Example

class Enclosing {
private:
int field;
public:
class Nested {
void function(Enclosing *ptr) {
ptr->field = 10;
}
};
};

Solution

Declare the nested class a friend.

class Enclosing {
private:
int field;
public:
friend class Nested;
class Nested {
void function(Enclosing *ptr) {
ptr->field = 10;
}
};
};