Why does my union not show the correct values?

Question

union
{ int i;
  bool b;
} x;

x.i = 20000;
x.b = true;
cout << x.i;

It prints out 19969. Why does it not print out 20000?

Answer 1

A union is not a struct. In a union, all of the data occupies the same space and can be treated as different types via its field names. When you assign true to x.b, you are overwriting the lower-order bits of 20000.

More specifically:

20000 in binary: 100111000100000

19969 in binary: 100111000000001

What happened here was that you put a one-byte value of 1 (00000001) in the 8 lower-order bits of 200000.

If you use a struct instead of a union, you will have space for both an int and a bool, rather than just an int, and you will see the results you expected.

Answer 2

In a union, all data members start at the same memory location. In your example you can only really use one data member at a time. This feature can be used for some neat tricks however, such as exposing the same data in multiple ways:

union Vector3
{
  int v[3];
  struct
  {
    int x, y, z;
  };
};

Which allows you to access the three integers either by name (x, y and z) or as an array (v).