CODEWITHSUNDEEP
cincludetypedef
Art Swri
Art Swri

Reputation: 2804

How to partially declare a struct that is typedef'ed when in an include file

I'm trying to minimize interdependence of #include files as a general practice.

In xxx.h I have:

struct my_struct;  // partial decl to satisfy use of my_struct*
void funct(struct my_struct* ms);  // uses the partial def

How to do a similar partial decl with a typedef'd struct? I have an actual decl in some third #include that looks like (say in yyy.h):

typedef struct my_data_s {
  int ival;
  ... struct's other components ...
} my_data_t;

I just want a representative decl in xxx.h that reference the typedef:

typedef struct my_data_s  my_data_t;  // actual full decl is elsewhere
void funct2(my_data_t* md);

This attempt causes 'redefinition of typedef my_data_t' error. (Using gcc 4.4.3 / Ubuntu 10.4) Other random search attempts (e.g., add '{}' to typedef) also give errors.

I know the compiler only needs to know that the function requires a pointer, so it seems like this should be possible. So far, found nothing that compiles w/o errors/warnings.

I've looked at the other questions and answers, could not find this problem addressed. Seems like there should be a well-known way to do this(?!) (I know that I can #include yyy.y whenever I #include xxx.h - trying to avoid such dependencies.) Thanks.

Upvotes: 5

Views: 2590

Answers (3)

Art Swri
Art Swri

Reputation: 2804

Here's what our group decided to do. It represents a compromise of several conflicting requirements/desires. I am posting to show another approach, so readers of the post have some variety to choose from. It represents the best answer for our situation.

obj_a_defs.h

// contains the definition
// #include'd only by other .h files
...
#define ... // as needed for struct definition
...
typedef struct obj_a {
  ...
} obj_a;

obj_a.h

// contains the 'full info' about obj_a: data and behaviors
// #include'd by other .c files
...
#include "obj_a_defs.h"
...
// declares functions that implement 
// the behaviors associated with obj_a

obj_a.c

...
#include "obj_a.h"
...
// implementations of functions declared in obj_a.h

obj_b.h

// a 'user' of obj_a that uses obj_a as arg
#include "obj_a_defs.h"  // to get the typedef    
...
int some_b_funct(obj_a* obja, ...);
...

obj_b.c

// Defines the 'contract' that this implementation
// is meeting.
#include "obj_b.h"
...
// This .c file includes obj_a.h only if it
// uses the functions defined for obj_a.
// If obj_a is used only to 'pass through'
// to other modules, there's no need for 
// this include.
#include "obj_a.h"  // only if obj_b uses 
...
// obj_b's function implementations

Rationale / Conditions

  • typedef and struct kept together
  • a .c file that uses obj_X must #include "obj_X.h" to show that use
  • avoid .h files including other .h files in general; only 'defs.h' files are #include'd in .h files.
  • avoid #include'ing a file just to handle dependencies; IOW avoid #include'ing obj_a.h just because it's used in obj_b.h

Upvotes: 0

Jens Gustedt
Jens Gustedt

Reputation: 78943

C99 doesn't allow to repeat a typedef, C11 does.

Just do the typedef only once and always have it first:

typedef struct my_data  my_data;

There also is no need to chose different names for the struct tag and the typedef identifier.

Upvotes: 4

thiton
thiton

Reputation: 36059

Have you tried the simple approach:?

xxx.h

struct my_data_s;
typedef struct my_data_s my_data_t;

yyy.h

#include "decl.h"
struct my_data_s {
   int foo;
};

Upvotes: 3

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