Stuck trying to do first exercise in C++ Template Metaprogramming

[Patrick Horgan]

I'm reading through C++ Template Metaprogramming and thought I was 
understanding things pretty well until I tried to do exercise 2-0 which 
wants you to make a add_const_ref<T> that returns T if it's already a 
reference type, else return T const&. I include my floundering below 
inline, which gets these errors in their current state. I'm really stuck.

g++ -ggdb test.cpp -o test
In file included from test.cpp:1:
add_const_ref.h:21: error: explicit specialization in non-namespace 
scope ‘class mine::add_const_ref<T>’
add_const_ref.h:22: error: template parameters not used in partial 
specialization:
add_const_ref.h:22: error: ‘T’


#if !defined ADD_CONST_REF
#define ADD_CONST_REF
#include <boost/type_traits/add_reference.hpp>
#include <boost/type_traits/is_reference.hpp>

namespace mine {

template <bool is_const_ref>struct acr_impl;

template<typename T>
class add_const_ref
{
bool static const is_const_ref=boost::is_reference<T>::value;

template<bool x>
struct acr_impl
{
typedef T type;
};

template<>
struct acr_impl<true>
{
typedef T type;
};
#if 0
template<>
struct acr_impl<false>
{
typedef boost::add_reference<T> type;
};

typedef acr_impl<is_const_ref>::type type;
#endif
};

};
#endif // ADD_CONST_REF