Unions

Unions behave like an "or" type.

Defining an union

In the example below, Variant is either a char[10] type, an int type, OR a double type. An union's value is stored in the same memory address, and you are responsible to "keep track" which data type it is currently using.

#include <cstdio>

union Variant {
  char string[10];
  int int_number;
  double double_number;
};

int main() {
  Variant v;
  v.int_number = 42;
  printf("Current v: %d\n", v.int_number);
  //=> Current v: 42

  v.double_number = 2.7182818284;
  printf("Current v: %lf\n", v.double_number);
  //=> Current v: 2.718282

  // `v` is currently storing a double value
  // from the last assignment, so trying to read
  // it as `int` will give a gibberish result.
  printf("Current v: %d\n", v.int_number);
  //=> Current v: -1961734133
}

References

• C++ Crash Course (Josh Lospinoso) — 2. Types