using C struct that is declared later

Question

I want use a typedef struct that isn't already defined, but it is later. Is there anything like a struct prototype?

file container.h

// i would place a sort of struct prototype here
typedef struct 
{
 TheType * the_type;
} Container;

file thetype.h

typedef struct {......} TheType;

file main.c

#include "container.h"
#include "thetype.h"
...

Not sure what of the relationship you're trying to establish between TheType and Container. What is it specifically you are asking? — octopusgrabbus, Feb 05 '12 at 21:48
@octopusgrabbus he's wanting to "forward declare a `typedef`" — Seth Carnegie, Feb 05 '12 at 21:49

score 5 · Answer 1 · answered Feb 05 '12 at 21:45

5

In container.h:

struct _TheType;
typedef struct _TheType TheType;

Than in thetype.h:

struct _TheType { ..... };

answered Feb 05 '12 at 21:45

Joshua

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This is the correct answer; for the OP's further illumination, what is being done here is called a "forward declaration". – tbert Feb 05 '12 at 21:48
2

identifiers that begin with an `_` followed by an uppercase letter are reserved identifiers – ouah Feb 05 '12 at 21:57
Sorry. I recalled the rule being somewhat different. – Joshua Feb 05 '12 at 22:19

score 5 · Accepted Answer · answered Feb 05 '12 at 22:07

Replace this line:

// i would place a sort of struct prototype here

with these lines:

struct TheType;
typedef struct TheType TheType;

Since you need the type TheType to be defined before type Container is defined, you have to use forward declaration of type TheType - and to do so you also need forward declaration of struct TheType.

Then you will not define typedef TheType like this:

typedef struct {......} TheType;

but you will define struct TheType:

struct {......};

score 1 · Answer 3 · edited May 23 '17 at 10:34

You can declare a struct inside the typedef:

typedef struct TheType_Struct TheType;  // declares "struct TheType_Struct"
                                        // and makes typedef
typedef struct
{
    TheType * p;
} UsefulType;

Note though that you may only have at most one typedef in one translation unit in C89 and C99 (this differs from C11 and C++).

Later on you must define the actual struct TheType_Struct { /* ... */ }.

pmg · Answer 4 · 2012-02-05T21:57:55.193

You cannot define an object of an, as yet, undefined struct; but you can define a pointer to such a struct

struct one {
    struct undefined *ok;
    // struct undefined obj; /* error */
};

int foo(void) {
  volatile struct one obj;
  obj.ok = 0;               /* NULL, but <stddef.h> not included, so 0 */
  if (obj.ok) return 1;
  return 0;
}

The above module is legal (and compiles with gcc without warnings).

using C struct that is declared later

4 Answers4