoints_1 );
waitKey(0);
return 0;
}
void readme() {
std::cout << " Usage: ./detectTemplatePoints <img1>" << std::endl;
}
What is causing this error?

Answer 1

As the error message says: you have no main function. They have to have one of the following signatures:
int main()
or
int main(int, char**)

Answer 2

First of all, you should use g++ detectTemplatePoints.cpp -o SURF_TemplatePoints $(pkg-config --cflags --libs opencv ) in order to link opencv libraries.
Also, you haven't shown us the libraries you are including.
I can get it compiled so:
#include <iostream>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/nonfree/features2d.hpp>
using namespace std;
using namespace cv;
void readme();
int main (int argc, char** argv) {
if( argc != 2 ) {
readme(); return -1;
}
Mat img_1 = imread( argv[1], CV_LOAD_IMAGE_GRAYSCALE );
if( !img_1.data ) {
std::cout<< " --(!) Error reading images " << std::endl; return -1;
}
int minHessian = 400;
SurfFeatureDetector detector( minHessian );
std::vector<KeyPoint> keypoints_1;
detector.detect( img_1, keypoints_1 );
Mat img_keypoints_1;
drawKeypoints( img_1, keypoints_1, img_keypoints_1, Scalar::all(-1), DrawMatchesFlags::DEFAULT );
imshow("Keypoints 1", img_keypoints_1 );
waitKey(0);
return 0;
}
void readme() {
std::cout << " Usage: ./detectTemplatePoints <img1>" << std::endl;
}