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Smart Pointers to boost your code

, 27 Sep 2004 CPOL
A beginner's introduction to the smart pointers provided by the boost library.
#ifndef BOOST_WEAK_PTR_HPP_INCLUDED
#define BOOST_WEAK_PTR_HPP_INCLUDED

//
//  weak_ptr.hpp
//
//  Copyright (c) 2001, 2002, 2003 Peter Dimov
//
//  Permission to copy, use, modify, sell and distribute this software
//  is granted provided this copyright notice appears in all copies.
//  This software is provided "as is" without express or implied
//  warranty, and with no claim as to its suitability for any purpose.
//
//  See http://www.boost.org/libs/smart_ptr/weak_ptr.htm for documentation.
//

#include <boost/shared_ptr.hpp>

#ifdef BOOST_MSVC  // moved here to work around VC++ compiler crash
# pragma warning(push)
# pragma warning(disable:4284) // odd return type for operator->
#endif

namespace boost
{

template<class T> class weak_ptr
{
private:

    // Borland 5.5.1 specific workarounds
    typedef weak_ptr<T> this_type;

public:

    typedef T element_type;

    weak_ptr(): px(0), pn() // never throws in 1.30+
    {
    }

//  generated copy constructor, assignment, destructor are fine


//
//  The "obvious" converting constructor implementation:
//
//  template<class Y>
//  weak_ptr(weak_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws
//  {
//  }
//
//  has a serious problem.
//
//  r.px may already have been invalidated. The px(r.px)
//  conversion may require access to *r.px (virtual inheritance).
//
//  It is not possible to avoid spurious access violations since
//  in multithreaded programs r.px may be invalidated at any point.
//

    template<class Y>
    weak_ptr(weak_ptr<Y> const & r): pn(r.pn) // never throws
    {
        px = r.lock().get();
    }

    template<class Y>
    weak_ptr(shared_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws
    {
    }

#if !defined(BOOST_MSVC) || (BOOST_MSVC > 1200)

    template<class Y>
    weak_ptr & operator=(weak_ptr<Y> const & r) // never throws
    {
        px = r.lock().get();
        pn = r.pn;
        return *this;
    }

    template<class Y>
    weak_ptr & operator=(shared_ptr<Y> const & r) // never throws
    {
        px = r.px;
        pn = r.pn;
        return *this;
    }

#endif

    shared_ptr<T> lock() const // never throws
    {
#if defined(BOOST_HAS_THREADS)

        // optimization: avoid throw overhead
        if(expired())
        {
            return shared_ptr<element_type>();
        }

        try
        {
            return shared_ptr<element_type>(*this);
        }
        catch(bad_weak_ptr const &)
        {
            // Q: how can we get here?
            // A: another thread may have invalidated r after the use_count test above.
            return shared_ptr<element_type>();
        }

#else

        // optimization: avoid try/catch overhead when single threaded
        return expired()? shared_ptr<element_type>(): shared_ptr<element_type>(*this);

#endif
    }

    long use_count() const // never throws
    {
        return pn.use_count();
    }

    bool expired() const // never throws
    {
        return pn.use_count() == 0;
    }

    void reset() // never throws in 1.30+
    {
        this_type().swap(*this);
    }

    void swap(this_type & other) // never throws
    {
        std::swap(px, other.px);
        pn.swap(other.pn);
    }

    void _internal_assign(T * px2, detail::shared_count const & pn2)
    {
        px = px2;
        pn = pn2;
    }

    template<class Y> bool _internal_less(weak_ptr<Y> const & rhs) const
    {
        return pn < rhs.pn;
    }

// Tasteless as this may seem, making all members public allows member templates
// to work in the absence of member template friends. (Matthew Langston)

#ifndef BOOST_NO_MEMBER_TEMPLATE_FRIENDS

private:

    template<class Y> friend class weak_ptr;
    template<class Y> friend class shared_ptr;

#endif

    T * px;                     // contained pointer
    detail::weak_count pn;      // reference counter

};  // weak_ptr

template<class T, class U> inline bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b)
{
    return a._internal_less(b);
}

template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b)
{
    a.swap(b);
}

// deprecated, provided for backward compatibility
template<class T> shared_ptr<T> make_shared(weak_ptr<T> const & r)
{
    return r.lock();
}

} // namespace boost

#ifdef BOOST_MSVC
# pragma warning(pop)
#endif    

#endif  // #ifndef BOOST_WEAK_PTR_HPP_INCLUDED

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peterchen
Klippel
Germany Germany
Peter is tired of being called "Mr. Chen", even so certain individuals insist on it. No, he's not chinese.
 
Peter has seen lots of boxes you youngsters wouldn't even accept as calculators. He is proud of having visited the insides of a 16 Bit Machine.
 
In his spare time he ponders new ways of turning groceries into biohazards, or tries to coax South American officials to add some stamps to his passport.
 
Beyond these trivialities Peter works for Klippel[^], a small german company that wants to make mankind happier by selling them novel loudspeaker measurement equipment.
 

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