How to call multiple C++ functions that are in the same file

Hello everyone,

I am working on a project where I need OpenCV, which is in Cpp. I successfully made the Cpp file, the wrapper, and everything is working fine. However I would now like to display some variables from the Cpp code in the UI. To do so, I made a second function in my Cpp file (which returns the variables I want) and a second wrapper for that function, however I get this error:

Code Block 
Undefined symbols for architecture arm64:
  "_OBJC_CLASS_$_dataAccessBridge", referenced from:
      objc-class-ref in ViewController.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

This is how my code looks like:

• The cpp file, called imageProcessing.cpp:

Code Block 
#include "imageProcessing.hpp"
using namespace cv;
using namespace std;
Mat image_processing::detect_line(Mat image) {
    /*colouring the bright parts of the image in red, just to make sure everything is working*/
    Mat img;
    cvtColor(image, img, CV_RGB2GRAY);
    for(int i = 0; i < image.rows; i) {
        for(int j = 0; j < image.cols; j) {
            if (img.at<uchar>(i, j) > 127) {
                image.at<Vec3b>(i, j)[0] = 255;
                image.at<Vec3b>(i, j)[1] = 0;
                image.at<Vec3b>(i, j)[2] = 0;
            }
        }
    }
    return image;
}
string image_processing::dataAccess() {
    return "hello world from c++";
}

• The hpp file, called imageProcessing.hpp:

Code Block 
#ifndef imageProcessing_hpp
#define imageProcessing_hpp
#include <stdio.h>
#include <opencv2/opencv.hpp>
#include <string>
using namespace cv;
using namespace std;
class image_processing {
public:
    Mat detect_line(Mat image);
    string dataAccess();
};
#endif /* imageProcessing_hpp */

• The .h file of the first wrapper (which is working fine), called imageProcessingBridge.h:

Code Block 
#ifndef imageProcessingBridge_h
#define imageProcessingBridge_h
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
@interface imageProcessingBridge : NSObject
- (UIImage *) imageProcessingIn: (UIImage *) image;
@end
#endif /* imageProcessingBridge_h */

• The .mm file of the first wrapper, called imageProcessingBridge.mm:

Code Block 
#import <opencv2/opencv.hpp>
#import <opencv2/imgcodecs/ios.h>
#import <Foundation/Foundation.h>
#import "imageProcessingBridge.h"
#import "imageProcessing.hpp"
@implementation imageProcessingBridge
- (UIImage *) imageProcessingIn:(UIImage *)image {
    /* convert UI image to mat */
    cv::Mat opencvImage;
    UIImageToMat(image, opencvImage, true);
    /* convert colorspace */
    cv::Mat convertedColorSpaceImage;
    cv::cvtColor(opencvImage, convertedColorSpaceImage, CV_RGBA2RGB);
    /* Run image processing algorithm */
    image_processing imageProcessing;
    cv::Mat processedImage = imageProcessing.detect_line(convertedColorSpaceImage);
    /* convert mat to UI image and return it to the caller */
    return MatToUIImage(processedImage);
}
@end

• The .h file of the second wrapper (which is not working), called dataAccessBridge.h:

Code Block 
#ifndef dataAccessBridge_h
#define dataAccessBridge_h
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
@interface dataAccessBridge : NSObject
- (NSString *) dataAccesOut;
@end
#endif /* dataAccessBridge_h */

• The .mm file of the second wrapper (which is not working), called dataAccessBridge.mm:

Note that the words NSString, image(underscore)processing, stringWithCString, dataAccess(), c(underscore)str(), encoding:NSUTF8StringEncoding are not coloured, which they become if I copy paste this code in the first wrapper.

Code Block 
#import <Foundation/Foundation.h>
#import "dataAccessBridge.h"
#import "imageProcessing.hpp"
@implementation dataAccessBridge
- (NSString *) dataAccessOut {
    image_processing data_access;
    return [NSString
            stringWithCString:data_access.dataAccess().c_str()
            encoding:NSUTF8StringEncoding];
}
@end

• The ImageProcessingTest-Bridging-Header.h file:

Code Block 
#import "imageProcessingBridge.h"
#import "dataAccessBridge.h"

• How I call the functions in the ViewController.swift file:

Code Block 
let processed_image = imageProcessingBridge().imageProcessing(in: image)
self.frameRate.text = dataAccessBridge().imageProcessing()

This last line of code also gives me this error:

Code Block 
Value of type 'dataAccessBridge' has no member 'imageProcessing'

But I would assume this is a result of the first error. I also notice that the icons next the dataAccessBridge.h and dataAccessBridge.mm file are greyed out.

What am I doing wrong? Or is it not possible to have 2 wrappers for the same Cpp file? In which case, how should I call the second function?

Thanks in advance