Mikroelektronika MIKROE-738 Datenbogen

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mikoC PRO for PIC32

MikroElektronika

Union types are derived types sharing many of syntactic and functional features of structure types. The key difference 

is that a union members share the same memory space.

Note: The mikroC PRO for PIC supports anonymous unions. 

Union Declaration

Unions have the same declaration as structures, with the keyword 

union

 used instead of 

struct

:

union tag { member-declarator-list };

Unlike structures’ members, the value of only one of union’s members can be stored at any time. Here is a simple 

example:

union myunion {  // union tag is ‘myunion’

  int i;

  double d;

  char ch;

} mu, *pm;

The identifier 

mu

, of the type 

myunion

, can be used to hold a 2-byte

 int

, 4-byte 

double

 or single-byte 

char

, but only 

one of them at a certain moment. The identifier 

pm

 is a pointer to union 

myunion

.

Size of Union

The size of a union is the size of its largest member. In our previous example, both 

sizeof(union myunion)

 and 

sizeof(mu)

 return 4, but 2 bytes are unused (padded) when 

mu

 holds the 

int 

object, and 3 bytes are unused when 

mu

 holds

 char

.

Union Member Access

Union members can be accessed with the structure member selectors (

. 

and 

->

), be careful when doing this:

pm = μ

mu.d = 4.016;

tmp = mu.d;  // OK: mu.d = 4.016

tmp = mu.i;  // peculiar result

pm->i = 3;

tmp = mu.i;  // OK: mu.i = 3

The third line is legal, since 

mu.

i is an integral type. However, the bit pattern in 

mu.i

 corresponds to parts of the 

previously assigned 

double

. As such, it probably won’t provide an useful integer interpretation.

When properly converted, a pointer to a union points to each of its members, and vice versa.