Can a class definition contain that class' pointer in C++?

Question

I'm trying to define a class this way (simplified):

class Student
{
    private:
        Student* pointer;
    public:
        set_pointer(Student*);
}

Is this legal in C++ and is it consistent with object oriented design?

Yes, it's legal. Whether it is a good design depends on what you are going to do with it. — , Feb 18 '18 at 22:09
@NeilButterworth: Why are you answering in the comments section? — Lightness Races in Orbit, Feb 18 '18 at 22:11
@Lightness Because I wanted to. And because I assumed it was a dupe. — , Feb 18 '18 at 22:12
Could somebody please tell us in what way this could be *inconsistent* with OO design? — user207421, Feb 18 '18 at 22:17

score 2 · Accepted Answer · answered Feb 18 '18 at 22:10

2

Yes, a class may store a pointer to an object of the same type (which may be itself, or some other instance).

This is common in linked-list implementations, where a node stores a pointer to the next node.

As for whether it's "consistent with object oriented design", it is impossible to make a generalisation on the subject; it depends on the program. Personally I cast a suspicious eye on code of this form, but again you can't really generalise.

answered Feb 18 '18 at 22:10

Lightness Races in Orbit

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Such classes are "recursively defined". – ChiefTwoPencils Feb 18 '18 at 22:13
So you cast a suspicious eye on linked lists? – user207421 Feb 18 '18 at 22:18
@ChiefTwoPencils: No, they're not. – Lightness Races in Orbit Feb 18 '18 at 22:21
1

@EJP: Sometimes. – Lightness Races in Orbit Feb 18 '18 at 22:21
Sometimes *why*? How about trees? – user207421 Feb 18 '18 at 22:37
@EJP: Generally anything that isn't a data structure gets the suspicious eye. – Lightness Races in Orbit Feb 18 '18 at 22:44
@LightnessRacesinOrbit, yes they are. – ChiefTwoPencils Feb 18 '18 at 22:56
@ChiefTwoPencils: A recursively defined class is impossible, but you could write a non-compiling program that contains one like this: `struct T { T child; };` – Lightness Races in Orbit Feb 18 '18 at 23:34
@Lightness, data structures like a `tree` (or "linked-list") are said to be recursively defined because their backing `node` classes are indeed recursively defined (a `tree` can just be the root `node` in an application; the `tree` is just a convenience/generalization of a structure with a single node as a root). With the list we have `node *next;` inside the `node` class. I understand there are limitations as you suggest, however, because a `node` has a pointer to a `node` doesn't change that the class is defined using its own definition; meaning a `node` has one or more `node`s. – ChiefTwoPencils Feb 19 '18 at 02:58
@ChiefTwoPencils: This is a difference in terminology coming from different layers of abstraction. From the C++ perspective, it's what I said above. From a computer science/mathematical perspective I guess you could define it any number of ways. – Lightness Races in Orbit Feb 19 '18 at 09:57

Can a class definition contain that class' pointer in C++?

1 Answers1