Check if container of shared_ptr contains a pointer?
std::list<SomeSharedPointer> items_;
...
void Monitor::Update(Subject *subject)
{
    if(subject == something_)
    {
        DoSomething();
    }
    else if
    ??
    // if subject is one of the objects in our collection then do something..

}

auto i = std::find_if(items_.begin(), items_.end(),
    [=](const SomeSharedPointer& x) { return x.get() == subject; });

if (i != c.end())
{
    // Object found, and i is an iterator pointing to it
}

typedef std::list<SomeSharedPtr> ObserverCollection;

// You can also add a const version if needed
ObserverCollection::iterator find_observer(Subject* s)
{
    return std::find_if(items_.begin(), items_.end(),
        [=](const SomeSharedPointer& x) { return x.get() == s; });
}

auto i = find_observer(subject);
if (i != items_.end())
{
    // Object found
}

if (find_observer(subject) != items_.end())
{
    ...
}

for (auto iter = items_.begin(); iter != items_.end(); ++iter)
{
     if (subject == iter->get())
     {
         .. do stuff ..
     }
}

enum State{ STATE_1, STATE_2 };

void Monitor::Update(Subject *subject)
{
    switch( subject->getState() )
    {
      case STATE_1:
         // do something 1
         break;
      case STATE_2:
         // do something 2
         break;
      default:
         //error
    }
}