problems with typedef on C

I writing some ADT on C: I have two file date.c and date.h inside date.c I have:

typedef struct Date_t {
 int date;
 char* month;
 int year;

} Date;

inside date.h I have:

typedef Date pDate;

compiler gives me errors:

..\checking.h:15: error: syntax erro开发者_Go百科r before "pDate"

can somebody please explain what is wrong with my typedef, thanks in advance

EDIT:

with files all is ok, problem is, when I change my struct to:

struct Date_t {
     int date;
     char* month;
     int year;
    
    };

and pointer to:

typedef struct Date_t* pDate;

program works perfectly, so I want to understand the difference

---

You are probably declaring pDate as a typedef of Date before you even declared Date (since you are make pDate in the header).

You could do:

typedef struct Date_t {
 int date;
 char* month;
 int year;

} Date;

typedef Date pDate;

All in you .c file.

As for your edit:

You are declaring the struct in the typedef:

typedef struct Date_t* pDate;

It is working because you are declaring struct before you place pDate.

typedef /*see this struct here*/ struct /*huh?*/ Date_t* pDate;

The other has no struct in it.

---

Assuming that you include the date.h file before the struct definition of Date. The problem is you're attempting to typedef a type before it exists. The C compiler processes a file from top to bottom. So essentially what' you're saying is

typedef Date pDate;

typedef struct Date_t {
 int date;
 char* month;
 int year;

} Date;

At the point of the first typedef C has no idea what Date is and hence the error. You need to switch the definitions.

typedef struct Date_t {
 int date;
 char* month;
 int year;

} Date;

typedef Date pDate;

---

If you're including date.h before the structure definition in date.c, the compiler is complaining because it has no idea what 'Date' is referring to at the moment you say 'typedef Date pDate'.

---

You can use pointers to incomplete types without problems in C (the problems come later)

struct Date; /* declaration of incomplete type */
typedef struct Date Date;
typedef Date *pDate;

pDate p;
p = NULL; /* ok, even when Date hasn't been defined */
p = malloc(sizeof *p); /* wrong! you cannot know the sizes of incomplete types */

---

Probably part of your confusion is due to a weirdness in C (not C++). With this declaration:

struct Date_t {...};

You create a structure type, that to use you must qualify with struct:

struct Date_t date1; /* Valid. */
Date_t date2; /* Not valid */

However, it is a common idiom in C to fix this with a typedef:

typedef struct {...} Date_t;

struct Date_t date1; /* invalid */
Date_t date2; /* Valid */

It is also a common idiom in C to do both together like this:

typedef struct _Date_t {...} Date_t;

struct _Date_t date1; /* Valid */
Date_t date2; /* Also valid */

And just to add to the confusion it is common to add a pointer typedef too:

typedef struct _Date_t {...} Date_t, * DateP;

Really, it is a historical artifact that in fixed in C++.