share_ptr

描述

std::shared_ptr - cppreference.com

share_ptr 基于计数原理，和传统指针使用起来非常类似。

注意：std::share_ptr 即使基类写虚析构，也会调用多态的析构，而这里的手写没有实现

MySharePtr.hpp

/*** std::shared_ptr* https://zh.cppreference.com/w/cpp/memory/shared_ptr*/
#include 
#include template 
class MySharePtr {
private:T *m_p = nullptr;size_t * m_count = nullptr;public:// 构造MySharePtr(T *ptr = nullptr) : m_p(ptr), m_count(new size_t(0)) {*this->m_count = !!(nullptr != m_p);}// 析构~MySharePtr() {// 保证空指针在调用use_count()也能返回0if (nullptr == this->m_p) {return;}(*this->m_count) -= !!(this->m_p);// 保证m_count有实例if (0 == *(this->m_count)) {if (nullptr != this->m_p) {delete this->m_p;this->m_p = nullptr;}if (nullptr != this->m_count) {delete this->m_count;this->m_count = nullptr;}}}// 拷贝构造MySharePtr(const MySharePtr &obj) : MySharePtr() {*this = obj;}// 拷贝赋值MySharePtr &operator=(const MySharePtr &obj) {if (this == &obj) {return *this;}this->~MySharePtr();this->m_p = obj.m_p;this->m_count = obj.m_count;// 拷贝计数(*this->m_count) += 1;return *this;}// 移动构造MySharePtr(MySharePtr &&obj) : MySharePtr() {*this = ::std::move(obj);}// 移动赋值MySharePtr &operator=(MySharePtr &&obj) {if (this == &obj) {return *this;}this->~MySharePtr();this->m_p = obj.m_p;this->m_count = obj.m_count;// 移动抢占资源，计数不变obj.m_p = nullptr;return *this;}public:  // 修改器// 替换所管理的对象void reset(T *p = nullptr) {*this = p;}// 交换所管理的对象void swap(MySharePtr &other) {using ::std::swap;swap(this->m_p, other.m_p);swap(this->m_count, other.m_count);}public:// 返回存储的指针T *get() {return this->m_p;}// 解引用存储的指针T &operator*() {assert(this->get());return *this->m_p;}// 解引用存储的指针T *operator->() {assert(this->get());return this->m_p;}// 返回 shared_ptr 所指对象的引用计数size_t use_count() {return *this->m_count;}// 检查所管理对象是否仅由当前 shared_ptr 的实例管理bool unique() {return this->use_count() == 1;}// 检查是否有关联的管理对象operator bool() {return this->get() != nullptr;}// 提供基于拥有者的共享指针排序// 本简单样例不考虑删除器// bool owner_before(auto &&other);
};

MySharePtr_test.cpp

#include #include "MySharePtr.hpp"struct Base {Base() {std::cout << "Base::Base()\n";}// ::std::share_ptr 即使不是虚析构也能调用virtual ~Base() {std::cout << "Base::~Base()\n";}
};struct Derived : public Base {Derived() {std::cout << "Derived::Derived()\n";}~Derived() {std::cout << "Derived::~Derived()\n";}
};void fun(MySharePtr p) {MySharePtr lp = p;std::cout << "local pointer in a fun:\n"<< "  lp.get() = " << lp.get()<< ", lp.use_count() = " << lp.use_count() << '\n';
}int main() {MySharePtr p(new Derived);std::cout << "Created a shared Derived (as a pointer to Base)\n"<< "  p.get() = " << p.get()<< ", p.use_count() = " << p.use_count() << '\n';auto tmp = p;p.reset();std::cout << "Shared ownership released\n"<< "ownership from main:\n"<< "  p.get() = " << p.get()<< ", p.use_count() = " << p.use_count() << '\n';fun(tmp);std::cout << "main end\n";
}

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