iOS: Trouble initializing BOWImgDescriptorExtractor as top level variable

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Hi, I have a function generateBOW that returns a working BOWImgDescriptorExtractor. I want to set this as a top level variable in my ViewController.mm so that I can use it in another method (not viewDidLoad), but when I try, I am getting this error message: No matching constructor for cv::BOWImgDescriptorExtractor.

I tried just calling the function at the top level (outside of viewDidLoad), but it gave me this runtime error: Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)

How should I initialize bowDE so can I create the object with my function? Here is my code:

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

ImageProc *imgProc = [[ImageProc alloc] init];
BOWImgDescriptorExtractor bowDE; //Error here
Ptr<SVM> svm;

#pragma mark - System methods
- (void)viewDidLoad {
    [super viewDidLoad];

    bowDE = [imgProc generateBOW];
    svm = [imgProc generateSVM2: bowDE];
}

- (void)processImage:(Mat&)image {
    // I want to use bowDE here
}