深入学习c++--智能指针(一) shared_ptr

1. 几种智能指针

1. auto_ptr: c++11中推荐不使用他(放弃)

2. shared_ptr: 拥有共享对象所有权语义的智能指针 

3. unique_ptr: 拥有独有对象所有权语义的智能指针 

4. weaked_ptr: 到 std::shared_ptr 所管理对象的弱引用 

1.1 shared_ptr

参考：https://zh.cppreference.com/w/cpp/memory/shared_ptr

std::shared_ptr 是通过指针保持对象共享所有权的智能指针。多个 shared_ptr 对象可占有同一对象。下列情况之一出现时销毁对象并解分配其内存：

• 最后剩下的占有对象的 shared_ptr 被销毁；
• 最后剩下的占有对象的 shared_ptr 被通过 operator= 或 reset() 赋值为另一指针。

#include <iostream>
#include <memory>
#include <thread>
#include <chrono>
#include <mutex>

class Object
{
public:
    Object(int id) : m_id(id) {
        std::cout << "init obj " << m_id << std::endl;
    }    
    ~Object() {
        std::cout << "bye bye " << m_id << std::endl;
    }
    int id() const {
        return m_id;
    }
private:
    int m_id;
};

typedef std::shared_ptr<Object> ObjectPtr;

void print(ObjectPtr obj);
void printRef(const ObjectPtr& obj);

void interfaceOfSharedPtr()
{
    ObjectPtr null;
    std::cout << "ref count is " << null.use_count() << std::endl; // 0

    ObjectPtr obj(new Object(1));
    std::cout << "ref count is " << obj.use_count() << std::endl;  // 1

    ObjectPtr obj2(obj);
    std::cout << "ref count is " << obj.use_count() << std::endl;  // 2

    ObjectPtr obj3 = obj;
    std::cout << "ref count is " << obj.use_count() << std::endl;  // 3

    obj2.reset();        // 不需要了 
    //obj2 = nullptr;    // c++11写法
    std::cout << "ref count is " << obj.use_count() << std::endl;  // 2

    ObjectPtr obj4;
    obj3.swap(obj4);
    //std::swap(obj3, obj4);   // 本身管理资源，引用数没有变化 
    std::cout << "ref count is " << obj.use_count() << std::endl;  // 2

    auto p = obj.get();
    if (p)
    {
        std::cout << "id is " << p->id() << std::endl;
    }

    if (obj)
    {
         std::cout << "p id is " << obj->id() << std::endl;        // operator ->
         std::cout << "ref id is " << (*obj).id() << std::endl;    // operator *
    }

    obj4 = nullptr;
//   if (obj.unique())          //效率更高
//   if (obj.use_count() == 1)  // 会有效率损失   

    std::cout << "only one hold ptr " << obj.unique() << std::endl;
    // 值传入 
    print(obj);
    std::cout << "ref count is " << obj.use_count() << std::endl;

    // 引用传入, 推荐
    printRef(obj);
}

void print(ObjectPtr obj)         // 值传入，会临时改变引用数量+1
{
    std::cout << "count " << obj.use_count() << " id " << obj->id()
              << std::endl;       // 3
}

void printRef(const ObjectPtr& obj)  // 引用传递不会改变引用数量
{
    std::cout << "ref count " << obj.use_count() << " id " << obj->id()
              << std::endl;       // 2
}

void deleteOfObject(Object* obj)
{
    if (obj) {
        std::cout << "delete obj " << obj->id() << std::endl;
        delete obj;
    }
}

void useDeleter()
{
    // 管理 裸指针 和 处理这个指针的函数 -- 可以管理应用delete资源，也可以管理其他类似资源
    ObjectPtr obj(new Object(2), deleteOfObject);
    ObjectPtr obj2 = obj;
}

int main()
{

    interfaceOfSharedPtr();

    return 0;
}

ref count is 0
init obj 1
ref count is 1
ref count is 2
ref count is 3
ref count is 2
ref count is 2
id is 1
p id is 1
ref id is 1
only one hold ptr 1
count 2 id 1
ref count is 1
ref count 1 id 1
bye bye 1