C++11的智能指針確實帶來了很多方便，但是shared_ptr因為要維護很多元信息，會犧牲一些效率，如果大量使用智能指針，一定不要忘記開啟編譯器的優化選項。。

下面是仿照unique_ptr和shared_ptr的實現。

unique_ptr

#pragma once

namespace m{

template<typename T>
struct DefaultDeleter{
    void operator()(T *ptr){
        if(ptr!=nullptr){
            delete ptr;
            ptr = nullptr;
        }
    }
};

template<typename T,typename Deleter = DefaultDeleter<T>>
class UniquePtr{
    T *ptr;
    Deleter deleter;
    
    // delete copy constructor and operator=
    UniquePtr(const UniquePtr &){}
    UniquePtr &operator=(const UniquePtr &){}
public:
    UniquePtr():ptr(nullptr){}
    UniquePtr(T *_ptr):ptr(_ptr){}
    ~UniquePtr(){ deleter(ptr); } 

    //replaces the managed object 
    void reset(T *_ptr);
    //returns a pointer to the managed object and releases the ownership
    T *release();
    //returns a pointer to the managed object
    T *get();

    operator bool() const;
    T &operator*();
    T *operator->();
};

template<typename T,typename Deleter>
void UniquePtr<T,Deleter>::reset(T *_ptr){
    if(ptr==_ptr) return ;
    T *old_ptr = ptr;
    ptr = _ptr;
    deleter(old_ptr);
}

template<typename T,typename Deleter>
T *UniquePtr<T,Deleter>::release(){
    T *old_ptr = ptr;
    ptr = nullptr;
    return old_ptr;
}

template<typename T,typename Deleter>
T *UniquePtr<T,Deleter>::get(){
    return ptr;
}

template<typename T, typename Deleter>
UniquePtr<T,Deleter>::operator bool() const{
    return ptr!=nullptr;
}

template<typename T, typename Deleter>
T &UniquePtr<T,Deleter>::operator*(){
    return *ptr;
}

template<typename T, typename Deleter>
T *UniquePtr<T,Deleter>::operator->(){
    return ptr;
}

}

shared_ptr

#pragma once
namespace m{

template<typename T>
struct DefaultDeleter{
    void operator()(T *ptr){
        if(ptr!=nullptr){
            delete ptr;
            ptr = nullptr;
        }
    }
};

template<typename T,typename Deleter>
struct Control{
    int reference_count;
    Deleter deleter;
    Control():reference_count(1){}
};

template <typename T,typename Deleter = DefaultDeleter<T>>
class SharedPtr{
    T *ptr;
    Control<T,Deleter> *ctrl;
public:

    SharedPtr():ptr(nullptr),ctrl(new Control<T,Deleter>()){}
    SharedPtr(T *_ptr):ptr(_ptr),ctrl(new Control<T,Deleter>()){}
    SharedPtr(const SharedPtr<T> &other):ptr(other->ptr),ctrl(other->ctrl){
        ctrl->reference_count++;
    }
    ~SharedPtr() {
        ctrl->reference_count--;
        if(ctrl->reference_count == 0){
            ctrl->deleter(ptr);
            delete ctrl;
            ctrl = nullptr;
        }
    }
    T &operator*(){
        return *ptr;
    }
    T *operator->(){
        return ptr;
    }
    operator bool() const{
        return ptr!=nullptr;
    }
};
}