Linking C++ with Go

Question

I am trying to bind C++ OpenCV programs with Go. I am using the concept from 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.

Answer 1

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
}