CPP_SmartPtr_07.cpp

#include<iostream>
#include<string>
#include<memory>
using namespace std;

class Dog {
	weak_ptr<Dog> m_pFriend;
public:
	string m_name;
	Dog(string name) : m_name(name) { cout << "Dog: " << m_name << " is defined!" << endl; }
	void bark() { cout << "Dog " << m_name << " rules!" << endl; }
	~Dog() { cout << "Dog is destroyed: " << m_name << endl; };
	void makeFriend(shared_ptr<Dog> f) { m_pFriend = f; }
	void showFriend() {
		//Compile error. m_pFriend cannot direct access the object like the normal pointer.
		//cout << "My Friend is: " << m_pFriend->m_name << endl;


		// The following code fix the problem. lock() function will create 
		//a shared_ptr of weak_ptr. It will check whether the weak pointer 
		//is still pointing to a valid object, and make sure that when the
		// weak pointer is accessing the object, the object has not been 
		//deleted!
		//If the weak pointer is empty, the lock() will throw an exception.
		if (!m_pFriend.expired())
			cout << "My Friend is: " << m_pFriend.lock()->m_name << endl;
		cout << "He is owned by " << m_pFriend.use_count() << " pointers" << endl;
	}
};
int main() {
	shared_ptr<Dog> pD(new Dog("Gunner"));
	shared_ptr<Dog> pD1(new Dog("Smile"));
	pD->makeFriend(pD1);
	pD1->makeFriend(pD);
	pD->showFriend();
	return 0;
}