CascadeClassifier::detectMultiScale doesn't work with C++

Question

I'm using OpenCV and CascadeClassifier::detectMultiScale for facial detection. My problem is that it seems to cause memory corruption on the output vector<Rect>. The vector is correctly filled with Rects, but it causes a crash when the vector is deallocated.

This only occurs when compiling a Debug build. The error message is a Debug Assertion Failed, which makes me wonder if there is a problem that also occurs in Release build, and the assert simply isn't checked.

Could this be a bug with OpenCV? Or is it just that I'm doing something wrong with how I handle my vectors?

The following code snippet shows an example code to reproduce the bug:

#include <opencv2/opencv.hpp>
using namespace cv;

int main(array<System::String ^> ^args)
{
    VideoCapture video(0);
    Mat frame;
    CascadeClassifier classifier("haarcascade_frontalface_default.xml");

    while (waitKey(1000 / 30) != 'q')
    {
        video >> frame;

        vector<Rect> faces;
        classifier.detectMultiScale(frame, faces);
        for (int i = 0; i < faces.size(); i++)
            rectangle(frame, faces[i], Scalar(255, 255, 255));

        imshow("Camera", frame);
    } // <<< The crash occurs here when the faces vector is released
}

I get the following error message:

Debug Assertion Failed!

Program: MyProgram.exe File: minkernel\crts\ucrt\src\appcrt\heap\debug_heap.cpp Line: 892

Expression: is_block_type_valid(header->_block_use)

Answer 1

I had this same issue. I solved it by passing a dereferenced global pointer to the function.

I.e.

    std::vector<cv::Rect>* faces = nullptr;

    void init()
    {
        faces = new std::vector<cv::Rect>; //never call delete whatever you do
    }

    void findSomeFaces()
    {
        cascade->detectMultiScale(image_source, *faces);
    }