How to treat inherited classes as a member object? c++

Question

Assume the following:

class A {
    public: 
        A();
    protected:
        int a;
};

class B : public A {
    public: 
        B();
};

My questions:

Can I access (directly) the protected data members of A and maybe modify them without having to use public member functions?
Can I treat A inside B as an "existing" object so that I can return A by reference and be able to modify its data member?

I am new to C++. I tried couple things to treat "A" as an object but I keep getting error messages. Here is one thing I have tried:

A & B::getA() {
    return A; //error: "A does not refer to a value"
}

but how would I use `*this` to access the data member of that class? @πάνταῥεῖ — Olank1969, Apr 30 '16 at 11:08
Yes. Yes, except before A was constructed (in the initializer list of a `B()` constructor). — Ivan Aksamentov - Drop, Apr 30 '16 at 11:08
You don't need to use `this` to access data. Just type `a = ...` whatever. — Ivan Aksamentov - Drop, Apr 30 '16 at 11:09
You should really pick a book, it will answer all your questions: [The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — Ivan Aksamentov - Drop, Apr 30 '16 at 11:10
@πάνταῥεῖ Would you write it as an answer so I can mark it. Also, one last thing, if I passed an object of A by reference to a function inside B, can I still (directly) modify the data member of A that is inherited by B and assign it to the data member of the object being passed by reference? If yes, how? — Olank1969, Apr 30 '16 at 11:20

score 1 · Answer 1 · edited May 23 '17 at 12:16

Can I access (directly) the protected data members of A and maybe modify them without having to use public member functions?

A protected member variable or function is very similar to a private member but it provided one additional benefit that they can be accessed in child classes which are called derived classes.

Can I treat A inside B as an "existing" object so that I can return A by reference and be able to modify its data member?

In B you can directly access to A (public and protected) method and attributes.

Read here for more.

score 0 · Accepted Answer · answered Apr 30 '16 at 11:37

0

To return a reference to your base class you can write

A & B::getA() {
    return *this;
}

to access A's protected data member you can simply write

a = 5;

answered Apr 30 '16 at 11:37

πάντα ῥεῖ

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Would you please add an answer for this question too? If I passed an object of A by reference to a function inside B, can I still (directly) modify the data member of A that is inherited by B and assign it to the data member of the object being passed by reference? If yes, how? – Olank1969 Apr 30 '16 at 11:41
For example, `void B::setA(const A & obj) { a = obj.a }`. – Olank1969 Apr 30 '16 at 11:45
@Olank1969 http://stackoverflow.com/questions/36954049/how-to-treat-inherited-classes-as-a-member-object-c/36954405?noredirect=1#comment61465948_36954049 – πάντα ῥεῖ Apr 30 '16 at 11:58
The link doesn't work! Please I really need your answer to understand this last question. – Olank1969 Apr 30 '16 at 12:18
@Olank1969 The link works as intended. Why don't you simply try your code yourself? – πάντα ῥεῖ Apr 30 '16 at 12:19
I tried so much things, but nothing worked. Otherwise, I wont keep bothering you – Olank1969 Apr 30 '16 at 12:29
@Olank1969 Well `void B::setA(const A & obj) { a = obj.a; }` should work. Did you simply forget the semicolon? – πάντα ῥεῖ Apr 30 '16 at 12:30
no I have the semicolon, but I get this error message "a is a protected member of A" – Olank1969 Apr 30 '16 at 12:36
@Olank1969 Oh, yes. I always forget this, sorry. It's because `obj` is a different instance, and protected members cannot be accessed outside of the same instance. – πάντα ῥεῖ Apr 30 '16 at 12:39
@Olank1969 `void B::setA(const A & obj) { *this = obj; }` should work. – πάντα ῥεῖ Apr 30 '16 at 12:41
I tried that but it didn't work also. it says "no viable overloaded =" – Olank1969 Apr 30 '16 at 12:46
does this mean I have to overload the assignment operator ? – Olank1969 Apr 30 '16 at 12:51
@Olank1969 There should be a auto generated assignment operator.. May be a problem with proper casting `void B::setA(const A & obj) { *static_cast(this) = obj; }` – πάντα ῥεῖ Apr 30 '16 at 12:54
It worked. Do you know what was the problem exactly so I can learn more about it? – Olank1969 May 01 '16 at 01:31
@Olank1969 The problem was that you want to call `A:operator=(const A&)` but tried `B:operator=(const A&)` which isn't defined. Hence the cast to `A*`. – πάντα ῥεῖ May 01 '16 at 06:27
Thanks a lot. That really helps. Sorry for taking your time – Olank1969 May 01 '16 at 07:16

How to treat inherited classes as a member object? c++

2 Answers2