Union basics - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a union in C?

A union is a special data type in C that allows storing different data types in the same memory location. Only one member can hold a value at a time.

beginner

How does memory allocation work in a union?

The size of a union is equal to the size of its largest member because all members share the same memory space.

intermediate

What happens if you assign a value to one member of a union and then read from another member?

Reading from a different member than the one last assigned can lead to unexpected or garbage values because all members share the same memory.

beginner

Write a simple union declaration with an int and a float member.

Example:

union Number {
  int i;
  float f;
};

intermediate

Why might you use a union instead of a struct?

You use a union to save memory when you know only one of several types will be used at a time, unlike a struct which allocates memory for all members.

What is the size of a union with members: char c;, int i;, and double d;?

A Sum of sizes of all members

B Size of <code>int</code>

C Size of <code>double</code>

D Size of <code>char</code>

A union's size is the size of its largest member, here double.

If you store a value in one union member and then read from another, what is the result?

A Undefined or garbage value

B Zero

C Compiler error

D The value of the last stored member

Reading a different member than the one last assigned leads to undefined or garbage values.

Which keyword is used to declare a union in C?

A union

B struct

C enum

D typedef

The keyword union declares a union type.

Why use a union instead of a struct?

A To define functions

B To store multiple values at once

C To create arrays

D To save memory when only one value is needed at a time

Unions save memory by sharing space for members used one at a time.

How do you access members of a union variable named u?

A <code>u->member</code>

B <code>u.member</code>

C <code>*u.member</code>

D <code>u:member</code>

Use dot notation u.member to access union members.

Explain what a union is in C and how it differs from a struct.

Think about memory usage and how values are stored.

Describe a situation where using a union would be more efficient than a struct.

Consider saving memory in embedded systems or low-memory devices.