Visual C++ find line causing "Debug Assertion failed"

Question

I am trying to get a C++ program that works fine when compiled with gcc to work properly on Visual C++. My problem is that I am now getting the following error:

Debug Assertion Failed!

Program: C:\WINDOWS\SYSTEM32\MSVCP110D.dll
File: c:\program files (x86)\microsoft visual studio 11.0\vc\include\vector
Line: 1140

Expression: vector subscript out of range

My real problem is that I do not know when or where this happens. By pressing break in the error window I am merely taken to the part of the vector class where the exception ultimately happened. I want to find the place in my application which actually caused it. I have managed to narrow it down to this block of code:

    for(unsigned int i=0;i<openPolygonList.size();i++)//error somewhere in here
    {
        if (openPolygonList[i].size() < 1) continue;
        for(unsigned int j=0;j<openPolygonList.size();j++)
        {
            if (openPolygonList[j].size() < 1) continue;

            Point diff = openPolygonList[i][openPolygonList[i].size()-1] - openPolygonList[j][0];
            int64_t distSquared = vSize2(diff);

            if (distSquared < 2 * 2)
            {
                if (i == j)
                {
                    polygonList.push_back(openPolygonList[i]);
                    openPolygonList.erase(openPolygonList.begin() + i);
                }else{
                    for(unsigned int n=0; n<openPolygonList[j].size(); n++)
                        openPolygonList[i].push_back(openPolygonList[j][n]);

                    openPolygonList[j].clear();
                }
            }
        }
    }

Simply placing breakpoints at each line where a vector is used is not an option because the loop iterates thousands of times and having to press continue each time will literally take me hours. Is there any way I can tell the debugger to brake on the appropriate line once the error occurs. It can help me to inspect the variables and to determine which variable is out of range?

Answer 1

I think the problem is that you are erasing members of the vector you are iterating through. What happens if you erase the first element?

i 1 2 3 Ei v1 v2 v3

If we erase 1 when i = 1, our vector indices and values are below and now i = 2.

i 1 2 Ei v2 v3

Ultimately, I think you can iterate past the end of the vector causing you to have a pointer that points past the end of the vector. Complete guess work here, but there is probably an easier way to do what you're trying to do. I just can't figure out what you're trying to do.

It looks like you're trying to invert the rows and columns of a two dimensional array while storing the diagonal polygons int he array into a new array. Anyway, yes, but a red circle at the beginning of the for loop and walk through your code line by line.

I would create temp vectors and then modify those ones in the for loop and then replace the vector openPolygonList.

Answer 2

The debugger cannot predict the future, but you can tell it to break on all exceptions, (ctrl+atl+e and tick all the boxes under "Thrown"). When the assert happens walk down the call stack to your code it will tell you which line is causing the problem.