C++11新特性——智能指针（二）

C++11新特性——智能指针（二）

1、shared_ptr

• 初始化

  • 构造函数

    • class Person {
      public:
      	Person(int v) {
      		no = v;
      		cout << "Construct" << no << endl;
      	}
      	~Person() {
      		cout << "Destruct" << no << endl;
      	}
      	int no;
      };
      class DestructPerson {
      public:
      	void operator()(Person* pt) {
      		cout << "DestructPerson" << endl;
      		delete pt;
      	}
      };
      int main() {
      
      
      	shared_ptr<Person> sp1;//空的shared_ptr，可以指向类型为Person的对象
      	shared_ptr<Person> sp2(new Person(2)); //定义shared_ptr, 同时指向类型为T的对象
      	// 1
      	cout << "sp2.use_count(): " << sp2.use_count() << endl;//当前管控Person(2)的共享指针的数量
      
      	sp1 = sp2;
      	// 2
      	cout << "sp2.use_count(): " << sp1.use_count() << endl;//当前管控Person(2)的共享指针的数量
      
      	shared_ptr<Person> sp3(sp1);
      	// 3
      	cout << "sp2.use_count(): " << sp3.use_count() << endl;//当前管控Person(2)的共享指针的数量
      
      	//数组对象的管理
      	shared_ptr<Person[]> sp4(new Person[5]{3,4,5,6,7});//delete[]
      
      
      	shared_ptr<Person> sp8(new Person(8), DestructPerson());//调用DestructPerson()释放对象
      
      	
      	system("pause");
      	return 0;
      }
      

  • 使用make_shared初始化对象，分配内存效率更高

    • make_shared函数的主要功能是在动态内存中分配一个对象并初始化它，返回指向此对象的shared_ptr

    • //make_shared<类型>(构造类型对象需要的参数列表);
      shared_ptr<int> p4 = make_shared<int>(2);//多个参数以逗号隔开，最多接受十个
      shared_ptr<string> = make_shared<string>("字符串");
      //使用make_shared 函数模板初始化
      shared_ptr<Person> sp9;
      sp9 = make_shared<Person>(9);
      cout << "after sp9 = make_shared<Person>(9)，ref_counter: " << sp9.use_count() << endl;// 1
      

• 其他

  • 	//主动释放对象
    	shared_ptr<Person> up1 = make_shared<Person>(9);
    	//up1 = nullptr;
    	shared_ptr<Person> upx = up1;
    
    	Person* up = new Person(10);
    
    	//重置
    	//up1.reset();//将up1重置为空指针，所管理对象引用计数 减1
    	up1.reset(up);//将up1重置为up的值，up1管控的对象计数减1，up1接管对up指针的管控，up1被析构掉了
    	//up1.reset(up,d));//将up1重置为up的值，up1管控的对象计数减1 并使用d作为删除器
    
    	cout << up1.use_count() << endl;
    
    	cout << "交换前，up1: " << up1->no << "sp10: " << upx->no << endl;//10 9
    	swap(upx, up1);
    
    	cout << "交换后，up1: " << up1->no << "sp10: " << upx->no << endl;//9 10
    
    

• 使用陷阱

  • shared_ptr作为被管控的对象的成员时，小心因循环引用造成无法释放资源

  • class boy {
    public:
    	boy() {
    		cout << "boy construct!" << endl;
    	}
    	~boy() {
    		cout << "boy destruct!" << endl;
    	}
    	void set_girl_friend(shared_ptr<girl>& g) {
    		girl_friend = g;
    	}
    private:
    	shared_ptr<girl> girl_friend;
    };
    class girl {
    public:
    	girl() {
    		cout << "girl construct!" << endl;
    	}
    	~girl() {
    		cout << "girl destruct!" << endl;
    	}
    	void set_boy_friend(shared_ptr<boy>& b) {
    		boy_friend = b;
    	}
    private:
    	shared_ptr<boy> boy_friend;
    };
    
    void use_trap() {
    
    	shared_ptr<girl> sp_girl(new girl());//白娘子
    	shared_ptr<boy> sp_boy(new boy());//许仙
    
    	sp_girl->set_boy_friend(sp_boy);
    	sp_boy->set_girl_friend(sp_girl);
    	/*
    	girl construct!
    	boy construct!
    	*/
    	//没有被析构！！！
    }
    

weak_ptr

class girl;

class boy {
public:
	boy() {
		cout << "boy construct!" << endl;
	}
	~boy() {
		cout << "boy destruct!" << endl;
	}
	void set_girl_friend(shared_ptr<girl>& g) {
		girl_friend = g;
	}
private:
	weak_ptr<girl> girl_friend; //****************lock*******************
};
class girl {
public:
	girl() {
		cout << "girl construct!" << endl;
	}
	~girl() {
		cout << "girl destruct!" << endl;
	}
	void set_boy_friend(shared_ptr<boy>& b) {
		boy_friend = b;
	}
private:
	shared_ptr<boy> boy_friend;
};

void use_trap() {

	shared_ptr<girl> sp_girl(new girl());//白娘子
	shared_ptr<boy> sp_boy(new boy());//许仙

	//弱指针的使用 
	weak_ptr<girl> wp_girl;			 //定义空的弱指针
	weak_ptr<girl> wp_girl2(sp_girl);//使用共享构造
	wp_girl = sp_girl;				 //允许共享指针赋值给弱指针  

	//weak_ptr<boy> wp_boy;

	cout << "sp_girl ref_count: " << sp_girl.use_count() << endl;// 1
	cout << "wp_girl ref_count: " << wp_girl.use_count() << endl;// 1

	//(*sp_girl).set_boy_friend(sp_boy); 弱指针不支持* 和 -> 对指针访问
	//sp_girl->set_boy_friend(sp_boy);

	//在必要时可以转成共享指针
	shared_ptr<girl> sp_girl1;
	sp_girl1 = wp_girl.lock();
	cout << "after lock, wp_girl ref_count: " << wp_girl.use_count() << endl;// 2

	sp_girl1 = NULL;


	sp_girl->set_boy_friend(sp_boy);
	sp_boy->set_girl_friend(sp_girl);
	/*
	girl construct!
	boy construct!
	*/
	//没有被析构！！！避免交叉使用或者把一个设置为弱指针
}