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

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);

shared_ptr<Thing> p (make_shared<Thing>(42, "I'm a Thing!"));
shared_ptr<Base> bp(make_shared<Derived1>());     // shared_ptr中的 template參數(shù)與make_shared中的tmeplate參數(shù)可以不一樣（繼承關(guān)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;
}

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
}