Examples of Union in C [closed]

One classic is to represent a value of “unknown” type, as in the core of a simplistic virtual machine:

typedef enum { INTEGER, STRING, REAL, POINTER } Type;

typedef struct
{
  Type type;
  union {
  int integer;
  char *string;
  float real;
  void *pointer;
  } x;
} Value;

Using this you can write code that handles “values” without knowing their exact type, for instance implement a stack and so on.

Since this is in (old, pre-C11) C, the inner union must be given a field name in the outer struct. In C++ you can let the union be anonymous. Picking this name can be hard. I tend to go with something single-lettered, since it is almost never referenced in isolation and thus it is always clear from context what is going on.

Code to set a value to an integer might look like this:

Value value_new_integer(int integer)
{
  Value v;
  v.type = INTEGER;
  v.x.integer = integer;
  return v;
}

Here I use the fact that structs can be returned directly, and treated almost like values of a primitive type (you can assign structs).