Question

I want to create a linked list, but I want the data field to be either int or float. I wrote some code like the following:

union int_or_float
{
    int int_member;
    float float_member;
};

struct node 
{
    union int_or_float data;
    struct node *next;
};

I want to write stuff like:

typedef struct node item; 

but how to specify which type in union do I like item to be?

Answer 1

That’s done automatically when you access the data. A union is nothing more than a slightly-less-kludgy way of casting a pointer.

#include <stdio.h>

union number {
  int i;
  float f;
};

struct node {
  union number data;
  struct node *next;
};

typedef struct node item;

int main(void) {
  item myItem;

  myItem.data.i = 12; /* Now contains an int */
  myItem.data.f = 12.0; /* Now contains a float */
  printf("location of myItem.data is %p\n", &myItem.data);
  printf("location of myItem.data.f is %p\n", &myItem.data.f);  
  printf("myItem.data is %f\n", *(float *)(&myItem.data));
  printf("myItem.data.f is %f\n", myItem.data.f);
  printf("sizeof(myItem.data) is %lu\n", sizeof(*(float *)(&myItem.data)));
  printf("sizeof(myItem.data.f) is %lu\n", sizeof(myItem.data.f));

  return 0;
}

This will output

location of myItem.data is 0x7fff5fbff9f0
location of myItem.data.f is 0x7fff5fbff9f0
myItem.data is 12.000000
myItem.data.f is 12.000000
sizeof(myItem.data) is 4
sizeof(myItem.data.f) is 4

run/tested on an Intel Mac with gcc version 4.2.1 (Apple Inc. build 5659)