Unions

Question 1

What is a union?

Answer 1

• Like a struct, contains one or more members, possibly of different types.
• The compiler allocates only enough space for the largest of the members, which overlay each other in this space
• Assigning a new value alters the values of the members as well

Question 2

How do you code a union?

Answer 2

Question 3

How do struct and Union differ in memory? Draw it.

Answer 3

Question 4

How are unions accessed?

Example for members i and d.

How would changing i affect d?

Answer 4

Question 5

How would you use unions to save space with the following structure?

How would you access a book’s title in the union?

Answer 5

If c is a catalog_item, we can print a book’s title with :

printf(“%s”, c.item.book.title);

Question 6

If we needed an array whose elements are a mixture of int and double values, how would we define the code for this union and array?

Answer 6

Question 7

In general, how can you tell which member of a union was last changed and therefore contains a meaningful value?

Answer 7

Question 8

How would you code a tag field for a union?

Answer 8

After the pictured code, each time we assign a value to a member of u, we’ll also change kind to remind us which member of u we modified.

Example:

n. kind = INT_KIND;
n. u.i = 82;

n is assumed to be a Number variable

Question 9

What is the code for a function that prints the correct number in a stuct based on it’s kind?

Answer 9