How do you detect and handle invalid pointers?

Hi,

How do you detect and handle invalid pointers to avoid segmentation fault?

I have a callback scenario that looks like this. (using C++0x)

#include <functional>
#include <string>
#include <iostream>
#include <vector>

class SomeObject
{
public:
    void fnc( const std::string& text )
    {
       std::cout << text << std::endl;
    }
};

int main()
{
    std::vector<std::function<void()> > callbacks;
    SomeObject* s = new SomeObject;

    callbacks.push_back( std::bind( &SomeObject::fnc, s, "Hello" ) );
    callbacks.push_back( std::bind( &SomeObject::fnc, s, "World" ) );
    callbacks.push_back( std::bind( &SomeObject::fnc, s, "The End" ) );

    std::vector<std::function<void()> >::iterator i = callbacks.begin();
    std::vector<std::function<void()> >::iterator end = callbacks.end();
    for( int n = 0; i != end; ++i, ++n )
    {
       (*i)(); // Run the callback.

       // At some point during this loop s is deleted. Could be from
       // another thread.
       if( n == 1 )
          delete s;
    }

    return 0;
}

This callback trouble is just an example, but how are invalid pointers
detected in general? Is there a smart pointer that can handle this? Like
throwing an exception or just not run the callback.

Thanks,
Daniel