In C++, nested classes are not connected to any instance of the outer class. If you want bar to access non-static members of foo, then bar needs to have access to an instance of foo. Maybe something like:
class bar {
public:
int getA(foo & f ) {return foo.a;}
};
Or maybe
class bar {
private:
foo & f;
public:
bar(foo & g)
: f(g)
{
}
int getA() { return f.a; }
};
In any case, you need to explicitly make sure you have access to an instance of foo.