ACE Value_Ptr的用法

       ACE的Value_Ptr和auto_ptr基本一致，不过就是多实现了一个clone的方法。使用Value_Ptr不需要有太多的注意,只要知道这个Smart pointer只能Swap数据，不能Share数据。

  /**
   * @struct VP_traits
   *
   * @brief @c Value_Ptr traits template structure.
   *
   * The @c Value_Ptr template class delegates some operations to this
   * template traits structure.
   *
   * Specialize this trait template if cloning through copy
   * construction is not sufficient.  For example, to avoid slicing
   * when copying an object through a base class pointer, one can
   * implement a virtual "clone" method that can be used to
   * polymorphically invoke the appropriate cloning operation(s).
   * That virtual method would then be invoked by the @c VP_traits/</>
   * specialization.
   */
  template <typename T>
  struct VP_traits
  {
    /// Copy the given object.
    static T * clone (T const * p) { return new T (*p); }
  };

  /**
   * @class Value_Ptr
   *
   * @brief Smart pointer implementation designed for use as a class
   *        member.
   *
   * Using a @c std::auto_ptr/</> as a class member is sometimes
   * problematic since ownership of memory is transferred when copying
   * such members.  This @c Value_Ptr class is explicitly designed to
   * avoid such problems by performing copies of the underlying object
   * rather than transfer ownership.  This, for example, allows it to
   * be readily used as a member in classes placed inside STL
   * containers.
   *
   * @see Item 31 in "More Exceptional C++" by Herb Sutter.
   */
  template <typename T>
  class Value_Ptr
  {
  public:

    /// Constructor.
    explicit Value_Ptr (T * p = 0) : p_ (p) { }

    /// Destructor.
    ~Value_Ptr (void) { delete this->p_; }

    /// Deference operator.
    T & operator* (void) const { return *this->p_; }

    /// Pointer operator.
    T * operator-> (void) const { return this->p_; }

    /// Non-throwing swap operation used to make assignment strongly
    /// exception-safe.
    /**
     * @note As implemented, the swap operation may not work correctly
     *       for @c auto_ptr/</>s, but why would one use an @c
     *       auto_ptr/</> as the template argument for this particular
     *       template class!?
     */
    void swap (Value_Ptr & other) { std::swap (this->p_, other.p_); }

    /// Copy constructor.
    Value_Ptr (Value_Ptr const & other)
      : p_ (create_from (other.p_)) { }

    /// Assignment operator.
    Value_Ptr & operator= (Value_Ptr const & other)
    {
      // Strongly exception-safe.
      Value_Ptr temp (other);
      this->swap (temp);
      return *this;
    }

#ifndef ACE_LACKS_MEMBER_TEMPLATES

    // Compiler can't handle member templates so we lose converting
    // copy operations.

    /// Converting copy constructor.
    template <typename U>
    Value_Ptr (Value_Ptr<U> const & other)
      : p_ (create_from (other.p_)) { }

    /// Converting assignment operator.
    template <typename U>
    Value_Ptr & operator= (Value_Ptr<U> const & other)
    {
      // Strongly exception-safe.
      Value_Ptr temp (other);
      this->swap (temp);
      return *this;
    }

#endif  /* !ACE_LACKS_MEMBER_TEMPLATES */

  private:

#ifndef ACE_LACKS_MEMBER_TEMPLATES

    /// Copying method invoked when copy constructing.
    template <typename U>
    T * create_from (U const * p) const
    {
      return p ? VP_traits<U>::clone (p) : 0;
    }

#else

    // Compiler can't handle member templates so we lose converting
    // copy operations.

    /// Copying method invoked when copy constructing.
    T * create_from (T const * p) const
    {
      return p ? VP_traits<T>::clone (p) : 0;
    }

#endif  /* !ACE_LACKS_MEMBER_TEMPLATES */

  private:

#ifndef ACE_LACKS_MEMBER_TEMPLATES
    template <typename U> friend class Value_Ptr;
#endif  /* !ACE_LACKS_MEMBER_TEMPLATES */

    /// Object owned by this @c Value_Ptr.
    T * p_;

  };

}