Forward declare a struct in C

Question

How do I forward declare the following treeNodeListCell struct?

I tried writing struct treeNodeListCell before the struct definitions and the code doesn't compile.

Does anyone have an idea?

struct treeNodeListCell;

typedef struct _treeNode {
    treeNodeListCell *next_possible_positions;
} treeNode;

typedef struct _treeNodeListCell {
    treeNode *node;
    struct _treeNodeListCell *next;
} treeNodeListCell;

Your actual structure name is `_treeNodeListCell` which is typedef-ed to `treeNodeListCell` . You sure this is not a mistake ? — Rahul Bharadwaj, May 02 '20 at 18:41
@RahulBharadwaj umm yes, I copy pasted the definition that are professor gave us, and usually thats how I write structs — Yaniv Avrahami, May 02 '20 at 18:44
Greetings to your professor, he should read https://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier — Werner Henze, May 02 '20 at 19:23

user3386109 · Accepted Answer · 2020-05-02T19:43:29.770

You can forward declare a struct, but when you do, you need to use the struct keyword with the forward-declared struct tag.

struct _treeNodeListCell;

typedef struct _treeNode {
    struct _treeNodeListCell *next_possible_positions;
} treeNode;

typedef struct _treeNodeListCell {
    treeNode *node;
    struct _treeNodeListCell *next;
} treeNodeListCell;

The alternative is a forward-declared typedef. C allows you to typedef an incomplete type, which is to say that you can typedef the structure before defining the structure. That allows you to use the typedef in the structure definition.

typedef struct _treeNodeListCell treeNodeListCell;

typedef struct _treeNode {
    treeNodeListCell *next_possible_positions;
} treeNode;

struct _treeNodeListCell {
    treeNode *node;
    treeNodeListCell *next;
};

If you want to use the structures in the question without changing them, all you need is the typedef before the structure definitions.

typedef struct _treeNodeListCell treeNodeListCell;

typedef struct _treeNode {
    treeNodeListCell *next_possible_positions;
} treeNode;

typedef struct _treeNodeListCell {
    treeNode *node;
    struct _treeNodeListCell *next;
} treeNodeListCell;

John Bollinger · Answer 2 · 2020-05-02T19:33:52.580

Quick question, how do I forward declare the following treeNodeListCell struct.

You don't need to.

You must in the first place discriminate between identifying a structure type by tag and identifying it via a typedefed alias. And in particular, you need to appreciate that typedef is completely optional. Where you use it to define aliases for structure types, it may be more clear to separate the typedef declaration from the structure declaration.

Here are your declarations without any typedef:

struct _treeNode {
    struct _treeNodeListCell *next_possible_positions;
};

struct _treeNodeListCell {
    struct _treeNode *node;
    struct _treeNodeListCell *next;
};

No forward declarations are required for structure types expressed in struct <tag> form.

You may add typedefs, too. They can be associated with the definitions above by adding the typedef keyword and one or more identifiers, or they may simply be written separately, as I previously recommended:

typedef struct _treeNode treeNode;
typedef struct _treeNodeListCell treeNodeListCell;

Personally, I think typedefs are much overused. I usually do not define typedef aliases for my structure and union types.

BUT if you really want to do then you can declare a typedef of an incomplete type, such as an as-yet undefined structure type. This is a regular declaration, not a forward declaration, but it would allow you to use the aliases in the definitions of the structures, which I take to be your objective:

typedef struct _treeNode treeNode;
typedef struct _treeNodeListCell treeNodeListCell;

struct _treeNode {
    treeNodeListCell *next_possible_positions;
};

struct _treeNodeListCell {
    treeNode *node;
    treeNodeListCell *next;
};

In fact, starting in in C11, you can write multiple declarations of the same typedef name in the same scope, as long as they all define the name to identify the same type. This provision can be leveraged to allow the typedef / structure declarations presented in the question to compile. Note well that designating the same type does not require the type to be expressed the same way. Since this is supposed to be an exercise, I will leave it to you to work out the few remaining details.

I like your answer, but the thing is that our lecturer in university gave us those struct definitions and he doesn't want us to change them, part of the assignment is to figure out what to write before the structs to make the code work — Yaniv Avrahami, May 02 '20 at 19:12
@YanivAvrahami, I have added a few additional remarks in that direction at the end. The task presented by the lecturer is achievable, and this answer contains just about everything you need except the final answer itself. — John Bollinger, May 02 '20 at 19:36

score -1 · Answer 3 · answered May 02 '20 at 18:43

-1

You cannot omit struct in C.

You should use

struct treeNodeListCell *next_possible_positions;

instead of

treeNodeListCell *next_possible_positions;

answered May 02 '20 at 18:43

MikeCAT

73,922
11
45
70

You can omit the "struct" keyword as long as you used typedef. – Bitmapped Apr 02 '22 at 02:14

Forward declare a struct in C

3 Answers3