Can there be functions inside structures?

Question

Can we have functions in structures in C language?
Could someone please give an example of how to implement it and explain?

Answer 1

No, structures contain data only. However, you can define a pointer to a function inside of a struct as below:

struct myStruct {
    int x;
    void (*anotherFunction)(struct foo *);
}

Answer 2

The answer is no, but there is away to get the same effect.

Functions can only be found at the outermost level of a C program. This improves run-time speed by reducing the housekeeping associated with function calls.

As such, you cannot have a function inside of a struct (or inside of another function) but it is very common to have function pointers inside structures. For example:

#include <stdio.h>

int get_int_global (void)
{
  return 10;
}

double get_double_global (void)
{
  return 3.14;
}

struct test {
  int a;
  double b;
};

struct test_func {
  int (*get_int) (void);
  double (*get_double)(void);
};

int main (void)
{
  struct test_func t1 = {get_int_global, get_double_global};
  struct test t2 = {10, 3.14};

  printf("Using function pointers: %d, %f\n", t1.get_int(), t1.get_double());
  printf("Using built-in types: %d, %f\n", t2.a, t2.b);

  return 0;
}

A lot of people will also use a naming convention for function pointers inside structures and will typedef their function pointers. For example you could declare the structure containing pointers like this:

typedef int (*get_int_fptr) (void);
typedef double (*get_double_fptr)(void);

struct test_func {
  get_int_fptr get_int;
  get_double_fptr get_double;
};

Everything else in the code above will work as it is. Now, get_int_fptr is a special type for a function returning int and if you assume that *_fptr are all function pointers then you can find what the function signature is by simply looking at the typedef.

Answer 3

No, it has to be implemented like this :

typedef struct S_House {
    char* name;
    int opened;
} House;

void openHouse(House* theHouse);

void openHouse(House* theHouse) {
   theHouse->opened = 1;
}

int main() {
  House myHouse;
  openHouse(&myHouse);
  return 0;
}