Q:

How to refer to enclosing instance from C++ inner class? 

In C++, an object refers to itself via this. 
But how does an instance of an inner class refer to the instance of its enclosing class? 
class Zoo
{
    class Bear 
    {
        void runAway()
        {
            EscapeService::helpEscapeFrom (
                this, /* the Bear */ 
                ??? /*  I need a pointer to the Bear's Zoo here */);
        }
    };
};

EDIT
My understanding of how non-static inner classes work is that Bear can access the members of its Zoo, therefore it has an implicit pointer to Zoo.  I don't want to access the members in this case; I'm trying to get that implicit pointer.  

A:

Unlike Java, inner classes in C++ do not have an implicit reference to an instance of their enclosing class. 
You can simulate this by passing an instance, there are two ways :
pass to the method :  
class Zoo
{
    class Bear 
    {
        void runAway( Zoo & zoo)
        {
            EscapeService::helpEscapeFrom (
                this, /* the Bear */ 
                zoo );
        }
    };
}; 

pass to the constructor :  
class Zoo
{
    class Bear
    {
        Bear( Zoo & zoo_ ) : zoo( zoo_ ) {}
        void runAway()
        {
            EscapeService::helpEscapeFrom (
                this, /* the Bear */ 
                zoo );
        }

        Zoo & zoo;
    };
}; 

A:

Inner classes are not special, and don't have any link to their outer class built-in.  If you want to access the outer class, then pass a pointer or reference, just as you would with any other class. 

A:

An inner class has access to all members of the outer class, but it does not have an implicit reference to a parent class instance.   
To answer your modifed Q:
No you cannot access that implicit pointer.  I believe one can do so in Java but not in C++. 
You will have to pass the outer class object explicitly through constructor or some other function to acheive this.   
Technically as per C++03 standard(sec 11.8.1), a nested class does NOT have special access to its enclosing class.  
But there is also this standard defect: openstd.org/jtc1/sc22/wg21/docs/cwg_defects.html#45  Not sure if this is closed.