C nested structs in correlation

Question

I have the following code:

#define MAX_VIDEOS 100

typedef struct video_t {
    int likes;
    int dislikes;
    char* title;
    user_t* uploader;
} video_t;

typedef struct user_t {
    char* username;
    char* password;
    video_t videos[MAX_VIDEOS];
} user_t;

I want to use user_t in video_t and vice versa.

In every case, gcc just says "unknown type name":

youtube.c:9:5: error: unknown type name ‘user_t’
user_t* uploader;
  ^

which is normal, but I can't think of a way to solve this problem.

you need to declare them beforehand (as prototypes) – UnholySheep Feb 03 '17 at 09:28 — UnholySheep, Feb 03 '17 at 09:28

score 3 · Accepted Answer · answered Feb 03 '17 at 09:29

3

You need to forward-declare user_t:

#define MAX_VIDEOS 100

typedef struct user_t user_t;

typedef struct video_t {
    int likes;
    int dislikes;
    char* title;
    user_t* uploader;
} video_t;

typedef struct user_t {
    char* username;
    char* password;
    video_t videos[MAX_VIDEOS];
} user_t;

answered Feb 03 '17 at 09:29

Vittorio Romeo

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Thanks to everyone for the fast answers, it solved my problem perfectly. – wencakisa Feb 03 '17 at 09:32
Isn't forward-declare a bit misleading here? I'd say _define_. – Sourav Ghosh Feb 03 '17 at 09:36
@SouravGhosh: it's [not a *definition*](http://stackoverflow.com/questions/1410563/what-is-the-difference-between-a-definition-and-a-declaration). It's a *forward-declaration* + `typedef`. – Vittorio Romeo Feb 03 '17 at 09:39
Right, but `user_t` is defined (as a result of typedef), I'm not talking about `struct user_t`. – Sourav Ghosh Feb 03 '17 at 09:42

score 1 · Answer 2 · answered Feb 03 '17 at 09:29

1

Forward declare the user_t type-alias:

typedef struct user_t user_t;

After that you can use user_t *uploader in the video_t structure.

answered Feb 03 '17 at 09:29

Some programmer dude

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Sourav Ghosh · Answer 3 · 2017-02-03T09:47:52.820

Move the type definition for the missing type at the beginning. This makes sure compiler is aware of the type while it is being used.

Something like

 typedef struct user_t user_t;

and then, later, just declare the structure, the typedef is already there.

at the beginning of the shown snippet can solve the issue. In this way

While declaring struct video_t, user_t is already known and the typedef video_t is defined.
While declaring struct user_t, video_t is known.

So, both end meets.

score 1 · Answer 4 · answered Feb 03 '17 at 09:29

The first struct doesn't know the signature of user_t (declare it forward)

Change to

#define MAX_VIDEOS 100

typedef struct user_t user_t;

typedef struct video_t {
    int likes;
    int dislikes;
    char* title;
    user_t* uploader;
} video_t;

struct user_t {
    char* username;
    char* password;
    video_t videos[MAX_VIDEOS];
};

C nested structs in correlation

4 Answers4