标签：c++

一、智能指针

二、智能指针的一般实现

　　智能指针通常使用类模板来实现。模拟类指针的各种行为。但是，其最重要的作用是对类指针成员的管理，防止悬垂指针的出现。

template<class T>  
class SmartPointer{  
    public:  
        SmartPointer(T *t):pt(t){}  
        T& operator *(){ return *pt; }  
        T* operator ->() { return pt; }  
    private:  
        T *pt;  
};  

三、引用计数的实现

　　为了实现引用计数，我们定义一个_counter类来记录引用次数，把_counter类的所有成员设定为private，因为其他的类型并不需要访问_counter，只有SmartPointer对其进行操作就行了，SmartPointer将设为其友元类。

class _counter{  
    template<class T> friend class SmartPointer;  
    _counter(int u):use(u){}  
    ~_counter(){}  
    int use;  
};  

在SmartPointer类中，保留_counter的指针。

template<class T>  
class SmartPointer{  
    public:  
        SmartPointer(T *t):pc(new _counter(1)){  
            cout<<"SmartPointer::SmartPointer() invoded use is: "<<pc->use<<endl;  
            this->pt = t;  
        }  
  
        SmartPointer(SmartPointer<T> &rhs){  
            this->pc = rhs.pc;  
            this->pt = rhs.pt;  
            this->pc->use++;  
            cout<<"SmartPointer copy invoked use is: "<<pc->use<<endl;  
        }  
  
        ~SmartPointer(){  
            pc->use--;  
            cout<<"SmartPointer::~SmartPointer() invoded use is: "<<pc->use<<endl;  
            if(pc->use == 0)  
            {  
                delete pt;  
                delete pc;  
            }  
        }  
  
        SmartPointer<T>& operator=(SmartPointer<T> rhs){  
            if(rhs == *this){  
                return *this;  
            }  
          
        if(--pc->use==0){
            delete pt;
            delete pc;
        }
            this->pt = rhs.pt;  
            this->pc = rhs.pc;  
            this->pc->use++;  
            cout<<"SmartPointer::operator=() invoked  use is: "<<pc->use<<endl;  
            return *this;  
        }  
  
    private:  
        T *pt;  
        _counter* pc;  
};  

例如：我们有一个HasPtr类，其类成员中有一个为指针*p。

class HasPtr{  
    public:  
        HasPtr(int val):value(val),p(new int(3)){  
            cout<<"HasPtr::HasPtr() invoked"<<endl;  
        }  
        ~HasPtr(){ delete p; cout<<"HasPtr::~HasPtr() invoded"<<endl;}  
  
    private:  
        int *p;  
        int value;  
};  

如果如下调用：

HasPtr *php = new HasPtr(3);  
SmartPointer<HasPtr> psp(php);  
SmartPointer<HasPtr> npsp(psp);

我们现在有两个智能指针对象，指向同一个HasPtr对象，其模型如下：

_counter的use成员（引用计数）为2.

四、测试

int main(void)  
{  
    HasPtr *php = new HasPtr(3);  
    SmartPointer<HasPtr> psp(php);  
    SmartPointer<HasPtr> npsp(psp);  
    SmartPointer<HasPtr> nnpsp = npsp;  
  
  
    return 0;  
}  

使用gcc编译器，运行结果如下：

 再找一份实现：

#include <iostream>
#include <windows.h>
using namespace std;
 
#define SAFE_DELETE(p) if (p) { delete p; p = NULL; }
 
class KRefCount
{
public:
    KRefCount():m_nCount(0){}
 
public:
    unsigned AddRef(){ return InterlockedIncrement(&m_nCount); }
    unsigned Release(){ return InterlockedDecrement(&m_nCount); }
    void Reset(){ m_nCount = 0; }
 
private:
    unsigned long m_nCount;
};
 
template <typename T>
class SmartPtr
{
public:
    SmartPtr(void)
        : m_pData(NULL)
    {
        m_pReference = new KRefCount();
        m_pReference->AddRef();
    }
 
    SmartPtr(T* pValue)
        : m_pData(pValue)
    {
        m_pReference = new KRefCount();
        m_pReference->AddRef();
    }
 
    SmartPtr(const SmartPtr<T>& sp)
        : m_pData(sp.m_pData)
        , m_pReference(sp.m_pReference)
    {
        m_pReference->AddRef();
    }
 
    ~SmartPtr(void)
    {
        if (m_pReference && m_pReference->Release() == 0)
        {
            SAFE_DELETE(m_pData);
            SAFE_DELETE(m_pReference);
        }
    }
 
    inline T& operator*()
    {
        return *m_pData;
    }
 
    inline T* operator->()
    {
        return m_pData;
    }
 
    SmartPtr<T>& operator=(const SmartPtr<T>& sp)
    {
        if (this != &sp)
        {
            if (m_pReference && m_pReference->Release() == 0)
            {
                SAFE_DELETE(m_pData);
                SAFE_DELETE(m_pReference);
            }
 
            m_pData = sp.m_pData;
            m_pReference = sp.m_pReference;
            m_pReference->AddRef();
        }
 
        return *this;
    }
 
    SmartPtr<T>& operator=(T* pValue)
    {
        if (m_pReference && m_pReference->Release() == 0)
        {
            SAFE_DELETE(m_pData);
            SAFE_DELETE(m_pReference);
        }
 
        m_pData = pValue;
        m_pReference = new KRefCount;
        m_pReference->AddRef();
        return *this;
    }
 
    T* Get()
    {
        T* ptr = NULL;        
        ptr = m_pData;
 
        return ptr;
    }
 
    void Attach(T* pObject)
    {
        if (m_pReference->Release() == 0)
        {
            SAFE_DELETE(m_pData);
            SAFE_DELETE(m_pReference);
        }
 
        m_pData = pObject;
        m_pReference = new KRefCount;
        m_pReference->AddRef();
    }
 
    T* Detach()
    {
        T* ptr = NULL;
 
        if (m_pData)
        {          
            ptr = m_pData;
            m_pData = NULL;
            m_pReference->Reset();
        }
        return ptr;
    }
 
private:
    KRefCount* m_pReference;
    T* m_pData;
};
 
class CTest
{
public:
    CTest(int b) : a(b) {}
private:
    int a;
};
 
int main()
{
    SmartPtr<CTest> pSmartPtr1(new CTest(10));
    SmartPtr<CTest> pSmartPtr2(new CTest(20));
 
    pSmartPtr1 = pSmartPtr2;
}

智能指针的实现--使用引用计数实现以及原理

标签：c++

原文地址：http://blog.csdn.net/yusiguyuan/article/details/42131183