Structure vs union comparison - Quick Revision & Key Differences

Recall & Review

beginner

What is a structure in C?

A structure is a user-defined data type in C that groups different variables under one name. Each member has its own memory location.

beginner

What is a union in C?

A union is a user-defined data type where all members share the same memory location. Only one member can hold a value at a time.

intermediate

How does memory allocation differ between structures and unions?

Structures allocate separate memory for each member, so total size is sum of all members (plus possible padding). Unions allocate memory equal to the largest member only.

intermediate

Can you access multiple members of a union at the same time?

No, because all members share the same memory, changing one member overwrites others. Only one member's value is valid at a time.

beginner

When should you use a structure vs a union?

Use a structure when you need to store multiple values simultaneously. Use a union to save memory when only one value is needed at a time.

Which of the following is true about structures in C?

A Each member has its own memory location.

B All members share the same memory location.

C Structures can only have members of the same type.

D Structures cannot contain other structures.

Structures allocate separate memory for each member, allowing multiple members to hold values simultaneously.

What is the size of a union in C?

A Sum of sizes of all members.

B Size of the largest member.

C Size of the smallest member.

D Always 4 bytes.

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

If you assign a value to one member of a union, what happens to other members?

A They keep their previous values.

B They cause a compile error.

C They automatically update to the same value.

D They become invalid or overwritten.

Since all members share memory, assigning one member overwrites the others' data.

Which is a good reason to use a union instead of a structure?

A When you want to save memory and only use one value at a time.

B When you need to store multiple values at once.

C When you want to group unrelated variables.

D When you want to create a new data type with fixed size.

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

Can a structure contain a union as a member in C?

A No, structures cannot contain unions.

B Only if the union has one member.

C Yes, structures can contain unions.

D Only in C++ but not in C.

Structures can contain unions as members, allowing flexible data grouping.

Explain the main differences between a structure and a union in C.

Think about how memory is used and when you want to store multiple values or just one.

Describe a scenario where using a union is better than a structure.

Imagine you have a variable that can be an int or a float but never both at once.