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重構ATL中的CAutoVectorPtr, CAutoPtr和CAutoStackPtr

看到ATL中有3個類的代碼比較比較重復，在atlbase.h中，分別是CAutoVectorPtr, CAutoPtr和CAutoStackPtr，他們的功能其實很類似STL中的autoptr, 但是這里因為針對不同的分配對象而用了3個不同的類，其中CAutoVectorPtr是針對數組類型的，CAutoPtr是針對普通的非數組類型，而CAutoStackPtr針對的是_malloca分配的類型，因為最后釋放方式的不同，它這里用了3份代碼來實現。

CAutoVectorPtr:

template< typename T >
class CAutoVectorPtr
{
public:
    CAutoVectorPtr() throw() :
        m_p( NULL )
    {
    }
    CAutoVectorPtr( CAutoVectorPtr< T >& p ) throw()
    {
        m_p = p.Detach();  // Transfer ownership
    }
    explicit CAutoVectorPtr( T* p ) throw() :
        m_p( p )
    {
    }
    ~CAutoVectorPtr() throw()
    {
        Free();
    }

    operator T*() const throw()
    {
        return( m_p );
    }

    CAutoVectorPtr< T >& operator=( CAutoVectorPtr< T >& p ) throw()
    {
        if(*this==p)
        {
            if(m_p == NULL)
            {
                // This branch means both two pointers are NULL, do nothing.
            }
            else if(this!=&p)
            {
                // If this assert fires, it means you attempted to assign one CAutoVectorPtr to another when they both contained
                // a pointer to the same underlying vector. This means a bug in your code, since your vector will get
                // double-deleted.
                ATLASSERT(FALSE);

                // For safety, we are going to detach the other CAutoVectorPtr to avoid a double-free. Your code still
                // has a bug, though.
                p.Detach();
            }
            else
            {
                // Alternatively, this branch means that you are assigning a CAutoVectorPtr to itself, which is
                // pointless but permissible

                // nothing to do
            }
        }
        else
        {
            Free();
            Attach( p.Detach() );  // Transfer ownership
        }
        return( *this );
    }

    // basic comparison operators
    bool operator!=(CAutoVectorPtr<T>& p) const
    {
        return !operator==(p);
    }

    bool operator==(CAutoVectorPtr<T>& p) const
    {
        return m_p==p.m_p;
    }

    // Allocate the vector
    bool Allocate( size_t nElements ) throw()
    {
        ATLASSUME( m_p == NULL );
        ATLTRY( m_p = new T[nElements] );
        if( m_p == NULL )
        {
            return( false );
        }

        return( true );
    }
    // Attach to an existing pointer (takes ownership)
    void Attach( T* p ) throw()
    {
        ATLASSUME( m_p == NULL );
        m_p = p;
    }
    // Detach the pointer (releases ownership)
    T* Detach() throw()
    {
        T* p;

        p = m_p;
        m_p = NULL;

        return( p );
    }
    // Delete the vector pointed to, and set the pointer to NULL
    void Free() throw()
    {
        delete[] m_p;
        m_p = NULL;
    }

public:
    T* m_p;
};

CAutoPtr：

template< typename T >
class CAutoPtr
{
public:
    CAutoPtr() throw() :
        m_p( NULL )
    {
    }
    template< typename TSrc >
    CAutoPtr( CAutoPtr< TSrc >& p ) throw()
    {
        m_p = p.Detach();  // Transfer ownership
    }
    CAutoPtr( CAutoPtr< T >& p ) throw()
    {
        m_p = p.Detach();  // Transfer ownership
    }
    explicit CAutoPtr( T* p ) throw() :
        m_p( p )
    {
    }
    ~CAutoPtr() throw()
    {
        Free();
    }

    // Templated version to allow pBase = pDerived
    template< typename TSrc >
    CAutoPtr< T >& operator=( CAutoPtr< TSrc >& p ) throw()
    {
        if(m_p==p.m_p)
        {
            // This means that two CAutoPtrs of two different types had the same m_p in them
            // which is never correct
            ATLASSERT(FALSE);
        }
        else
        {
            Free();
            Attach( p.Detach() );  // Transfer ownership
        }
        return( *this );
    }
    CAutoPtr< T >& operator=( CAutoPtr< T >& p ) throw()
    {
        if(*this==p)
        {
            if(this!=&p)
            {
                // If this assert fires, it means you attempted to assign one CAutoPtr to another when they both contained
                // a pointer to the same underlying object. This means a bug in your code, since your object will get
                // double-deleted.
#ifdef ATL_AUTOPTR_ASSIGNMENT_ASSERT
                ATLASSERT(FALSE);
#endif

                // For safety, we are going to detach the other CAutoPtr to avoid a double-free. Your code still
                // has a bug, though.
                p.Detach();
            }
            else
            {
                // Alternatively, this branch means that you are assigning a CAutoPtr to itself, which is
                // pointless but permissible

                // nothing to do
            }
        }
        else
        {
            Free();
            Attach( p.Detach() );  // Transfer ownership
        }
        return( *this );
    }

    // basic comparison operators
    bool operator!=(CAutoPtr<T>& p) const
    {
        return !operator==(p);
    }

    bool operator==(CAutoPtr<T>& p) const
    {
        return m_p==p.m_p;
    }

    operator T*() const throw()
    {
        return( m_p );
    }
    T* operator->() const throw()
    {
        ATLASSUME( m_p != NULL );
        return( m_p );
    }

    // Attach to an existing pointer (takes ownership)
    void Attach( T* p ) throw()
    {
        ATLASSUME( m_p == NULL );
        m_p = p;
    }
    // Detach the pointer (releases ownership)
    T* Detach() throw()
    {
        T* p;

        p = m_p;
        m_p = NULL;

        return( p );
    }
    // Delete the object pointed to, and set the pointer to NULL
    void Free() throw()
    {
        delete m_p;
        m_p = NULL;
    }

public:
    T* m_p;
};

CAutoStackPtr:

/* Automatic cleanup for _malloca objects */
template< typename T >
class CAutoStackPtr
{
public:
    CAutoStackPtr() throw() :
        m_p( NULL )
    {
    }
    template< typename TSrc >
    CAutoStackPtr( CAutoStackPtr< TSrc >& p ) throw()
    {
        m_p = p.Detach();  // Transfer ownership
    }
    CAutoStackPtr( CAutoStackPtr< T >& p ) throw()
    {
        m_p = p.Detach();  // Transfer ownership
    }
    explicit CAutoStackPtr( T* p ) throw() :
        m_p( p )
    {
    }
    ~CAutoStackPtr() throw()
    {
        Free();
    }

    // Templated version to allow pBase = pDerived
    template< typename TSrc >
    CAutoStackPtr< T >& operator=( CAutoStackPtr< TSrc >& p ) throw()
    {
        if(m_p==p.m_p)
        {
            // This means that two CAutoPtrs of two different types had the same m_p in them
            // which is never correct
            ATLASSERT(FALSE);
        }
        else
        {
            Free();
            Attach( p.Detach() );  // Transfer ownership
        }
        return( *this );
    }
    CAutoStackPtr< T >& operator=( CAutoStackPtr< T >& p ) throw()
    {
        if(*this==p)
        {
            if(this!=&p)
            {
                // If this assert fires, it means you attempted to assign one CAutoPtr to another when they both contained
                // a pointer to the same underlying object. This means a bug in your code, since your object will get
                // double-deleted.
                ATLASSERT(FALSE);

                // For safety, we are going to detach the other CAutoPtr to avoid a double-free. Your code still
                // has a bug, though.
                p.Detach();
            }
            else
            {
                // Alternatively, this branch means that you are assigning a CAutoPtr to itself, which is
                // pointless but permissible

                // nothing to do
            }
        }
        else
        {
            Free();
            Attach( p.Detach() );  // Transfer ownership
        }
        return( *this );
    }

    // basic comparison operators
    bool operator!=(CAutoStackPtr<T>& p) const
    {
        return !operator==(p);
    }

    bool operator==(CAutoStackPtr<T>& p) const
    {
        return m_p==p.m_p;
    }

    operator T*() const throw()
    {
        return( m_p );
    }
    T* operator->() const throw()
    {
        ATLASSUME( m_p != NULL );
        return( m_p );
    }

    // Attach to an existing pointer (takes ownership)
    void Attach( T* p ) throw()
    {
        ATLASSUME( m_p == NULL );
        m_p = p;
    }
    // Detach the pointer (releases ownership)
    T* Detach() throw()
    {
        T* p;

        p = m_p;
        m_p = NULL;

        return( p );
    }
    // Delete the object pointed to, and set the pointer to NULL
    void Free() throw()
    {
        /* Note: _freea only actually does anything if m_p was heap allocated
           If m_p was from the stack, it wouldn't be possible to actually free it here
           [wrong function] unless we got inlined. But really all we do if m_p is
           stack-based is ignore it and let its alloca storage disappear at the end
           of the outer function.
        */
        _freea(m_p);
        m_p = NULL;
    }

public:
    T* m_p;
};

可以看到上面代碼明顯非常重復，不知道ATL這樣寫是不是歷史原因，我們下面嘗試對它進行重構。

可以看到其實他們只是最終釋放(Free)的時候稍微有些差別，我們明顯可以把寫差別提取出來，作為一個釋放的Policy。

struct DeleteFunctor
{
    template<typename T> static void Release(T* p) { delete p; }
};

struct DeleteArrayFunctor
{
    template<typename T> static void Release(T* p) { delete []p; }
};

struct DeleteStackFunctor
{
    template<typename T> static void Release(T* p) { _freea p; }
};

然后我們把上面的各種釋放行為作為一個模板參數傳進去就可以了，代碼如下:

template< typename T, typename ReleasePolicy>
class CAutoReleasePtr
{
public:
    CAutoReleasePtr() throw() :
      m_p( NULL )
      {
      }
      template< typename TSrc >
      CAutoReleasePtr( CAutoReleasePtr< TSrc >& p ) throw()
      {
          m_p = p.Detach();  // Transfer ownership
      }
      CAutoReleasePtr( CAutoReleasePtr< T >& p ) throw()
      {
          m_p = p.Detach();  // Transfer ownership
      }
      explicit CAutoReleasePtr( T* p ) throw() :
      m_p( p )
      {
      }
      ~CAutoReleasePtr() throw()
      {
          Free();
      }

      // Templated version to allow pBase = pDerived
      template< typename TSrc >
      CAutoReleasePtr< T >& operator=( CAutoReleasePtr< TSrc >& p ) throw()
      {
          if(m_p==p.m_p)
          {
              // This means that two CAutoPtrs of two different types had the same m_p in them
              // which is never correct
              ATLASSERT(FALSE);
          }
          else
          {
              Free();
              Attach( p.Detach() );  // Transfer ownership
          }
          return( *this );
      }
      CAutoReleasePtr< T >& operator=( CAutoReleasePtr< T >& p ) throw()
      {
          if(*this==p)
          {
              if(this!=&p)
              {
                  // If this assert fires, it means you attempted to assign one CAutoPtr to another when they both contained
                  // a pointer to the same underlying object. This means a bug in your code, since your object will get
                  // double-deleted.
#ifdef ATL_AUTOPTR_ASSIGNMENT_ASSERT
                  ATLASSERT(FALSE);
#endif

                  // For safety, we are going to detach the other CAutoPtr to avoid a double-free. Your code still
                  // has a bug, though.
                  p.Detach();
              }
              else
              {
                  // Alternatively, this branch means that you are assigning a CAutoPtr to itself, which is
                  // pointless but permissible

                  // nothing to do
              }
          }
          else
          {
              Free();
              Attach( p.Detach() );  // Transfer ownership
          }
          return( *this );
      }

      // basic comparison operators
      bool operator!=(CAutoReleasePtr<T>& p) const
      {
          return !operator==(p);
      }

      bool operator==(CAutoReleasePtr<T>& p) const
      {
          return m_p==p.m_p;
      }

      operator T*() const throw()
      {
          return( m_p );
      }
      T* operator->() const throw()
      {
          ATLASSUME( m_p != NULL );
          return( m_p );
      }

      // Attach to an existing pointer (takes ownership)
      void Attach( T* p ) throw()
      {
          ATLASSUME( m_p == NULL );
          m_p = p;
      }
      // Detach the pointer (releases ownership)
      T* Detach() throw()
      {
          T* p;

          p = m_p;
          m_p = NULL;

          return( p );
      }
      // Delete the object pointed to, and set the pointer to NULL
      void Free() throw()
      {
          ReleasePolicy::Release(m_p);
          m_p = NULL;
      }

public:
    T* m_p;
};

可以看到我們上面其實就改了一行代碼，改了下最終的釋放策略.

好，現在我們可以這樣用了:

CAutoReleasePtr<T, DeleteFunctor> p1(new int);
CAutoReleasePtr<T, DeleteArrayFunctor> p2(new int[10]);

功能是可以了，但是是不是覺得上面這樣用起來不方便，typedef一下就好了:

typedef CAutoReleasePtr<T, DeleteFunctor> CSimplePtr<T>;
typedef CAutoReleasePtr<T, DeleteArrayFunctor> CArrayPtr<T>;
typedef CAutoReleasePtr<T, DeleteStackFunctor> CStackPtr<T>;

但是我們很塊發現上面的代碼編譯都過不了。

既然typedef不行，那我們就通過繼承來生成一個新類：

template<typename T> class CSimplePtr: public CAutoReleasePtr<T, DeleteFunctor> {};
template<typename T> class CArrayPtr: public CAutoReleasePtr<T, DeleteArrayFunctor> {};
template<typename T> class CStackPtr: public CAutoReleasePtr<T, DeleteStackFunctor> {};

我們很快又發現，用不起來，我們新類的構造函數需要重寫才行。

接下來我們考慮生成一個新類，然后在內部typedef:

template<typename T>
struct CSimplePtr
{
     typedef CAutoReleasePtr<T, DeleteFunctor> type;
};

template<typename T>
struct CArrayPtr
{
     typedef CAutoReleasePtr<T, DeleteArrayFunctor> type;
};

template<typename T>
struct CStackPtr
{
     typedef CAutoReleasePtr<T, DeleteStackFunctor> type;
};

然后這樣用:

CSimplePtr<int>::type p(new int);
CArrayPtr<int>::type p1(new int[20]);

可是這樣用和最初的用法似乎又沒多少改進....

再最后想到了用宏:

#define CSimplePtr(T) CAutoReleasePtr<T, DeleteFunctor>
#define CArrayPtr(T) CAutoReleasePtr<T, DeleteArrayFunctor>

但是用的時候太嘔心了:

CSimplePtr(int) p(new int);
CArrayPtr(int) p1(new int[20]);

最后，實在沒有什么辦法了....
不知道大家有沒有什么好方法？？？

posted on 2012-09-24 23:10 Richard Wei 閱讀(1610) 評論(5) 收藏舉報

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