智能指针之句柄类

对于上一篇提到的方法，可以不用引入辅助类，可以直接把指针封装起来。然后，重载操作符，定义为一个指针的行为，使得可以像使用指针一样使用它。这种方法叫做句柄类，查下了资料，对这个的解释是存储指向动态分配对象指针的类。可以在适当的时间自动删除指向的对象，同时，它的行为也像一个指针。由此可以看出本文标题不太合适。

同样的先上一张图。



1.定义一个SmartPtr对象P1，传入实际指针对象，调用构造函数，初始化计数为1；

2.定义一个SmartPtr对象P2，调用拷贝构造函数，此时，P1和P2的ptr指向相同的地址，pUse指向相同的地址，引用计数自加；

3.定义一个SmartPtr对象P3，调用赋值，操作符右操作数引用计数自加，左操作数自减，并判断原引用计数是否为0，如果是0，则删除原ptr指针指向的地址内容，赋值ptr和pUse，指向相同的ptr和pUse

4.析构时，引用计数自减，并判断计数值是否为0，如果是0，则自动删除指针对象。
示例代码如下：

template<typename T>
class SmartPtr
{
public:
SmartPtr(T* p= 0)
:ptr(p),pUse(new size_t(1))
{}

~SmartPtr()
{
decrUse();
}

SmartPtr(const SmartPtr<T>& src)
:ptr(src.ptr),pUse(src.pUse)
{
++*pUse;
}

SmartPtr<T>& operator=(const SmartPtr<T>& rhs)
{
if (rhs.ptr != ptr)
{
++*rhs.pUse;
decrUse();
ptr = rhs.ptr;
pUse = rhs.pUse;
}
return *this;
}

T* operator->()
{
if(ptr)
return ptr;
throw std::runtime_error("access through NULL pointer");
}

const T* operator->() const
{
if(ptr)
return ptr;
throw std::runtime_error("access through NULL pointer");
}

T& operator*()
{
if(ptr)
return *ptr;
throw std::runtime_error("dereference of NULL pointer");
}

const T& operator*() const
{
if(ptr)
return *ptr;
throw std::runtime_error("dereference of NULL pointer");
}

private:

void decrUse()
{
if(--*pUse == 0)
{
delete ptr;
delete pUse;
}
}

private:
T* ptr;
size_t* pUse;
};

顺便贴上一点Cocos2d的智能指针类的代码，它使用的方式是引入辅助类，辅助类里有引用计数，当然这个辅助类不仅限于在智能指针的使用。省略一部分代码和注释。

class CC_DLL Ref
{
public:
void retain();
void release();
Ref* autorelease();
unsigned int getReferenceCount() const;

protected:
Ref();

public:
virtual ~Ref();

protected:
/// count of references
unsigned int _referenceCount;

friend class AutoreleasePool;

// Memory leak diagnostic data (only included when CC_USE_MEM_LEAK_DETECTION is defined and its value isn't zero)
#if CC_USE_MEM_LEAK_DETECTION
public:
static void printLeaks();
#endif
};

下面是RefPtr，Cocos2d的智能指针类

template <typename T> class RefPtr
{
public:

inline RefPtr()
:
_ptr(nullptr)
{

}

inline RefPtr(RefPtr<T> && other)
{
_ptr = other._ptr;
other._ptr = nullptr;
}

inline RefPtr(T * ptr)
:
_ptr(const_cast<typename std::remove_const<T>::type*>(ptr)) // Const cast allows RefPtr<T> to reference objects marked const too.
{
CC_REF_PTR_SAFE_RETAIN(_ptr);
}

inline RefPtr(std::nullptr_t ptr)
:
_ptr(nullptr)
{

}

inline RefPtr(const RefPtr<T> & other)
:
_ptr(other._ptr)
{
CC_REF_PTR_SAFE_RETAIN(_ptr);
}

inline ~RefPtr()
{
CC_REF_PTR_SAFE_RELEASE_NULL(_ptr);
}

inline RefPtr<T> & operator = (const RefPtr<T> & other)
{
if (other._ptr != _ptr)
{
CC_REF_PTR_SAFE_RETAIN(other._ptr);
CC_REF_PTR_SAFE_RELEASE(_ptr);
_ptr = other._ptr;
}

return *this;
}

inline RefPtr<T> & operator = (RefPtr<T> && other)
{
if (&other != this)
{
CC_REF_PTR_SAFE_RELEASE(_ptr);
_ptr = other._ptr;
other._ptr = nullptr;
}

return *this;
}

inline RefPtr<T> & operator = (T * other)
{
if (other != _ptr)
{
CC_REF_PTR_SAFE_RETAIN(other);
CC_REF_PTR_SAFE_RELEASE(_ptr);
_ptr = const_cast<typename std::remove_const<T>::type*>(other); // Const cast allows RefPtr<T> to reference objects marked const too.
}

return *this;
}

inline RefPtr<T> & operator = (std::nullptr_t other)
{
CC_REF_PTR_SAFE_RELEASE_NULL(_ptr);
return *this;
}

// Note: using reinterpret_cast<> instead of static_cast<> here because it doesn't require type info.
// Since we verify the correct type cast at compile time on construction/assign we don't need to know the type info
// here. Not needing the type info here enables us to use these operations in inline functions in header files when
// the type pointed to by this class is only forward referenced.

inline operator T * () const { return reinterpret_cast<T*>(_ptr); }

inline T & operator * () const
{
CCASSERT(_ptr, "Attempt to dereference a null pointer!");
return reinterpret_cast<T&>(*_ptr);
}

inline T * operator->() const
{
CCASSERT(_ptr, "Attempt to dereference a null pointer!");
return reinterpret_cast<T*>(_ptr);
}

inline T * get() const { return reinterpret_cast<T*>(_ptr); }

inline bool operator == (const RefPtr<T> & other) const { return _ptr == other._ptr; }

inline bool operator == (const T * other) const { return _ptr == other; }

inline bool operator == (typename std::remove_const<T>::type * other) const { return _ptr == other; }

inline bool operator == (const std::nullptr_t other) const { return _ptr == other; }

inline bool operator != (const RefPtr<T> & other) const { return _ptr != other._ptr; }

inline bool operator != (const T * other) const { return _ptr != other; }

inline bool operator != (typename std::remove_const<T>::type * other) const { return _ptr != other; }

inline bool operator != (const std::nullptr_t other) const { return _ptr != other; }

inline bool operator > (const RefPtr<T> & other) const { return _ptr > other._ptr; }

inline bool operator > (const T * other) const { return _ptr > other; }

inline bool operator > (typename std::remove_const<T>::type * other) const { return _ptr > other; }

inline bool operator > (const std::nullptr_t other) const { return _ptr > other; }

inline bool operator < (const RefPtr<T> & other) const { return _ptr < other._ptr; }

inline bool operator < (const T * other) const { return _ptr < other; }

inline bool operator < (typename std::remove_const<T>::type * other) const { return _ptr < other; }

inline bool operator < (const std::nullptr_t other) const { return _ptr < other; }

inline bool operator >= (const RefPtr<T> & other) const { return _ptr >= other._ptr; }

inline bool operator >= (const T * other) const { return _ptr >= other; }

inline bool operator >= (typename std::remove_const<T>::type * other) const { return _ptr >= other; }

inline bool operator >= (const std::nullptr_t other) const { return _ptr >= other; }

inline bool operator <= (const RefPtr<T> & other) const { return _ptr <= other._ptr; }

inline bool operator <= (const T * other) const { return _ptr <= other; }

inline bool operator <= (typename std::remove_const<T>::type * other) const { return _ptr <= other; }

inline bool operator <= (const std::nullptr_t other) const { return _ptr <= other; }

inline operator bool() const { return _ptr != nullptr; }

inline void reset()
{
CC_REF_PTR_SAFE_RELEASE_NULL(_ptr);
}

inline void swap(RefPtr<T> & other)
{
if (&other != this)
{
Ref * tmp = _ptr;
_ptr = other._ptr;
other._ptr = tmp;
}
}

/**
* This function assigns to this RefPtr<T> but does not increase the reference count of the object pointed to.
* Useful for assigning an object created through the 'new' operator to a RefPtr<T>. Basically used in scenarios
* where the RefPtr<T> has the initial ownership of the object.
*
* E.G:
* RefPtr<cocos2d::Image> image;
* image.weakAssign(new cocos2d::Image());
*
* Instead of:
* RefPtr<cocos2d::Image> image;
* image = new cocos2d::Image();
* image->release(); // Required because new'd object already has a reference count of '1'.
*/
inline void weakAssign(const RefPtr<T> & other)
{
CC_REF_PTR_SAFE_RELEASE(_ptr);
_ptr = other._ptr;
}

private:
Ref * _ptr;
};