c++智能指针-shared_ptr

#include <iostream>
#include <memory>

static void interfaceOfSharedPtr();

int main(void) {
    interfaceOfSharedPtr();
    return 0;
}

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 = 0;
};
//目前c++ 的4种指针
// auto_ptr  c++98 智能指针,使用了
// shared_ptr //共享指针
// unique_ptr //只能由一个使用者使用
// weaked_ptr //与share_ptr搭配使用

//使用智能指针
using ObjectPtr = std::shared_ptr<Object>;


//作为参数的obj,相当于拷贝了一次,use_count 会+ 1,但当函数结束时候,智能指针析构,use_count会恢复原来的个数
void print(ObjectPtr obj) {
    //-------------------------智能指针的成员函数
    std::cout << "count " << obj.use_count() << " id " << obj->id() << std::endl;
}
//以引用方式,不会进行copy, use_count不变
void printRef(const ObjectPtr &obj) {
    std::cout << "ref count " << obj.use_count() << "id " << obj->id() << std::endl;
}


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

    //使用智能指针来储存对象的指针
    ObjectPtr obj (new Object(1));  
    std::cout << "ref count is " << obj.use_count() << std::endl;

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

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

    obj2.reset(); //What dose means of reset ?
    std::cout << "ref count is " << obj.use_count() << std::endl;

    ObjectPtr obj4;
    obj3.swap(obj4); //把obj3的管理资源相互交换
    std::cout << "obj4 ref count is " << obj4.use_count() << std::endl;
    std::swap(obj3, obj4); //采用另一种方式交换 
    std::cout << "obj4 ref count is " << obj4.use_count() << std::endl;

    //把智能指针存储的指针传出来
    auto p = obj.get();
    if(p) {
        std::cout << "id is" << p->id() << std::endl;   
    }
    if(obj) { //重载了 operator bool
        std::cout << "p id is " << obj->id() << std::endl; //重载: operator ->
        std::cout << "ref id is " << (*obj).id() << std::endl; //重载了: operator *
    }
    
    obj4 = nullptr;
    //----------------------------->obj.use_count() == 1 可能效率比较底一点
    std::cout << "only one hold ptr " << obj.unique() << std::endl; //判断是否是一个人在使用
    std::cout << "judge differnt: " << std::endl;
    std::cout << "use_count: " << obj.use_count() << std::endl;
    printRef(obj);
    print(obj);
    std::cout << "use_count: " << obj.use_count() << std::endl;
}

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

void useDeleter() {
    //当智能指针出了作用域后,析构函数就被调用, 哪怕中途抛出异常
    ObjectPtr obj(new Object(2), deleterOfObject);
    ObjectPtr obj2 = obj;
}

c++智能指针-shared_ptr

原文：https://www.cnblogs.com/lyxf/p/12358272.html