Why is using the structure variable itself in its element not allowed?

Question

Why declaring a structure with its own identifier as an element of it is not allowed? where as a self referential structure which is declared by the same variable of the struct with a pointer symbol!

this is the code I tried out:

 #include<stdio.h>
    #include<conio.h>
    struct am
    {
     int a;
     struct am k; // this reported an error
    };

This code reported me a error where as when i used it as a pointer it accepted it , i searched the web and i came to know that it was called the self referential structure

#include<stdio.h>
    #include<conio.h>
    struct ko
    {
     int a;
     struct ko * op; // this was allowed by my compiler
    };

this structure worked please brief me !

I'm using a TurboC++ version 3.0 by borland international inc.

Think about it, compute the size of am, which contains itself. — Borgleader, Dec 11 '14 at 14:07
no I'm asking that why a self referential structure is allowed where as a structure with its own variable declaration is not allowed something like this struct ko { struct ko l; }; why is this not allowed? — satabios, Dec 11 '14 at 14:08
Try placing a 2'x 2'x 2' box inside a similar 2' x 2' x 2' box that's inside another box and so on. That's what you're trying to do in the first example. It can't work. But you can take a 2' x 2' x 2' box and place a note inside it where you write the location of another box. That's what you're doing in the 2. example with a pointer. — nos, Dec 11 '14 at 14:12
Thank you nos i get what you're trying to say and sorry if this post was done before im just a beginner — satabios, Dec 12 '14 at 17:06

score 9 · Accepted Answer · answered Dec 11 '14 at 14:08

9

An object cannot contain an object of the same type as itself; that's a logical impossibililty, as it would need to be larger than itself.

As you say, it could contain a pointer or reference, but not an object.

I'm using a TurboC++ version 3.0 by borland international inc.

I've no idea why you'd use a twenty-year old compiler, and an ancient dialect of C++ that few now remember. But this was just as impossible then as it is now.

answered Dec 11 '14 at 14:08

Mike Seymour

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*I've no idea why you'd use a twenty-year old compiler*, statistically speaking, his teacher probably told him to use it. – Borgleader Dec 11 '14 at 14:14
it's not even standard C++ – phuclv Dec 11 '14 at 14:30
our syllabus includes only these and not the modern ones. sorry to have them – satabios Dec 12 '14 at 17:08
@SathyaNarayanan: I'd drop the course then. It will only waste your time. If you still want to learn C++, teach yourself with a [good book](http://stackoverflow.com/questions/388242). – Mike Seymour Dec 12 '14 at 17:12
advise me a book then – satabios Dec 14 '14 at 14:10
@SathyaNarayanan: I did: http://stackoverflow.com/questions/388242 – Mike Seymour Dec 14 '14 at 16:15

score 1 · Answer 2 · answered Dec 11 '14 at 15:13

Two reasons:

The struct am type isn't complete until the closing } of the struct definition; when you try to declare k as an instance of struct am, the compiler still doesn't know how big struct am needs to be.
A struct cannot contain an instance of itself, since that member would also have to contain an instance of itself, which would contain an instance of itself, ad inifinitum; the resulting object would be inifinitely large.

A struct can contain a pointer to an instance of the same type because a) you can create pointers to incomplete types, and b) all struct pointer types have the same size and representation.

Why is using the structure variable itself in its element not allowed?

2 Answers2