gcc with boost for tr1, hash functor specialization

Question

I am using boost for tr1 instead of gcc 4.5's native tr1 libraries (by prioritizing boost tr1 headers over gcc headers). There is a compile problem specializing std::tr1::hash.

#include <functional>
#include <memory>

template <class A>
struct std::tr1::hash< std::tr1::shared_ptr<A> >
    : public unary_function< std::tr1::shared_ptr<A>, size_t>
{
    typedef std::tr1::shared_ptr<A> _Kty;

    size_t operator()(const _Kty& p) const
    {
        return std::tr1::hash<A*>()(p.get());
    }
};

The gcc error is:

error: specialization of 'template<class T> struct boost::hash' in different namespace

What will be the correct method to specialize the hash functor in tr1 namespace? Currently I am using a preprocessor directive to switch between specializing hash functor and overloading:

template<class A>
size_t hash_value(const shared_ptr<A> &ptr);

But I will prefer to specialize hash functor as hash_value is not standard.