cannot make outer class as member object inside inner class - c++

Question

I have 3 questions about below code:
class cb
{
public:
int y_;
class iterator
{
public:
//void func() { y_ = 5; } // (1)
private:
int x_;
//cb a; // (2)
};
void funcCB() { }
};
class Human
{
public:
void func() const {
cb c; // (3)
c.funcCB();
}
// (4)
};
1- Why i can't use Outer class's member variable y_ inside Inner class as in (1)?
2- Why i can't create object from Outer class inside inner class as in(2) but i can only create pointer and reference?
3- Why if i moved the line at (3) cb c; to line at (4) i get compile error ?

Answer 1

You can't use outer class field because it's not like your class iterator have direct access to it. First of all why would you want to have access to that field? I would think about redesign your solution first. But as you wish here is what you could do to solve the issue. You would need to have reference to the object which is your parent
class cb
{
public:
cb(): it(*this) {} // Passing the reference to ourself for iterator object init to have access in iterator class access to y_ field.
int y_;
class iterator
{
public:
iterator(cb& ob): a(ob) {}
void func() { a.y_ = 5; }
private:
int x_;
cb& a; // Reference to parent object
};
iterator it; // Added because I don't see the point where you don't want to have that object in your cb class.
void funcCB() { }
};
I'm not sure what would you like to achieve here. It will throw incomplete type because you will want to create cb object in which you will create iterator object in which you will create cb object in which again you will create iterator so you would get infinite recursion (if I understand that correctly). When you point iterator to concrete object it will stop the cycle.
You should change in class cb funcCB to be void funcCB() const {} because otherwise in class Human it is understanded that in func() which is const function you want to call funcCB() which will not modify the object c from Human class and there is a difference between qualifiers because you don't have const function in that case. You can also delete const qualifier in Human class for func and this will also work. Error here happens when you don't change qualifiers, because it is different when you have const function in which you create some object to further use and when you have object as a class field and you call function which cannot change the state of Human class and CB class don't have equivalent of const function for funcCB.

Answer 2

Why i can't use Outer class's member variable y_ inside Inner class as in (1)?
Because y_ is a non-static data member of class cb, meaning we must access it on a particular cb object. But here comes the problem. The assignment statement y_ = 5; is equivalent to:
vvvv--------------->this points to an object of type iterator and not cb
this->y_ = 5;
In the above shown equivalent statement, the this pointer points to the current instance of type iterator and not cb. But since to access y_ we must use a cb type object, we get the mentioned error.
Basically y_ should be accessed on an object of type cb. For example, you can make func to have a parameter of type cb& and then access y_ as shown below:
class cb
{
public:
int y_;
class iterator
{
public:
//------------vvv------------------->pass object of type cb by reference
void func(cb& it) { it.y_ = 5; } // (1) OK NOW
//----------------------^^---------->access member y_ on object it
private:
int x_;
};
void funcCB() { }
};
Why i can't create object from Outer class inside inner class as in(2) but i can only create pointer and reference?
Because at point #2 the class cb is incomplete and so at point #2 we cannot create a non-static data member of type cb. This can be seen from complete type documentation which states:
Any of the following contexts requires type T to be complete:
declaration of a non-static class data member of type T;
This means that at point #2 we cannot have a declaration for a non-static data member of type cb but we can still have a declaration for a non-static data member of type cb& or cb* as we can have a pointer or a reference to an incomplete type like cb.
Why if i moved the line at (3) cb c; to line at (4) i get compile error ?
If you moved line #3(cb c;) to line #4, you won't get any compile time error. Demo
class Human
{
public:
void func() const {
//cb c; // (3)
//c.funcCB();
}
cb c; // (4) perfectly fine
};

Answer 3

Forgive me if I don't quite get this -- my C++ is a little rusty.
Regarding question (1) -- Have you tried the "this" keyword?
E.g.,:
void func() { this->y_ = 5; }
Here is an example almost identical to yours:
'this' pointer in C++
For question 2, you're asking for a recursive inclusion of an object. That's like saying, What is an Onion? It is a Peel that contains an Onion. Well, that Onion inside is also a Peel that contains an Onion, then a Peel, then an Onion, etc. Without some mechanism to halt (an Onion might be a Core, which contains nothing), the first instantiation of the Onion object would loop infinitely until all memory was used up.
As a reference, the memory can be retrieved only when needed, and not before.
For question 3, I'm taking the lazy way out to say, them's just the rules.
UPDATE:
I'm not at work now, so I had more time to do a little more research. As I've said, my C++ is a little rusty.
Essentially for Q 1, what you're attempting to do is access the members of class "cb" from class "iterator". My research indicates that, just because "iterator" is a subclass of "cb", it doesn't get any special access privileges. So you can not access "cb::y_" from "iterator" and you can not use "this->y_" either. Methods (functions) inside the class can access "y_" directly. Sub-classes can not.
Note the following which compiles successfully:
class cb {
public:
int y_;
class iterator {
public:
void func() {
cb *z = new cb();
z->y_ = 5;
} // (1)
private:
int x_;
cb *a; // (2)
};
void funcCB() { }
};
class Human
{
public:
void func() const {
cb c; // (3)
c.funcCB();
}
// (4)
cb *z;
};
This probably is not what you're attempting to accomplish, but I expect that what you want to do can not be done the way you want. Your best bet would be to implement the "iterator" as its own class distinct from "cb" and make it a "friend" class of "cb". Or maybe better yet, and simpler, just include a set of iterator methods inside the class. The methods will have full access to all of the class attributes.
Also note the pointer reference to "cb". It does not compile as a straight object instantiation in a sub-class. Again, the Onion issue.
On a side note, remember that C++ is not Pascal. It's not Java. It can do sub-classes, but as you can see, it doesn't do them very well, or at least, it doesn't do them the way other languages do them. You're better off creating every class completely distinct from every other class, other than normal inheritance. That's sort of unofficially "The C++ Way."