Re: why boost:shared_ptr so slower?

From:

"Chris M. Thomasson" <no@spam.invalid>

Newsgroups:

comp.lang.c++

Date:

Sat, 22 Aug 2009 04:37:15 -0700

Message-ID:

<h6ol94$4s4$1@news.ett.com.ua>

"Juha Nieminen" <nospam@thanks.invalid> wrote in message
news:gqyjm.62$lD3.48@read4.inet.fi...

Sam wrote:

- Non-intrusive. We want to be able to use it for existing object types
(and even basic types if so desired).

Not a showstopper. Can be used with existing object types, and basic
types, by subclassing them.

Care to show me how you subclass a basic type?

- Works with incomplete types (the only place where the type must be
complete is when constructing the smart pointer).

Done.

Care to show me how you make an intrusive smart pointer work with
incomplete types?

For example, when the intrusive smart pointer needs to increment the
reference counter, how do you do it when the object type is incomplete?

Perhaps something along the lines of:
____________________________________________________________________
struct ref_base {
  unsigned m_count;
  virtual ~ref_base() = 0;
};

ref_base::~ref_base() {}

template<typename T>
class ptr {
  ref_base* m_ptr;

  void prv_init() const {
    if (m_ptr) m_ptr->m_count = 1;
  }

  void prv_inc() const {
    if (m_ptr) ++m_ptr->m_count;
  }

  void prv_dec() const {
    if (m_ptr) {
      if (! --m_ptr->m_count) {
        delete m_ptr;
      }
    }
  }

public:
  ptr(T* ptr = NULL) : m_ptr(static_cast<ref_base*>(ptr)) {
    prv_init();
  }

  ptr(ptr const& rhs) : m_ptr(rhs.m_ptr) {
    prv_inc();
  }

  ~ptr() {
    prv_dec();
  }

  ptr& operator = (ptr const& rhs) {
    rhs.prv_inc();
    prv_dec();
    m_ptr = rhs.m_ptr;
    return *this;
  }

  T* operator ->() {
    return static_cast<T*>(m_ptr);
  }
};
____________________________________________________________________

* The shared pointer becomes just a native pointer.

It's not like non-intrusive reference-counting smart pointers cannot
be made the size of one single pointer.