#include <iostream>
#include <memory>

using namespace std;

/**
 * 如何对shared_ptr, weak_ptr空指针进行判断?
 * 除了常规的shared_ptr::get, use_count(), 测试直接用if语句进行判断
 */
int main()
{
	std::shared_ptr<int> p1;
	std::shared_ptr<int> p2(new int);

	cout << "p1.use_count = " << p1.use_count() << endl; // p1.use_count = 0
	cout << "p2.use_count = " << p2.use_count() << endl; // p2.use_count = 1

	if (p1) cout << "p1 is not null" << endl;
	else cout << "p1 is null" << endl; // p1 is null

	if (p2) cout << "p2 is not null" << endl; // p2 is not null
	else cout << "p2 is null" << endl;

	std::weak_ptr<int> p3;
	std::weak_ptr<int> p4(p1);
	std::weak_ptr<int> p5(p2);

	cout << "p3.use_count = " << p3.use_count() << endl; // p3.use_count = 0
	cout << "p4.use_count = " << p4.use_count() << endl; // p4.use_count = 0
	cout << "p5.use_count = " << p5.use_count() << endl; // p5.use_count = 1

	if (p3.lock()) cout << "p3 is not null" << endl;
	else cout << "p3 is null" << endl; // p3 is null

	if (p4.lock()) cout << "p4 is not null" << endl;
	else cout << "p4 is null" << endl; // p4 is null

	if (p5.lock()) cout << "p5 is not null" << endl; // p5 is not null
	else cout << "p5 is null" << endl;
	return 0;
}