Unions

Unions

A union is like a struct, but all members share the same memory. Only one member has a value at a time.

Defining a union

union MyUnion {
    int intNum;
    float floatNum;
    char str[20];
};

All members overlap

All fields start at the same address. Writing one overwrites the others:

union MyUnion u;
u.intNum = 42;
u.floatNum = 3.14;   // intNum's value is now garbage

Why use unions?

• Save memory when only one form of data is needed at a time.
• Interpret the same bytes differently.
• Compact storage for variant data.

Union size

A union is as big as its largest member, not the sum.

TL;DR

• A union shares one memory block among all members.
• Only one member holds a meaningful value at a time.
• Writing one member destroys the others.
• Union size = its largest member.