boost::shared_ptr

    element_type * px;                 // contained pointer
    boost::detail::shared_count pn;    // reference counter

　　两个重要的成员：

px， 保存被包裹的指针

pn， 保存引用计数，pn._pi是类型为sp_counted_base的对象

class sp_counted_base
{
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)

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

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

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

    virtual void * get_deleter( sp_typeinfo const & ti ) = 0;
    virtual void * get_untyped_deleter() = 0;

    void add_ref_copy()
    {
        BOOST_INTERLOCKED_INCREMENT( &use_count_ );
    }

    bool add_ref_lock() // true on success
    {
        for( ;; )
        {
            long tmp = static_cast< long const volatile& >( use_count_ );
            if( tmp == 0 ) return false;

#if defined( BOOST_MSVC ) && BOOST_WORKAROUND( BOOST_MSVC, == 1200 )

            // work around a code generation bug

            long tmp2 = tmp + 1;
            if( BOOST_INTERLOCKED_COMPARE_EXCHANGE( &use_count_, tmp2, tmp ) == tmp2 - 1 ) return true;

#else

            if( BOOST_INTERLOCKED_COMPARE_EXCHANGE( &use_count_, tmp + 1, tmp ) == tmp ) return true;

#endif
        }
    }

    void release() // nothrow
    {
        if( BOOST_INTERLOCKED_DECREMENT( &use_count_ ) == 0 )
        {
            dispose();
            weak_release();
        }
    }

    void weak_add_ref() // nothrow
    {
        BOOST_INTERLOCKED_INCREMENT( &weak_count_ );
    }

    void weak_release() // nothrow
    {
        if( BOOST_INTERLOCKED_DECREMENT( &weak_count_ ) == 0 )
        {
            destroy();
        }
    }

    long use_count() const // nothrow
    {
        return static_cast<long const volatile &>( use_count_ );
    }
};