如何在另一个.c文件中使用前向声明的struct数据？

Question

I have a struct which is forward declared in file.h. The structure is defined in file1.c. I'm trying to use the struct in file2.c. But it is giving me "error: dereferencing pointer to incomplete type" in file2.c.

file.h

typedef struct foo foo;

file1.c

#include <file.h>

typedef struct foo {
   int val;
} foo;

file2.c

#include <file.h>

struct foo *f;
.
.
.
printf("%d", f->val);   <--Error here

I don't have any issue if I define the struct in file.h. Is there any way I can use val in file2?

Answer 1

This is called an opaque struct , is useful when you want to protect the access to the members (a kind of private specifier).

in this way, only file1.c can access the members of the struct , to make it visible to the rest of the .c files you need to

1) Define the struct inside the .h file

or

2) Access the members through a function:

//file.h

typedef struct foo foo;
int foo_val(const foo *);

//file1.c

#include "file.h" // Always prefer "" instead of <> for local headers

struct foo { // Notice that you don't need to retypedef the struct
   int val;
};

int foo_val(const foo *f)
{
    return f->val;
}

//file2.c

#include "file.h"

struct foo *f;

printf("%d", foo_val(f));