Is it safe to use negative integers with size_t?

I just saw some C++ code like this. It was using a condition to decide whether to walk forward or backward through a std::vector. The compiler doesn't complain, but I thought size_t was unsigned. Is this dangerous?

vector<int> v { 1,2,3,4,5 };    
bool rev = true;

size_t start, end, di;
if (rev) {
    start = v.size()-1;
    end = -1;
    di = -1;
}
else {
    start = 0;
    end = v.size();
    di = 1;
}

for (auto i=start; i!=end; i+=di) {
    cout << v[i] << endl;
}