C++ Design pattern for extending an arbitrary standard conform allocator

I'm currently searching for the best way to extend an arbitrary standard conform allocator type. To be clear: I don't want to write a custom allocator. I just want to "add" a specific extension or behavior to an already existing one. I've crea开发者_如何学Cted a sample how this could look like. Please note, that the following code is just for illustration purpose.

#ifndef HPP_SMART_ALLOCATOR_INCLUDED
#define HPP_SMART_ALLOCATOR_INCLUDED


#include <memory>
#include <map>


template<typename T>
struct allocator_traits;

template<typename T, class allocator_type = std::allocator<T>>
class smart_allocator;


template<>
struct allocator_traits<void>
{
    typedef std::allocator<void>::const_pointer const_pointer;
    typedef std::allocator<void>::pointer       pointer;
    typedef std::allocator<void>::value_type    value_type;
};

template<typename T>
struct allocator_traits
{
    typedef typename std::allocator<T>::const_pointer   const_pointer;
    typedef typename std::allocator<T>::const_reference const_reference;
    typedef typename std::allocator<T>::difference_type difference_type;
    typedef typename std::allocator<T>::pointer         pointer;
    typedef typename std::allocator<T>::reference       reference;
    typedef typename std::allocator<T>::size_type       size_type;
    typedef typename std::allocator<T>::value_type      value_type;
};


template<class allocator_type>
class smart_allocator<void, allocator_type>
    : public allocator_traits<void>
{
public:
    template<typename U> struct rebind { typedef smart_allocator<U, typename allocator_type::rebind<U>::other> other; };
};

template<typename T, class allocator_type>
class smart_allocator
    : public  allocator_traits<T>,
      private allocator_type
{
public:
    using typename allocator_traits<T>::const_pointer;
    using typename allocator_traits<T>::const_reference;
    using typename allocator_traits<T>::difference_type;
    using typename allocator_traits<T>::pointer;
    using typename allocator_traits<T>::reference;
    using typename allocator_traits<T>::size_type;
    using typename allocator_traits<T>::value_type;
    template<typename U> struct rebind { typedef smart_allocator<U, typename allocator_type::rebind<U>::other> other; };

    smart_allocator() throw() /*noexcept*/;
    smart_allocator(allocator_type const&) throw() /*noexcept*/;
    virtual ~smart_allocator() throw();

    virtual ~smart_allocator()
    {
        std::map<pointer, size_type>::iterator i = this->m_map.begin();
        while (i != this->m_map.end())
        {
            this->allocator_type::deallocate(i->first, i->second);
            ++i;
        }
    }

    pointer allocate(size_type n, allocator_traits<void>::const_pointer hint = 0)
    {
        pointer p = this->allocator_type::allocate(n, hint);
        this->m_map.insert(std::pair<pointer, size_type>(p, n));
        return p;
    }

    void deallocate(pointer p, size_type n) /*noexcept*/
    {
        std::map<pointer, size_type>::iterator iter = this->m_map.find(p);
        if (iter != this->m_map.end())
            this->allocator_type::deallocate(iter->first, iter->second);
    }

    using allocator_type::address;
    using allocator_type::construct;
    using allocator_type::destroy;
    using allocator_type::max_size;

private:
    smart_allocator(smart_allocator const&) throw();
    smart_allocator& operator=(smart_allocator const&);

    std::map<pointer, size_type> m_map;
};


#endif /* HPP_SMART_ALLOCATOR_INCLUDED */

Please consider the following notes:

• The template argument allocator_type could be any standard conform type. It is not restricted to std::allocator. This is the same technique all STL implementations are using.
• We need to use private inheritance when deriving from allocator_type, because non of the std::allocator member functions are virtual. However, std::allocator& alloc = smart_allocator() wouldn't do what you might expect.

Do you think this is applicable?

You'd certainly need to implement a copy constructor and copy assignment operator, or your map may get mangled when containers pass the allocator around by value (specifically you could wind up double-deleting). There may be other considerations I didn't notice.

What came to mind immediately was the Decorator; as the reference notes, "Decorators are useful for adapting objects to new situations without re-writing the original object's code." Which, if I understand your question, sounds like what you are after.