重点：
1.引用计数在堆上（原本应为原子变量）
2.引用计数增加减少需要加锁保证线程安全。
3.内部实现Release函数用于释放资源
4.未实现，增加自定义删除器可以将Release修改为模板函数，传入可调用参数。对于shared_ptr，删除器应当作为构造函数参数传入。故而成员函数需要有std::function对象。
#ifndef SHARED_PTR_SHARED_PTR_H
#define SHARED_PTR_SHARED_PTR_H
#include <mutex>template<typename T>
class Shared_ptr{
public:Shared_ptr(T * row) :_row_ptr(row),_ref_count(new int(1)),_ref_mux(new std::mutex){}~Shared_ptr(){ release(); }//拷贝构造Shared_ptr(const Shared_ptr<T> &sp) :_row_ptr(sp._row_ptr),_ref_count(sp._ref_count),_ref_mux(sp._ref_mux){ add_ref(); }//拷贝赋值Shared_ptr<T>& operator=(const Shared_ptr<T> &sp){if (this->_row_ptr != sp._row_ptr){release(); //释放管理的旧资源_row_ptr = sp._row_ptr;_ref_count = sp._ref_count;_ref_mux = sp._ref_mux;add_ref();}return *this;}public:int& use_count(){ return *_ref_count; }T& operator*(){ return *_row_ptr; }T* operator->(){ return _row_ptr; }T* get(){ return _row_ptr; };void reset(T *sp){if (this->_row_ptr != sp){release(); //释放旧资源,减少引动计数_row_ptr = sp;_ref_count = new int(1);_ref_mux = new std::mutex;} elseadd_ref();}private:T *_row_ptr;int *_ref_count;std::mutex *_ref_mux;//减少引用计数或释放内存void release(){std::unique_lock<std::mutex> lock(*_ref_mux);if (--(*_ref_count) == 0){delete _row_ptr;delete _ref_count;_row_ptr = nullptr; //避免悬空指针_ref_count = nullptr;}}//保证线程安全的前提下加锁增加引用计数void add_ref(){std::unique_lock<std::mutex> lock(*_ref_mux);++(*_ref_count);}
};#endif //SHARED_PTR_SHARED_PTR_H测试程序：
#include <iostream>
#include <string>
#include "Shared_ptr.h"int main() {Shared_ptr<std::string> ptr1(new std::string("hello"));Shared_ptr<std::string> ptr2(ptr1);std::cout << "ptr2 use_count = " << ptr2.use_count() << "ptr1 use_count = " << ptr1.use_count() << std::endl;Shared_ptr<std::string> ptr3 = ptr2;std::cout << "ptr3 use_count = " << ptr3.use_count() << std::endl;std::cout << *ptr2 << std::endl;auto str = new std::string("ni hao");ptr2.reset(str);std::cout << *ptr2 <<" ptr2 usecount = " << ptr2.use_count() << std::endl;std::cout << ptr1.use_count() << std::endl;return 0;
}
