Variants

In this exercise, you will discover a way of writing generic data structures in C: unions.

Unions

A union can have a large amount of fields, but contain only one at a time. It is declared the same way as a structure, like shown below:

union int_or_string
{
  int int_t;
  char *str_t;
};

The union can then be used like this:

union int_or_string value;

value.int_t = 12;
printf("The value is now: %i\n", value.int_t);

value.str_t = "acu";
printf("The value is now: %s\n", value.str_t);

The size of an union is the maximum of the size of its members. Consequently, the following union's size will be sizeof(int64_t), 8 bytes.

union integer
{
  int8_t smallest;
  int16_t small;
  int32_t big;
  int64_t biggest;
};

Tagged unions

Since there is no way to know which field was last assigned to a union, we have to add a tag to know the stored type.

Taking our previous union, we could write:

enum integer_values
{
  smallest,
  small,
  big,
  biggest
};

struct variant
{
  enum integer_values tag;
  union integer value;
};