Can I define a function inside a C structure? [duplicate]
I am trying to convert some C++ code to C and I am facing some problems. How can I define inside a structure a function?
Like this:
typedef struct {
double x, y, z;
struct Point *next;
struct Point *prev;
void act() {sth. to do here};
} Point;
Solution 1:
No, you cannot define a function within a struct
in C.
You can have a function pointer in a struct
though but having a function pointer is very different from a member function in C++, namely there is no implicit this
pointer to the containing struct
instance.
Contrived example (online demo http://ideone.com/kyHlQ):
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
struct point
{
int x;
int y;
void (*print)(const struct point*);
};
void print_x(const struct point* p)
{
printf("x=%d\n", p->x);
}
void print_y(const struct point* p)
{
printf("y=%d\n", p->y);
}
int main(void)
{
struct point p1 = { 2, 4, print_x };
struct point p2 = { 7, 1, print_y };
p1.print(&p1);
p2.print(&p2);
return 0;
}
Solution 2:
You can have a function pointer in a struct though. but not in this way
you can define it in this way
example:
typedef struct cont_func
{
int var1;
int (*func)(int x, int y);
void *input;
} cont_func;
int max (int x, int y)
{
return (x > y) ? x : y;
}
int main () {
struct cont_func T;
T.func = max;
}
Solution 3:
In C
it is not allowed to define a method inside a struct
. You could define a function pointer inside a struct as follows:
typedef struct {
double x, y, z;
struct Point *next;
struct Point *prev;
void (*act)();
} Point;
You will have to assign the pointer to a specific function whenever you instantiate the struct
.
Solution 4:
No, it is not possible to declare a function inside a struct in C.
That is (one of) the fundamental differences between C and C++.
See this thread: https://web.archive.org/web/20121024233849/http://forums.devshed.com/c-programming-42/declaring-function-in-structure-in-c-545529.html