Error in function sort - c++

Question

I'm trying to use the sort function from STL, but it gives me an error during execution.
My compare function returns true if v is smaller then e:
bool smallerThan(VertexEntry &v, VertexEntry &e) {
if(v.v[0] < e.v[0]) return true;
else if(v.v[1] < e.v[1]) return true;
else if(v.v[2] < e.v[2]) return true;
return false;
}
and here is the call:
sort(vertices.begin(),vertices.end(),smallerThan);
The size of the vector is aprox 400 elements.
Can somebody help me solve my problem?
Thank you!!

Answer 1

Your comparison function is incorrect - it doesn't enforce strict weak ordering.
Use this:
bool smallerThan(VertexEntry const & v, VertexEntry const & e) {
if (v.v[0] < e.v[0])
return true;
else if(v.v[0] > e.v[0])
return false;
else if(v.v[1] < e.v[1])
return true;
else if(v.v[1] > e.v[1])
return false;
else if(v.v[2] < e.v[2])
return true;
return false;
}

Answer 2

Your comparison operator doesn't enforce strict weak ordering. If you're able to use boost one trick I've seen is to bind your object to a boost::tuple and use its strict weak operator<.
If you need to write it yourself, something like this should work:
bool smallerThan(const VertexEntry &v, const VertexEntry &e)
{
if(v.v[0] != e.v[0]) return v.v[0] < e.v[0];
else if(v.v[1] != e.v[1]) return v.v[1] != e.v[1];
else return v.v[2] < e.v[2];
}