Can a struct have a member which is a pointer to a struct of the same type?

Question

Sounds super convoluted, but it's a simple concept. Say you have some struct "foo" one if it's members is a pointer to another foo struct (like a linked list)

Which seems to work.

struct foo {
   struct foo* ptr;
};

But what if I wanted foo to be a type?

Like how would I do the following?

typedef struct foo {
   foo* ptr;
} foo;

The declaration of ptr fails since foo is not a qualifier yet.

To amplify the previous comment: C and C++ differ in this respect, so the two get different answers. — Jerry Coffin, Aug 04 '14 at 00:07
Looks like C, amended the tag, feel free to revert if you meant otherwise. — Casper Beyer, Aug 04 '14 at 00:08
There's nothing stopping you doing `typedef struct foo { struct foo *ptr; } foo;` - you can have both a `struct _` and a `typedef` name for the same struct. — sapi, Aug 04 '14 at 00:08
Are you a new `C` programmer? Adding semicolons should be a reflex. — Fiddling Bits, Aug 04 '14 at 00:11
If you are asking about C http://stackoverflow.com/questions/506366/c-pointer-to-struct-in-the-struct-definition — 101010, Aug 04 '14 at 00:11
`Like how would I do the following?` In the definition just write `struct foo* ptr;` instead of `foo* ptr;`. — 101010, Aug 04 '14 at 00:16
This is an *extremely* common thing to do, needed to define linked lists and binary trees, for example. Any decent C textbook or tutorial should cover it. — Keith Thompson, Aug 04 '14 at 00:30
Thanks guys :) Yea, C was preferable, but I added a C++ tag because if this was easier in C++ I wouldn't mind, since I'm sorta learning both C and C++ right now. And yes, I am new to C, and programming in general. — Reuben Crimp, Aug 04 '14 at 00:49

score 5 · Accepted Answer · answered Aug 04 '14 at 00:17

5

Forward declare the definition.

typedef struct foo {
    struct foo* ptr
} foo;

Or you could forward declare the type declaration.

typedef struct node_t node_t;

typedef struct node_t {
   node_t *next;
} node_t;

answered Aug 04 '14 at 00:17

Casper Beyer

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For further reading see http://stackoverflow.com/questions/1675351/typedef-struct-vs-struct-definitions – Casper Beyer Aug 04 '14 at 00:18

score 1 · Answer 2 · answered Aug 04 '14 at 00:56

To go further in implementing Casper Von B's answer:

If you have a few variable types like int and maybe a char array in the struct, you will need to use malloc:

node_t *ptr = malloc(sizeof(node_t));

You can use the -> operator and . operator to choose the variable:

ptr -> x = 12;

Then, when finished use free() for garbage collection:

free(ptr);

My final note is you can continue to link structs together with pointers and then traverse through them.

Can a struct have a member which is a pointer to a struct of the same type?

2 Answers2