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Smart Pointers to boost your code

, 27 Sep 2004
A beginner's introduction to the smart pointers provided by the boost library.
Prize winner in Competition "MFC/C++ Aug 2004"
#ifndef BOOST_DETAIL_SHARED_COUNT_HPP_INCLUDED
#define BOOST_DETAIL_SHARED_COUNT_HPP_INCLUDED

// MS compatible compilers support #pragma once

#if defined(_MSC_VER) && (_MSC_VER >= 1020)
# pragma once
#endif

//
//  detail/shared_count.hpp
//
//  Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd.
//
//  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.
//

#include <boost/config.hpp>

#if defined(BOOST_SP_USE_STD_ALLOCATOR) && defined(BOOST_SP_USE_QUICK_ALLOCATOR)
# error BOOST_SP_USE_STD_ALLOCATOR and BOOST_SP_USE_QUICK_ALLOCATOR are incompatible.
#endif

#include <boost/checked_delete.hpp>
#include <boost/throw_exception.hpp>
#include <boost/detail/lightweight_mutex.hpp>

#if defined(BOOST_SP_USE_QUICK_ALLOCATOR)
#include <boost/detail/quick_allocator.hpp>
#endif

#include <memory>           // std::auto_ptr, std::allocator
#include <functional>       // std::less
#include <exception>        // std::exception
#include <new>              // std::bad_alloc
#include <typeinfo>         // std::type_info in get_deleter
#include <cstddef>          // std::size_t

#ifdef __BORLANDC__
# pragma warn -8026     // Functions with excep. spec. are not expanded inline
# pragma warn -8027     // Functions containing try are not expanded inline
#endif

namespace boost
{

// Debug hooks

#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)

void sp_scalar_constructor_hook(void * px, std::size_t size, void * pn);
void sp_array_constructor_hook(void * px);
void sp_scalar_destructor_hook(void * px, std::size_t size, void * pn);
void sp_array_destructor_hook(void * px);

#endif


// The standard library that comes with Borland C++ 5.5.1
// defines std::exception and its members as having C calling
// convention (-pc). When the definition of bad_weak_ptr
// is compiled with -ps, the compiler issues an error.
// Hence, the temporary #pragma option -pc below. The version
// check is deliberately conservative.

#if defined(__BORLANDC__) && __BORLANDC__ == 0x551
# pragma option push -pc
#endif

class bad_weak_ptr: public std::exception
{
public:

    virtual char const * what() const throw()
    {
        return "boost::bad_weak_ptr";
    }
};

#if defined(__BORLANDC__) && __BORLANDC__ == 0x551
# pragma option pop
#endif

namespace detail
{

class sp_counted_base
{
private:

    typedef detail::lightweight_mutex mutex_type;

public:

    sp_counted_base(): use_count_(1), weak_count_(1)
    {
    }

    virtual ~sp_counted_base() // nothrow
    {
    }

    // dispose() is called when use_count_ drops to zero, to release
    // the resources managed by *this.

    virtual void dispose() = 0; // nothrow

    // destruct() is called when weak_count_ drops to zero.

    virtual void destruct() // nothrow
    {
        delete this;
    }

    virtual void * get_deleter(std::type_info const & ti) = 0;

    void add_ref_copy()
    {
#if defined(BOOST_HAS_THREADS)
        mutex_type::scoped_lock lock(mtx_);
#endif
        ++use_count_;
    }

    void add_ref_lock()
    {
#if defined(BOOST_HAS_THREADS)
        mutex_type::scoped_lock lock(mtx_);
#endif
        if(use_count_ == 0) boost::throw_exception(boost::bad_weak_ptr());
        ++use_count_;
    }

    void release() // nothrow
    {
        {
#if defined(BOOST_HAS_THREADS)
            mutex_type::scoped_lock lock(mtx_);
#endif
            long new_use_count = --use_count_;

            if(new_use_count != 0) return;
        }

        dispose();
        weak_release();
    }

    void weak_add_ref() // nothrow
    {
#if defined(BOOST_HAS_THREADS)
        mutex_type::scoped_lock lock(mtx_);
#endif
        ++weak_count_;
    }

    void weak_release() // nothrow
    {
        long new_weak_count;

        {
#if defined(BOOST_HAS_THREADS)
            mutex_type::scoped_lock lock(mtx_);
#endif
            new_weak_count = --weak_count_;
        }

        if(new_weak_count == 0)
        {
            destruct();
        }
    }

    long use_count() const // nothrow
    {
#if defined(BOOST_HAS_THREADS)
        mutex_type::scoped_lock lock(mtx_);
#endif
        return use_count_;
    }

private:

    sp_counted_base(sp_counted_base const &);
    sp_counted_base & operator= (sp_counted_base const &);

    long use_count_;        // #shared
    long weak_count_;       // #weak + (#shared != 0)

#if defined(BOOST_HAS_THREADS) || defined(BOOST_LWM_WIN32)
    mutable mutex_type mtx_;
#endif
};

#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)

template<class T> void cbi_call_constructor_hook(sp_counted_base * pn, T * px, checked_deleter<T> const &, int)
{
    boost::sp_scalar_constructor_hook(px, sizeof(T), pn);
}

template<class T> void cbi_call_constructor_hook(sp_counted_base *, T * px, checked_array_deleter<T> const &, int)
{
    boost::sp_array_constructor_hook(px);
}

template<class P, class D> void cbi_call_constructor_hook(sp_counted_base *, P const &, D const &, long)
{
}

template<class T> void cbi_call_destructor_hook(sp_counted_base * pn, T * px, checked_deleter<T> const &, int)
{
    boost::sp_scalar_destructor_hook(px, sizeof(T), pn);
}

template<class T> void cbi_call_destructor_hook(sp_counted_base *, T * px, checked_array_deleter<T> const &, int)
{
    boost::sp_array_destructor_hook(px);
}

template<class P, class D> void cbi_call_destructor_hook(sp_counted_base *, P const &, D const &, long)
{
}

#endif

//
// Borland's Codeguard trips up over the -Vx- option here:
//
#ifdef __CODEGUARD__
# pragma option push -Vx-
#endif

template<class P, class D> class sp_counted_base_impl: public sp_counted_base
{
private:

    P ptr; // copy constructor must not throw
    D del; // copy constructor must not throw

    sp_counted_base_impl(sp_counted_base_impl const &);
    sp_counted_base_impl & operator= (sp_counted_base_impl const &);

    typedef sp_counted_base_impl<P, D> this_type;

public:

    // pre: initial_use_count <= initial_weak_count, d(p) must not throw

    sp_counted_base_impl(P p, D d): ptr(p), del(d)
    {
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        detail::cbi_call_constructor_hook(this, p, d, 0);
#endif
    }

    virtual void dispose() // nothrow
    {
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        detail::cbi_call_destructor_hook(this, ptr, del, 0);
#endif
        del(ptr);
    }

    virtual void * get_deleter(std::type_info const & ti)
    {
        return ti == typeid(D)? &del: 0;
    }

#if defined(BOOST_SP_USE_STD_ALLOCATOR)

    void * operator new(std::size_t)
    {
        return std::allocator<this_type>().allocate(1, static_cast<this_type *>(0));
    }

    void operator delete(void * p)
    {
        std::allocator<this_type>().deallocate(static_cast<this_type *>(p), 1);
    }

#endif

#if defined(BOOST_SP_USE_QUICK_ALLOCATOR)

    void * operator new(std::size_t)
    {
        return quick_allocator<this_type>::alloc();
    }

    void operator delete(void * p)
    {
        quick_allocator<this_type>::dealloc(p);
    }

#endif
};

#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)

int const shared_count_id = 0x2C35F101;
int const   weak_count_id = 0x298C38A4;

#endif

class weak_count;

class shared_count
{
private:

    sp_counted_base * pi_;

#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
    int id_;
#endif

    friend class weak_count;

public:

    shared_count(): pi_(0) // nothrow
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
    {
    }

    template<class P, class D> shared_count(P p, D d): pi_(0)
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
    {
#ifndef BOOST_NO_EXCEPTIONS

        try
        {
            pi_ = new sp_counted_base_impl<P, D>(p, d);
        }
        catch(...)
        {
            d(p); // delete p
            throw;
        }

#else

        pi_ = new sp_counted_base_impl<P, D>(p, d);

        if(pi_ == 0)
        {
            d(p); // delete p
            boost::throw_exception(std::bad_alloc());
        }

#endif
    }

#ifndef BOOST_NO_AUTO_PTR

    // auto_ptr<Y> is special cased to provide the strong guarantee

    template<class Y>
    explicit shared_count(std::auto_ptr<Y> & r): pi_(new sp_counted_base_impl< Y *, checked_deleter<Y> >(r.get(), checked_deleter<Y>()))
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
    {
        r.release();
    }

#endif 

    ~shared_count() // nothrow
    {
        if(pi_ != 0) pi_->release();
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        id_ = 0;
#endif
    }

    shared_count(shared_count const & r): pi_(r.pi_) // nothrow
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
    {
        if(pi_ != 0) pi_->add_ref_copy();
    }

    explicit shared_count(weak_count const & r); // throws bad_weak_ptr when r.use_count() == 0

    shared_count & operator= (shared_count const & r) // nothrow
    {
        sp_counted_base * tmp = r.pi_;
        if(tmp != 0) tmp->add_ref_copy();
        if(pi_ != 0) pi_->release();
        pi_ = tmp;

        return *this;
    }

    void swap(shared_count & r) // nothrow
    {
        sp_counted_base * tmp = r.pi_;
        r.pi_ = pi_;
        pi_ = tmp;
    }

    long use_count() const // nothrow
    {
        return pi_ != 0? pi_->use_count(): 0;
    }

    bool unique() const // nothrow
    {
        return use_count() == 1;
    }

    friend inline bool operator==(shared_count const & a, shared_count const & b)
    {
        return a.pi_ == b.pi_;
    }

    friend inline bool operator<(shared_count const & a, shared_count const & b)
    {
        return std::less<sp_counted_base *>()(a.pi_, b.pi_);
    }

    void * get_deleter(std::type_info const & ti) const
    {
        return pi_? pi_->get_deleter(ti): 0;
    }
};

#ifdef __CODEGUARD__
# pragma option pop
#endif


class weak_count
{
private:

    sp_counted_base * pi_;

#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
    int id_;
#endif

    friend class shared_count;

public:

    weak_count(): pi_(0) // nothrow
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(weak_count_id)
#endif
    {
    }

    weak_count(shared_count const & r): pi_(r.pi_) // nothrow
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
    {
        if(pi_ != 0) pi_->weak_add_ref();
    }

    weak_count(weak_count const & r): pi_(r.pi_) // nothrow
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
    {
        if(pi_ != 0) pi_->weak_add_ref();
    }

    ~weak_count() // nothrow
    {
        if(pi_ != 0) pi_->weak_release();
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        id_ = 0;
#endif
    }

    weak_count & operator= (shared_count const & r) // nothrow
    {
        sp_counted_base * tmp = r.pi_;
        if(tmp != 0) tmp->weak_add_ref();
        if(pi_ != 0) pi_->weak_release();
        pi_ = tmp;

        return *this;
    }

    weak_count & operator= (weak_count const & r) // nothrow
    {
        sp_counted_base * tmp = r.pi_;
        if(tmp != 0) tmp->weak_add_ref();
        if(pi_ != 0) pi_->weak_release();
        pi_ = tmp;

        return *this;
    }

    void swap(weak_count & r) // nothrow
    {
        sp_counted_base * tmp = r.pi_;
        r.pi_ = pi_;
        pi_ = tmp;
    }

    long use_count() const // nothrow
    {
        return pi_ != 0? pi_->use_count(): 0;
    }

    friend inline bool operator==(weak_count const & a, weak_count const & b)
    {
        return a.pi_ == b.pi_;
    }

    friend inline bool operator<(weak_count const & a, weak_count const & b)
    {
        return std::less<sp_counted_base *>()(a.pi_, b.pi_);
    }
};

inline shared_count::shared_count(weak_count const & r): pi_(r.pi_)
#if defined(BOOST_SP_ENABLE_DEBUG_HOOKS)
        , id_(shared_count_id)
#endif
{
    if(pi_ != 0)
    {
        pi_->add_ref_lock();
    }
    else
    {
        boost::throw_exception(boost::bad_weak_ptr());
    }
}

} // namespace detail

} // namespace boost

#ifdef __BORLANDC__
# pragma warn .8027     // Functions containing try are not expanded inline
# pragma warn .8026     // Functions with excep. spec. are not expanded inline
#endif

#endif  // #ifndef BOOST_DETAIL_SHARED_COUNT_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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