Linking C++ with Go

I am trying to bind C++ OpenCV programs with Go. I am using the concept from <https://stackoverflow.com/questions/16747021/how-do-you-statically-link-a-c-library-in-go-using-cgo>[1]. My myimgcom.cpp file is:

/**
* @file SURF_FlannMatcher
* @brief SURF detector + descriptor + FLANN Matcher
* @author A. Huaman
Edited: nnnnn
*/
#include <stdio.h>
#include <string>
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/nonfree/features2d.hpp"
using namespace cv;
void readme();
int mysurf( int myarg)
{
string option1= "./Lenna.png";
string option2= "./Lenna.png";
if( myarg != 3 )
{ readme(); return -1; }
std::cout << option1 << option2;
Mat img_1 = imread( option1, CV_LOAD_IMAGE_GRAYSCALE );
Mat img_2 = imread( option2, CV_LOAD_IMAGE_GRAYSCALE );
if( !img_1.data || !img_2.data )
{ std::cout<< " --(!) Error reading images " << std::endl; return -1; }
//-- Step 1: Detect the keypoints using SURF Detector
int minHessian = 400;
SurfFeatureDetector detector( minHessian );
std::vector<KeyPoint> keypoints_1, keypoints_2;
detector.detect( img_1, keypoints_1 );
detector.detect( img_2, keypoints_2 );
//-- Step 2: Calculate descriptors (feature vectors)
SurfDescriptorExtractor extractor;
Mat descriptors_1, descriptors_2;
extractor.compute( img_1, keypoints_1, descriptors_1 );
extractor.compute( img_2, keypoints_2, descriptors_2 );
//-- Step 3: Matching descriptor vectors using FLANN matcher
FlannBasedMatcher matcher;
std::vector< DMatch > matches;
matcher.match( descriptors_1, descriptors_2, matches );
double max_dist = 0; double min_dist = 100;
//-- Quick calculation of max and min distances between keypoints
for( int i = 0; i < descriptors_1.rows; i++ )
{ double dist = matches[i].distance;
if( dist < min_dist ) min_dist = dist;
if( dist > max_dist ) max_dist = dist;
}
printf("-- Max dist : %f \n", max_dist );
printf("-- Min dist : %f \n", min_dist );
std::vector< DMatch > good_matches;
for( int i = 0; i < descriptors_1.rows; i++ )
{ if( matches[i].distance <= max(4*min_dist, 0.02) )
{ good_matches.push_back( matches[i]); }
}
//-- Draw only "good" matches
Mat img_matches;
drawMatches( img_1, keypoints_1, img_2, keypoints_2,
good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
//-- Show detected matches
imshow( "Good Matches", img_matches );
for( int i = 0; i < (int)good_matches.size(); i++ )
{ printf( "-- Good Match [%d] Keypoint 1: %d  -- Keypoint 2: %d  \n", i,    good_matches[i].queryIdx, good_matches[i].trainIdx ); }
waitKey(0);
return 0;
}
void readme()
{ std::cout << " Usage: ./SURF_FlannMatcher <img1> <img2>" << std::endl; }

Next, my header file myimgcom.h file is:

int mysurf( int);
void readme();

Next, my imagetest.go file is:

package main
import "fmt"
// #include "myimgcom.h"
import "C"
func main() {
fmt.Printf("Invoking c library...\n")
C.mysurf( 3)
}

I am doing the following commands:

 gcc -O2 -c myimgcom.cpp  -> runs successfully
ar q libgb.a myimgcom.o  -> runs successfully
go build imagetest.go    -> Here I get the following error:
# command-line-arguments
Undefined symbols for architecture x86_64:
"_mysurf", referenced from:
__cgo_8db0969da00f_Cfunc_mysurf in imagetest.cgo2.o
(maybe you meant: __cgo_8db0969da00f_Cfunc_mysurf)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I think the problem is linking the pkg config and cflags with cgo. Can anybody suggest me how can I solve the issue? I am working with Mac OS.

C++ isn't C, and call conventions are different. It's almost impossible to call C++ directly. If you want to call it from Go, first do a C wrapper. In your case by adding "extern C" to function declarations should work.

extern "C"
int mysurf(int myarg)
{
// code here
}