智能指针shared_ptr的实现

Posted on Edited on

我们今天来实现一个简易版的智能指针吧！拖了好久啦，以后会把vector、string的简易版也给补上，gigigi

我们可以先想想shared_ptr最大的特点，就是当他的引用计数为0时，便会自动释放所指对象和析构。所以一个关键点就是这个引用计数怎么设置？static？不可以！static变量同属于一个类的所有对象，这样就会导致不管指的是不是同一个对象，引用计数都相同。所以我们这里决定采用一个指针来进行引用计数。具体实现如下：

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#include <iostream>
#include <mutex>
#include <string>

template <typename T>
class shared_ptr{
public:
    explicit shared_ptr(T* sPtr = nullptr): sPtr_(sPtr){
        if(sPtr_){
            useCount_ = new int(1);
            sMutex_ = new std::mutex;
        }
    }

    ~shared_ptr(){
        release();
    }

    shared_ptr(const shared_ptr<T>& sp){
        useCount_ = sp.useCount_;
        sPtr_ = sp.sPtr_;
        sMutex_ = sp.sMutex_;
        addUsecount();
    }

    shared_ptr<T>& operator=(const shared_ptr<T>& sp){
        if(sPtr_ != sp.sPtr_){
            release();
            sPtr_ = sp.sPtr_;
            useCount_ = sp.useCount_;
            sMutex_ = sp.sMutex_;
            addUsecount();
        }
        return *this;
    }

    T& operator*(){
        return *sPtr_;
    }

    T* operator->(){
        return sPtr_;
    }

    int useCount(){
        return *useCount_;
    }

    T* get(){
        return sPtr_;
    }
private:
    void addUsecount(){
        sMutex_->lock();
        ++(*useCount_);
        sMutex_->unlock();
    }

    void release(){
        bool deleteFlag = false;
        sMutex_->lock();
        if(--(*useCount_) == 0){
            delete useCount_;
            delete sPtr_;
            deleteFlag = true;
        }
        sMutex_->unlock();
        if(deleteFlag){
            delete sMutex_;
        }
    }
private:
    int* useCount_;
    T* sPtr_;
    std::mutex* sMutex_;
};


int main(){
    shared_ptr<std::string> p1(new std::string("hello"));
    shared_ptr<std::string> p2 = p1;
    //std::cout << p2.useCount() << std::endl;
    shared_ptr<std::string> p3(p2);
    std::cout << p3.useCount() << std::endl;
    std::cout << p2.useCount() << std::endl;
    p2 = p3;
    std::cout << p3.useCount() << std::endl;
    std::cout << p2.useCount() << std::endl;
    return 0;
}

插入结果图片有点问题，最近没时间弄，各位自己跑一下验证一下，有错误记得说！！！