How to get value of a private c++ typed field in a oc class?

I have objective-c class in iOS, there is a field whose type is c++ class. I don't have the definition of the c++ class.
How to get the value of this field ?
And how to change the field value of this c++ typed field ?
For example.

@interface A 
    B b // c++ type
@end

class B {
    int c;
}

How to get the value of b ? How to change the value of c?