function pointer not working for int - c

Question

Im trying to use the power of function pointers, it all went fine until i tried to make the function pointer use a 2nd argument as type int.
The code below generates an error, which is displayed below
In an header file:
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct UnitTag {
int x;
int y;
void (*move)(Unit, int);
} Unit;
Error:
error: expected ‘)’ before ‘int’
void (*move)(Unit, int);
^
void (*move)(Unit); works all fine, which surprises me how adding an argument can cause an error.
I call my struct in a C file, by including header and then doing:
Unit units[UNITCOUNT];
units[0].move(&units[0], 1);
Update:
adding:
typedef struct UnitTag Unit
Causes the error to dissapear, however I can no longer use the function as before.
error: incompatible type for argument 1 of ‘units[i].move’
units[0].move(&units[0], 0);
^
note: expected ‘Unit’ but argument is of type ‘struct UnitTag *’

Answer 1

If I'm getting you, you can simply use struct keyword:
#include <stdio.h>
typedef struct UnitTag {
int x;
int y;
void (*move)(struct UnitTag, int);
} Unit;
void Test (struct UnitTag test1, int test2)
{
printf("Test1.x: %d\n", test1.x);
printf("Test1.y: %d\n", test1.y);
printf("Test2 : %d\n", test2);
}
int main(void)
{
Unit units[100];
units[0].move = Test;
units[0].x = 1;
units[0].y = 2;
units[0].move(units[0], 3);
}
Output:
Test1.x: 1
Test1.y: 2
Test2 : 3
If you want to pass struct by referebce, simply:
#include <stdio.h>
typedef struct UnitTag {
int x;
int y;
void (*move)(struct UnitTag*, int);
} Unit;
void Test (struct UnitTag *test1, int test2)
{
test1->x = 4;
test1->y = 5;
}
int main(void)
{
Unit units[100];
units[0].move = Test;
units[0].x = 1;
units[0].y = 2;
units[0].move(&units[0], 3);
printf("units[0].x: %d\n", units[0].x);
printf("units[0].y: %d\n", units[0].y);
}
Output is:
units[0].x: 4
units[0].y: 5

Answer 2

You need the prototype for Unit before using it.
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct UnitTag Unit;
typedef struct UnitTag {
int x;
int y;
void (*move)(Unit, int);
} Unit;
int main(void)
{
return 0;
}
After the clarification what you wanted to do. It probably makes more sense to give a pointer to Unit, so that the move command which returns void can change something about your object.
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct UnitTag Unit;
typedef struct UnitTag {
int x;
int y;
void (*move)(Unit *, int);
} Unit;
Unit test;
/* some function that corresponds to the interface */
void myMove(Unit *u, int i)
{
u->x = u->x + i;
}
int main(void)
{
/* initialize test struct */
test.x = 0;
test.y = 0;
test.move = myMove;
test.move(&test, 5);
printf("Values after move are (x, y) = (%i, %i).\n", test.x, test.y);
return 0;
}