What Determines the Size of a C/C Union?

Determining the Size of a Union in C/C

In C/C , unions are datatypes that store different data types at the same memory location. One commonly encountered question pertains to the sizeof a union. Does it correspond to the sizeof the largest datatype within the union?

The answer is yes. A union always occupies memory equivalent to the size of its largest member, regardless of the currently active datatype. This is because the compiler performs optimization to ensure that sufficient space is allocated for the largest potential value.

Consider the following example union:

union {
  short x;
  int y;
  long long z;
};

An instance of this union will always require at least the space of a long long variable, which is its largest member.

It's worth noting, as mentioned by Stefano, that the actual size of a union in memory may depend on factors such as alignment requirements imposed by the compiler. However, it's important to understand that, conceptually, a union's sizeof corresponds to the size of its largest member.

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