Defining a hash function in TR1 unordered_map inside a struct

Question

According to this, it is possible to define an equality function in a TR1 unordered_map like this:

#include <tr1/unordered_map>
using namespace std;
using namespace std::tr1;
struct foo{
    ...
    bool operator==(const foo& b) const{
        return ..;
    }
};

unordered_map<foo,int> map;

Is it possible to define the hash function the same way?