Sorting a vector of struct causes Visual Studio to throw a popup with "Debug Assertion Failed"

I have a simple Rectangle struct with x, y, width, and height, which I thought would be easy to work with vectors but it turned out to be pretty messy.

This is what I have in main:

vector<CvRect> v;
v.push_back(cvRect(50,0,50, 50));
v.push_back(cvRect(150, 0, 50,50));
v.push_back(cvRect(10开发者_如何转开发0, 0, 50,50));
sort(v.begin(), v.end(), rectangleCmpByPosition);

I have this declared in my header

static int rectangleCmpByPosition(const CvRect &a, const CvRect &b);

with this as definition:

static int rectangleCmpByPosition(const CvRect& a, const CvRect &b){
    if (a.y != b.y){
        return a.y - b.y;
    }else{
        return a.x - b.x;
    }
}

And visual studio throws this error message at me

I spent the whole afternoon googling to see what I did wrong but I can't find the cause. Please help.

Your comparator is incorrect. The comparator needs to return a bool: true if a is "less than" b and false otherwise. It needs to provide a strict weak ordering.