Problem with functor, maybe I need operator== for my struct?

Hello, I am very rusty in C++ and I'm having some problems: I have a
std::set<SomeClass> and I need to look through the set to find if any of
the instances of SomeClass that it contains has a particular value for
one of its data members. I know I could simply iterate through the set
myself and check every SomeClass instance if any of them is a match, but
I tried to do it using functors instead.
The following simplified test program does not build, and it seems to be
that operator== is missing for the struct s1. Could you gurus help me
explain why my test program does not build and how I should fix it?

#include <algorithm>
#include <functional>
#include <set>

using namespace std;

struct s1
{
    s1(int u, int v) : u(u), v(v) {}

    int u, v;
};

struct OnV : public unary_function<s1, bool>
{
    explicit OnV(int v) : v(v) {}

    bool operator()(s1 const& inst) const
    {
       return v == inst.v;
    }

    int v;
};

int main()
{
    set<s1> a_set;

    find(a_set.begin(), a_set.end(), OnV(4711));
}

$ g++ -Wall -Wextra -std=c++98 -pedantic functor.cpp -o functor.exe
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algo.h: In
function ?_InputIterator std::__find(_InputIterator, _InputIterator,
const _Tp&, std::input_iterator_tag) [with _InputIterator =
std::_Rb_tree_const_iterator<s1>, _Tp = OnV]?:
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algo.h:3814:
instantiated from ?_IIter std::find(_IIter, _IIter, const _Tp&) [with
_IIter = std::_Rb_tree_const_iterator<s1>, _Tp = OnV]?
functor.cpp:30: instantiated from here
/usr/lib/gcc/i686-pc-cygwin/4.3.4/include/c++/bits/stl_algo.h:151:
error: no match for ?operator==? in
?__first.std::_Rb_tree_const_iterator<_Tp>::operator* [with _Tp = s1]()
== __val?

Thank you for reading and thanks for any help! Even though I could use
the alternative solution I mentioned above that does not involve
functors, I wanted to know what I did wrong with my functor and
hopefully learn something from your replies.

- Fencer