Question

#include <stdio.h>
typedef struct {int a; int b; int c;} F;
typedef struct{
    int a;
    int (*ptr)(F*);
} A;

int set_a(F * obj){
    obj->a = 1;
}
int main(){
    F a;
    A b;
    b.a = 0;
    b.ptr = set_a;
    b.ptr(&a);
    printf("%d",a.a);

    getchar();
}

this seems correct

but why

#include <stdio.h>
typedef struct{
    int a;
    int (*ptr)(A*);
} A;

int set_a(A * obj){
    obj->a = 1;
}
int main(){
    A a;
    a.a = 0;
    a.ptr = set_a;
    a.ptr(&a);
    printf("%d",a.a);

    getchar();
}

this is incorrect? and

#include <stdio.h>
typedef struct{
    int a;
    int (*ptr)(A);
} A;

int set_a(A * obj){
    obj->a = 1;
}
int main(){
    A a;
    a.a = 0;
    a.ptr = set_a;
    a.ptr(&a);
    printf("%d",a.a);

    getchar();
}

this is correct?
I really wonder
thanks

environment Language C on Visual studio 2012 on Windows 7

Answer 1

In the 2nd and 3rd example you refer to A before (during) the declaration of it:

typedef struct{
    int a;
    int (*ptr)(A*); // < the compiler doesn't know A at this point.
} A;

Also, in the 3rd example, you declare ptr as function that receives A, but then assign it with set_a which receives A*, which is not valid as well.

You need to declare it before:

struct A;
typedef struct A{
    int a;
    int (*ptr)(struct A*);
} A;

In the first example you refer to F which already exists so it is OK.