How to wrap OpenCV C++ function in C code

Question

I am able to run my C++ code to display the image, but am trying to integrate it with some other C code.

I'm looking for a walkthrough of how to write a C wrapper for my C++ code in OpenCV. I will need to be able to call this C++ method in my C code in future!

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

using namespace cv;
using namespace std;

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

    Mat image;
    image = imread(argv[1], CV_LOAD_IMAGE_COLOR);   // Read the file

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

    namedWindow( "Display window", WINDOW_AUTOSIZE );// Create a window for display.
    imshow( "Display window", image );                   // Show our image inside it.

    waitKey(0);                                          // Wait for a keystroke in the window
    return 0;
}

Here is a sample OpenCV C++ code I am currently working with first.

robthebloke · Accepted Answer · 2019-12-11 04:53:33Z

To wrap C++ code so that it's callable by C can be done with a few C++ functions that present themselves as C funcs. As an example, Let's imagine I have a class called MyObj....

// MyObj.h
#pragma once
#include <iostream>

class MyObj
{
  int m_thing = 42;
public: 
  MyObj() = default;
  ~MyObj() = default;

  void printThing() const {
    std::cout << "MyObj: " << m_thing << std::endl;
  }
  int getThing() const {
    return m_thing;
  }
  void setThing(int v) {
    m_thing = v;
  }
};

I need to wrap this in some C functions (declared with C-linkage).

// MyObjWrapper.h
#pragma once

/* 
 * use C name mangling if compiling as C++ code. 
 * When compiling as C, this is ignored. 
 */
#ifdef __cplusplus
extern "C" {
#endif

struct ObjWrapper;

/* return a newly created object */
struct ObjWrapper* createObj();

/* delete an object */
void deleteObj(struct ObjWrapper*);

/* do something on an object */
void printThing(const struct ObjWrapper*);

/* get value from object */
int getThing(const struct ObjWrapper*);

/* set value on object */
void setThing(struct ObjWrapper*, int);

#ifdef __cplusplus
}
#endif

Now in the C++ wrapper file, we can encapsulate all of the C++, leaving only a C interface.

// MyObjWrapper.cpp
#include "MyObj.h"
#include "MyObjWrapper.h"
#include <cassert>

/* use C name mangling */
#ifdef __cplusplus
extern "C" {
#endif

struct ObjWrapper
{
  MyObj obj;
};

/* return a newly created object */
struct ObjWrapper* createObj()
{
  return new ObjWrapper;
}

/* delete an object */
void deleteObj(struct ObjWrapper* wrapper)
{
  assert(wrapper);
  delete wrapper;
}

/* do something on an object */
void printThing(const struct ObjWrapper* wrapper)
{
  assert(wrapper);
  wrapper->obj.printThing();
}

/* get value from object */
int getThing(const struct ObjWrapper* wrapper)
{
  assert(wrapper);
  return wrapper->obj.getThing();
}

/* set value on object */
void setThing(struct ObjWrapper* wrapper, int thing)
{
  assert(wrapper);
  wrapper->obj.setThing(thing);
}

#ifdef __cplusplus
}
#endif

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