Union basics in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a union in C++?

A union is a special data type where all members share 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.

beginner

Can you store values in multiple members of a union at the same time?

No, only one member can store a value at a time. Writing to one member overwrites the others.

beginner

How do you access members of a union?

You access union members using the dot (.) operator if you have a union variable, or arrow (->) if you have a pointer to a union.

intermediate

Why would you use a union instead of a struct?

Unions save memory when you need to store different types but only one at a time, unlike structs which allocate memory for all members.

What happens when you assign a value to one member of a union?

A It stores values in all members simultaneously.

B It creates a copy of the value in each member.

C It overwrites the value of other members.

D It causes a compilation error.

In a union, all members share the same memory, so assigning a value to one member overwrites the others.

What determines the size of a union?

A The size of its largest member.

B The sum of sizes of all members.

C The size of the first member.

D Always 4 bytes.

The union's size equals the size of its largest member because all members share the same memory.

Which operator is used to access a member of a union variable?

A & (ampersand)

B . (dot)

C * (asterisk)

D -> (arrow)

The dot operator is used to access members of a union variable directly.

Why might you choose a union over a struct?

A To enforce type safety.

B To store multiple values simultaneously.

C To create a class.

D To save memory when only one member is used at a time.

Unions save memory by sharing space among members, useful when only one member is needed at a time.

What will happen if you read a union member different from the one last written?

A You may get unexpected or garbage data.

B You get the last written value correctly.

C It causes a runtime error.

D It automatically converts the value.

Reading a different member than the one last written can produce unpredictable results because the memory is shared.

Explain what a union is and how it manages memory.

Think about how a single box can hold different items but only one at a time.

Describe a situation where using a union is better than using a struct.

Imagine you have a tool that can be a hammer or a screwdriver, but you only use one function at a time.