Managing the lifetime of member functions bound by `std::bind`

I am currently experimenting with writing an event queue in C++11. I am using std::bind to obtain std::function objects which are called when certain events happen. The code for this roughly looks like this:

class A
{
public:
  void handle();
};

class B { ... };

// Later on, somewhere else...
std::vector< std::function< void() > functions;

A a;
B b;

functions.push_back( std::bind( &A::handle, &a ) );
functions.push_back( std::bind( &B::handle, &b ) );

// Even later:
for( auto&& f : functions )
  f(); // <--- How do I know whether f is still "valid"?

Is there any way to guarantee the validity of the function object so that I can avoid stumbling over undefined behaviour here?

I have already taken a look at this question here, std::function to member function of object and lifetime of object, but it only discussed whether deleting a pointer to a bound object raises undefined behaviour. I am more interested in how to handle the destruction of such an object. Is there any way to detect this?

EDIT: To clarify, I know that I cannot guarantee a lifetime for non-static, non-global objects. It would be sufficient to be notified about their destruction so that the invalid function objects can be removed.