How to find a lower bound in a sorted vector

Question

I'm pretty new to C++ and do not understand all the concepts of the STL library, so bear with me. I wrote the following code snippet (pasted below) to find the lower_bound in a sorted vector. Although this code works fine in Release mode, it asserts in debug mode (VStudio-8). I believe this is because less_equal<int> is not a strictly weak ordering .

From the following thread: stl ordering - strict weak ordering

I do sort of understand that a weak ordering is imposed by the STL, but I'm still not very clear why?

In my case below I need to use less_equal<int> since I'm trying to find the nearest element to a given value in a sorted vector.

Is the code snippet below even valid? Also, is there a better way to do it? Also any insights/references to what exactly is weak and partial ordering would be helpful.

int main() {

  vector<int> dest;
  for(int i = 0;i <6;i++) {

     dest.push_back(i);
  }

  vector<int>::iterator i = 
  std::lower_bound(dest.begin(),dest.end(),4,less_equal< int >());

  return 1;

}