Errors risen on std::find over a vector of objects - c++

Question

In this function I would like to search 'vertex' in the vector queue.
bool PriorityQueue::contains(VertexPriority vertex) const {
return (std::find(queue.begin(), queue.end(), vertex) != queue.end());
}
The vector queue is an instance of this object:
std::vector<VertexPriority> queue;
And my operator overloading is this:
bool operator==(const VertexPriority& v){ return (v.vertex == vertex); }
How can I solve this error?
The errors risen are the next ones, and at the beginning of every error there is the following path:
C:\Dev-Cpp\include\c++\3.4.2\bits\stl_algo.h
In function `_RandomAccessIterator std::find(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Tp = VertexPriority]':
instantiated from `_InputIterator std::find(_InputIterator, _InputIterator, const _Tp&) [with _InputIterator = __gnu_cxx::__normal_iterator > >, _Tp = VertexPriority]'
passing const VertexPriority' asthis' argument of `bool VertexPriority::operator==(const VertexPriority&)' discards qualifiers

Answer 1

Make the operator const:
bool operator==(const VertexPriority& v) const
{
return (v.vertex == vertex);
}
Note that this makes the member function const itself, which enables the compiler to invoke it on a const object of VertexPriority type.
This is actually precisely what the compiler sais:
passing const VertexPriority' as this' argument of `bool VertexPriority::operator==(const VertexPriority&)' discards qualifiers
The VertexPriority object that is const, is being passed as the implicit thisargument to member functions.