std::shared_ptr<int> sp1 = new int(); // shared count = 1, weak count = 0 std::shared_ptr<int> sp2(sp1); // shared count = 2, weak count = 0 std::shared_ptr<int> sp3 = sp2; // shared count = 3, weak count = 0 std::weak_ptr<int> wp1(sp1); // shared count = 3, weak count = 1 std::weak_ptr<int> wp2(wp1); // shared count = 3, weak count = 2 std::weak_ptr<int> wp3 = wp2; // shared count = 3, weak count = 3
shared_ptr weak_ptr 使用 reset 或者指向另一个 managed object导致 shared count或weak count相应的减一。
class Base {}; class Derived : public Base {}; ...... shared_ptr<Derived> dp1(new Derived); shared_ptr<Base> bp1 = dp1; shared_ptr<Base> bp2(dp1); shared_ptr<Base> bp3(new Derived);
shared_ptr<Base> base_ptr (new Base); shared_ptr<Derived> derived_ptr; // if static_cast<Derived *>(base_ptr.get()) is valid, then the following is valid: derived_ptr = static_pointer_cast<Derived>(base_ptr);
使用shared_ptr
make_shared 用法:
shared_ptr<Thing> p (make_shared<Thing>(42, "I'm a Thing!")); shared_ptr<Base> bp(make_shared<Derived1>()); // shared_ptr中的 template参数与make_shared中的tmeplate参数可以不一样(继承关系)
void do_it(weak_ptr<Thing> wp){ shared_ptr<Thing> sp = wp.lock(); // get shared_ptr from weak_ptr if(sp) sp->defrangulate(); // tell the Thing to do something else cout << "The Thing is gone!" << endl; }
也可以直接从weak_ptr构建shared_ptr,这个时间如果weak_ptr过期(通过 weak_ptr::expired() 可以查询),则抛出异常:
void do_it(weak_ptr<Thing> wp){ shared_ptr<Thing> sp(wp); // construct shared_ptr from weak_ptr // exception thrown if wp is expired, so if here, sp is good to go sp->defrangulate(); // tell the Thing to do something }