std::shared_ptr<int> a = std::make_shared<int>(666);
std::shared_ptr<int> b = a;
std::shared_ptr<int> c = a;
std::shared_ptr<int> d = a;
std::cout << a << std::endl;
std::cout << b << std::endl;
std::cout << c << std::endl;
std::cout << d << std::endl;

use_count

std::shared_ptr<int> a = std::make_shared<int>(666);
std::shared_ptr<int> b = a;
std::shared_ptr<int> c = a;
std::shared_ptr<int> d = a;
// 打印出4，表示有4个指针指向666
std::cout << d.use_count() << std::endl;

reset释放内存，但是共享指针只有所有指针都不再指向666的时候，才会真正释放

std::shared_ptr<int> a = std::make_shared<int>(666);
std::shared_ptr<int> b = a;
std::shared_ptr<int> c = a;
std::shared_ptr<int> d = a;
a.reset();
// 打印出3，原来是4，因为a.reset了，所以还剩下bcd
std::cout << d.use_count() << std::endl;