Unhandled exception. Access violation reading location - c++

Question

I'm currently writing a BLOB detection algorithm. When I run it I get the error
Unhandled exception at 0x00007FF70476CDA7 in blobdetector.exe: 0xC0000005:
Access violation reading location 0x0000007BDDA0347D.
This is the part of the code where it breaks.
The debugger stops at the last line before the if(val == 255)
void connectivity(BlobBurnMat temp, size_t y, size_t x){
uchar* ptr = (uchar*)temp.getTemp().data;
size_t min_y = temp.getTemp().rows, min_x = temp.getTemp().cols, max_y = 0, max_x = 0;
for (int i = 0; i < 4; i++){
int kernel[4][2] = { { 0, 1 }, { 1, 0 }, { 0, -1 }, { -1, 0 } };
int temp_y = y + kernel[i][0];
int temp_x = x + kernel[i][1];
uchar val = ptr[temp.getTemp().step * temp_y + temp_x]; //breaks here
if (val == 255) {
temp.setValZero(x, y);
x = temp_x;
y = temp_y;
if (temp_x > max_x)
max_x = temp_x;
if(temp_x < min_x)
min_x = temp_x;
if (temp_y > max_y)
max_y = temp_y;
if (temp_y < min_y)
min_y = temp_y;
int pass_x = ((max_x - min_x) / 2);
int pass_y((max_y - min_y) / 2);
setCoordinates(pass_x, pass_y);
connectivity(temp, y, x);
}
}
}
Here is the rest of the code. The above connectivity() method is in the BLOB class.
#include "opencv2/opencv.hpp"
#include<stdio.h>
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
using namespace cv;
using namespace std;
class BlobBurnMat{
//Class used to store a copy of the source image where pixels get burned in BLOB finding.
private:
Mat temp;
public:
void initiate(Mat temp){
this -> temp = temp;
}
Mat getTemp(){
return temp;
}
void setValZero(int x, int y){
temp.at<uchar>(y, x) = 0;
}
};
class BLOB{
//BLOB class to store coordinates of the BLOBs before they are added to the vector-array-list-thingamajig
private:
int x, y;
public:
void setCoordinates(int x, int y){
this->x = x;
this->y = y;
}
int getX(){
return x;
}
int getY(){
return y;
}
void connectivity(BlobBurnMat temp){
connectivity(temp, x, y);
}
void connectivity(BlobBurnMat temp, int y, int x){
uchar* ptr = (uchar*)temp.getTemp().data;
int min_y = temp.getTemp().rows, min_x = temp.getTemp().cols, max_y = 0, max_x = 0;
for (int i = 0; i < 4; i++){
int kernel[4][2] = { { 0, 1 }, { 1, 0 }, { 0, -1 }, { -1, 0 } };
int temp_y = y + kernel[i][0];
int temp_x = x + kernel[i][1];
uchar val = ptr[temp.getTemp().step * temp_y + temp_x];
if (val == 255) {
temp.setValZero(x, y);
x = temp_x;
y = temp_y;
if (temp_x > max_x)
max_x = temp_x;
if(temp_x < min_x)
min_x = temp_x;
if (temp_y > max_y)
max_y = temp_y;
if (temp_y < min_y)
min_y = temp_y;
int pass_x = ((max_x - min_x) / 2);
int pass_y((max_y - min_y) / 2);
setCoordinates(pass_x, pass_y);
connectivity(temp, y, x);
}
}
}
};
vector<Point2i> getBlobCoordinates(Mat src){
BlobBurnMat *temp = new BlobBurnMat();
temp->initiate(src.clone());
vector<Point2i> blobCoordinates; //Vector holding all BLOB coordinates
Point2i blobCoords; //Coordinates of a single BLOB
//Go through the binary matrix looking for white pixels.
for (size_t y = 0; y < src.rows; y++) {
for (size_t x = 0; x < src.cols; x++) {
uchar* ptr = (uchar*)temp->getTemp().data;
uchar val = ptr[temp->getTemp().step * y + x];
if (val == 255){
BLOB *blob = new BLOB();
blob->setCoordinates(x, y);
blob->connectivity(*temp);
blobCoords.x = blob->getX();
blobCoords.y = blob->getY();
blobCoordinates.push_back(blobCoords); //add a new element to the vector.
}
}
}
return blobCoordinates;
}
int main(int, char){
Mat src;
String path = "C:/Users/Runagar/Desktop/segment.jpg";
src = imread(path, 0);
if (src.data && !src.empty()){
imshow("blobie", src);
}
else cout << "You fucked up ";
vector <Point2i> blobCoordinates = getBlobCoordinates(src);
for (int k = 0; k < blobCoordinates.size(); k++)
cout << blobCoordinates[k];
waitKey(0);
return 0;
}
Thanks in advance!

Answer 1

When i == 2 and x == 0, temp_x == -1 so you read 1 byte before ptr outside the memory allocaded.
You have to handle the case when x == 0, y == 0, x == src.cols and y == src.rows.

Answer 2

Mat getTemp() returns a copy of your Mat instance.
uchar* ptr = (uchar*)temp.getTemp().data; retrieves a pointer from a temporary Mat instance. This instance is then immediately deleted, and you're left with a pointer to (presumably) properly deleted data.
Try Mat & getTemp() instead.