C typedef function prototype with struct attempting to reference before defined.

Question

I need to reference a struct that's not yet defined because the struct actually conatins the typedef'd function prototype.

For example,

typedef int (MyCallbackFunction)(X * x, void * ctx);

typedef struct CallbackData {
  MyCallbackFunction * callback;
  void * ctx;
} CallbackData;

typedef struct X {
  char a;
  int b;
  int c;
  double d;

  CallbackData e;
} X;

What's the valid way to actually write this code/header ?

Kerrek SB · Accepted Answer · 2011-07-30 22:26:55Z

5

Just forward declare the relevant types - and you can make the function pointer part of the typedef:

struct X_;

typedef int (*MyCallbackFunction)(struct X_ * x, void * ctx);

typedef struct CallbackData_ {
  MyCallbackFunction callback;
  void * ctx;
} CallbackData;

typedef struct X_ {
  char a;
  int b;
  int c;
  double d;

  CallbackData e;
} X;

edited Jul 30, 2011 at 22:26

answered Jul 30, 2011 at 22:21

Kerrek SB

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2 Comments

Adam M-W Over a year ago

Will this work if I pass in a callback function that uses for example: int callback(X * x, void * ctx), or will the compiler complain that X is different from struct X?

Kerrek SB Over a year ago

Sorry, I fixed it -- I generally prefer to give the struct and its typedef different names so you can unambiguously refer to both as needed. See the edit.

Jens Gustedt · Accepted Answer · 2011-07-30 22:23:29Z

5

Just forward declare your typedefs

typedef struct X X;
typedef struct CallbackData CallbackData;

and then declare the structs later.

answered Jul 30, 2011 at 22:23

Jens Gustedt

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Comments

Lara Bailey · Accepted Answer · 2011-07-30 22:21:11Z

0

Yes, you can forward declare the struct and use it in the declaration of MyCallbackFunction where you don't need it to be a complete type.

struct X;
typedef int (MyCallbackFunction)(struct X * x, void * ctx);

answered Jul 30, 2011 at 22:21

Lara Bailey

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