How is the Size of a C/C Union Determined?

Understanding the Size of a Union in C/C

In C/C , a union can accommodate multiple data members of differing data types. The question arises: what determines the size of a union?

The Largest Member Rule

The fundamental rule governing the size of a union is that it always takes up as much space as its largest data member. This is because the compiler allocates memory for the union based on the size of its most extensive member.

Compiler Optimization

When an active member of the union is smaller than the largest member, the compiler employs an optimization technique. It calculates the memory offset between the start of the union and the active member. This offset is stored in a reserved space within the union.

For instance, consider the following union:

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

Even though the union contains a short, an int, and a long long, it will always reserve enough memory to accommodate a long long, which is its largest member.

Implications of Alignment

It's important to note that the actual size of a union in memory can be influenced by alignment requirements imposed by the compiler. Different architectures may have specific alignment constraints that can affect the overall size of the union. Therefore, the actual size may not always be solely determined by the size of its largest member.

The above is the detailed content of How is the Size of a C/C Union Determined?. For more information, please follow other related articles on the PHP Chinese website!