智能指针之 unique_ptr

　　对于动态申请的内存，C++语言为我们提供了new和delete运算符， 而没有像java一样，提供一个完整的GC机制，因此对于我们申请的动态内存，我们需要时刻记得释放，且不能重复释放，释放后不能再去使用...  因此在使用时造成很多不便，且容易出现很多问题，一旦出问题就造成core dump，程序直接挂掉 ， 这个时候，智能指针的优势就体现出来了，智能指针符合RAII原则，资源获取就是初始化，在对象析构时，将资源进行释放，对动态内存做到一个比较好的管理

　　unique_ptr 持有对对象的独有权—两个unique_ptr不能指向一个对象，不能进行复制操作只能进行移动操作

　　unique_ptr拥有所有auto_ptr的功能，且unique_ptr通过将复制构造和赋值操作符私有化，将对象的所有权独有，很好的将auto_ptr的安全问题给规避掉了，unique_ptr的其他特点包括：1.提供删除器释放对象，允许用户自定义删除器 2.添加了对象数组的偏特化实现，new[]，delete[] 3.使用C++ 11的右值引用特性，实现所有权转移 std::move()

　　本次实现，将unique_ptr的基本接口进行了实现，基本包括了unique_ptr的功能  （编译平台：Linux centos 7.0　编译器：gcc 4.8.5 )

　　使用std::unique_ptr时，需要#include <memory>头文件，具体使用代码如下（文件名：test_ptr.cpp）：

#include <memory>
#include <iostream>

using namespace std;

class Test
{
public:
    Test()
    {
        cout << "construct.." << endl;
    }
    
    ~Test()    
    {
        cout << "destruct.." << endl;
    }
};

void test()
{
    
}

int main()
{
    //auto_ptr
    Test* p = new Test(); 
    auto_ptr<Test> ap(p);
    
    //unique_ptr
    Test* p1 = new Test();
    unique_ptr<Test> up(new Test());
    unique_ptr<Test> up1(move(up));

    

    //unique_ptr<Test> up2 = up;
    
    unique_ptr<int> up3(new int(5));

    return 0;
}

　　具体实现代码如下，没有对动态对象数组及std::move()进行实现，动态对象数组实现代码和这差不多，写个模板偏特化即可，至于std::move()则和成员函数release()类似，转移所有权（文件名：unique_ptr_implement.cpp）:

#include <iostream>
#include <assert.h>

using namespace std;

#define PTR_ASSERT(x) assert(x)

template<class T>
struct defalute_deleter
{
    void defalute_deleter_method()
    {
        cout << "deleter method..." << endl;
    }
    void operator()(T* ptr)
    {
        if(ptr != NULL)
        {
            cout << "default deleter....." << endl;
            delete ptr;
            ptr = NULL;
        }
    }
};

template<typename T, typename deleter = defalute_deleter<T> >
class unique_ptr
{
public:
    explicit unique_ptr(T* ptr = NULL);
    
    unique_ptr(T* ptr, deleter d);

    ~unique_ptr();

    T* get();

    void reset(T* ptr = NULL);

    deleter& getDeleter();

    T* release();

public:
    T& operator*();    
    
    T* operator->();

    operator bool() const;
    
private:
    unique_ptr(unique_ptr& up);

    unique_ptr& operator = (unique_ptr& up);
    
private:
    T* m_ptr;

    deleter m_deleter;
};

template<typename T, typename deleter>
unique_ptr<T, deleter>::unique_ptr(T* ptr /* = NULL */, deleter d)
{
    if(ptr != NULL)
    {
        m_ptr = ptr;
        m_deleter = d;
    }
}

template<typename T, typename deleter>
unique_ptr<T, deleter>::unique_ptr(T* ptr /* = NULL */)
{
    if(ptr != NULL)
    {
        m_ptr = ptr;
    }    
}

template<typename T, typename deleter>
unique_ptr<T, deleter>::~unique_ptr()
{
    if(m_ptr != NULL)
    {
        m_deleter(m_ptr);
        m_ptr = NULL;
    }
}

template<typename T, typename deleter>
T& unique_ptr<T, deleter>::operator*()
{
    PTR_ASSERT(m_ptr != NULL);

    return *m_ptr;
}

template<class T, class deleter>
T* unique_ptr<T, deleter>::operator->()
{
    PTR_ASSERT(m_ptr != NULL);

    return m_ptr;
}

template<typename T, typename deleter>
T* unique_ptr<T, deleter>::get()
{
    return m_ptr;
}

template<typename T, typename deleter>
void unique_ptr<T, deleter>::reset(T* ptr)
{
    T* old_ptr = m_ptr;
    m_ptr = ptr;

    if(old_ptr != NULL)
    {
        m_deleter(old_ptr);
        old_ptr = NULL;
    }
}

template<typename T, typename deleter>
deleter& unique_ptr<T, deleter>::getDeleter()
{
    return m_deleter;
}

template<typename T, typename deleter>
T* unique_ptr<T, deleter>::release()
{
    T* pTemp = m_ptr;
    m_ptr = NULL;

    return pTemp;    
}

template<typename T, typename deleter>
unique_ptr<T, deleter>::operator bool() const
{
    return m_ptr != NULL;
}


//Test class
class Test
{
public:
    Test()
    {
        cout << "construct.." << endl;
    }
    
    ~Test()    
    {
        cout << "destruct.." << endl;
    }

    void method()
    {
        cout << "welcome Test.." << endl;
    }
};

//custom deleter
template <class T>
struct custom_deleter
{
    void deleter_method()
    {
        cout << "custom deleter method..." << endl;
    }

    void operator()(T* ptr)
    {
        cout << "custom deleter... " << endl;
        delete ptr;
        ptr = NULL;
    }
};


int main()
{
    //default deleter
    cout << "=======default deleter====interface test begin: ========== " << endl;
    unique_ptr<Test> up(new Test());
    
    cout << "operator ->: " << endl;
    up->method();
    
    cout << "operator *: " << endl;
    (*up).method();

    cout << "operator bool: " << endl;
    if(up){ cout<< "obj is exit" << endl;}

    cout << "get: " << endl;
    up.get()->method();

    cout << "getDeleter: " << endl;
    defalute_deleter<Test> del = up.getDeleter();
    del.defalute_deleter_method();

    cout << "release: " << endl;
    unique_ptr<Test> up1(up.release());

    //if take this,  will die
    //(*up).method();

    cout << "reset: " << endl;
    up1.reset();

    //Custom deleter
    cout << "=======Custom deleter====interface test begin: ========== " << endl;
    custom_deleter<Test> d;
    unique_ptr<Test, custom_deleter<Test> > up_custom_dele(new Test(), d);
    
    cout << "operator ->: " << endl;
    up_custom_dele->method();
    
    cout << "operator *: " << endl;
    (*up_custom_dele).method();

    cout << "operator bool: " << endl;
    if(up_custom_dele){ cout<< "obj is exit" << endl;}

    cout << "get: " << endl;
    up_custom_dele.get()->method();

    cout << "getDeleter: " << endl;
    custom_deleter<Test> custom_del = up_custom_dele.getDeleter();
    custom_del.deleter_method();

    cout << "release: " << endl;
    unique_ptr<Test> up3(up_custom_dele.release());

    //if take this,  will die
    //(*up_custom_dele).method();

    cout << "reset: " << endl;
    up3.reset();
    

    return 0;
}

　　执行代码，打印如下：

[root@localhost code]# g++ -o unique unique_ptr_implement.cpp 
[root@localhost code]# ./unique
=======default deleter====interface test begin: ========== 
construct..
operator ->: 
welcome Test..
operator *: 
welcome Test..
operator bool: 
obj is exit
get: 
welcome Test..
getDeleter: 
deleter method...
release: 
reset: 
default deleter.....
destruct..
=======Custom deleter====interface test begin: ========== 
construct..
operator ->: 
welcome Test..
operator *: 
welcome Test..
operator bool: 
obj is exit
get: 
welcome Test..
getDeleter: 
custom deleter method...
release: 
reset: 
default deleter.....
destruct..
[root@localhost code]# 

　　从实现结果可以看出，unique_ptr保证一个动态对象只有一个unique_ptr对象所拥有，unique_ptr对象之间无法copy和赋值，只能进行动态内存对象的转移，转移时，原有的unique_ptr对象将不再拥有动态内存的访问权限，这样可以保证动态内存使用的安全性。在实现中构造函数提供了两个，一个使用默认删除器，另一个则需要我们构造时传入删除器，这样可以很好的进行资源自定义删除。 对于多个指针对象指向同一个动态内存对象，unique_ptr不适用，需使用share_ptr