Variable packing in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is variable packing in blockchain smart contracts?

Variable packing is a technique to store multiple smaller variables together in a single storage slot to save space and reduce gas costs.

beginner

Why is variable packing important in blockchain development?

Because storage operations are expensive in blockchain, packing variables reduces storage slots used, lowering gas fees and improving contract efficiency.

intermediate

Which data types can be packed together in a single storage slot?

Smaller data types like uint8, bool, and bytes1 can be packed together if their total size fits within 32 bytes (256 bits).

intermediate

How does Solidity store variables in storage slots when using variable packing?

Solidity stores variables sequentially in 32-byte slots. Smaller variables declared consecutively are packed tightly into one slot if they fit.

intermediate

What happens if variables exceed the 32-byte storage slot size during packing?

If variables together exceed 32 bytes, Solidity places the extra variables in the next storage slot, so packing only works within slot size limits.

What is the main benefit of variable packing in blockchain smart contracts?

A Reduces gas costs by saving storage space

B Makes code run faster on the CPU

C Increases contract size

D Improves user interface design

Variable packing reduces the number of storage slots used, which lowers gas costs.

Which of these data types can be packed together in one storage slot?

A string, uint256, address

B uint8, bool, bytes1

C uint256, uint256, uint256

D mapping, array, struct

Small fixed-size types like uint8, bool, and bytes1 can be packed together if they fit in 32 bytes.

How many bytes does one storage slot hold in Solidity?

A 32 bytes

B 128 bytes

C 64 bytes

D 16 bytes

Each storage slot in Solidity holds 32 bytes (256 bits).

What happens if packed variables exceed the 32-byte slot size?

A They are truncated

B They cause a compile error

C Extra variables go to the next storage slot

D They are compressed automatically

Variables exceeding the slot size are stored in the next slot.

Which of these is NOT a benefit of variable packing?

A Lower gas fees

B Reduced storage usage

C More efficient contract storage

D Faster transaction confirmation

Variable packing reduces storage and gas costs but does not directly speed up transaction confirmation.

Explain variable packing and why it matters in blockchain smart contracts.

Think about how storing many small variables together helps save money.

Describe how Solidity packs variables into storage slots and what limits this packing.

Imagine fitting small boxes into a bigger box without overflowing.

Practice

1.

What is the main benefit of variable packing in blockchain smart contracts?

easy

A. It reduces storage space and lowers gas fees.

B. It increases the number of variables allowed.

C. It makes the code run faster on all blockchains.

D. It automatically encrypts stored data.

Solution

1. Step 1: Understand variable packing purpose

   Variable packing groups smaller variables to use less storage space.

2. Step 2: Connect storage saving to gas fees

   Less storage means lower gas fees because blockchain charges for storage.

3. Final Answer:

   It reduces storage space and lowers gas fees. -> Option A

4. Quick Check:

   Variable packing = saves space and gas [OK]

Hint: Variable packing saves storage and gas fees [OK]

Common Mistakes:

• Thinking it increases variable count
• Assuming it speeds up all code execution
• Believing it encrypts data automatically

2.

Which of the following Solidity variable declarations best uses variable packing?

uint8 a;
uint16 b;
uint32 c;

easy

A. Declare all as uint256 for best packing.

B. Declare as uint32 c; uint16 b; uint8 a; in this order.

C. Declare as uint8 a; uint16 b; uint32 c; in this order.

D. Declare as uint16 b; uint8 a; uint32 c; in this order.

Solution

1. Step 1: Order variables from smallest to largest

   Variable packing works best when smaller variables come first to fill storage slots efficiently.

2. Step 2: Check given orders

   Declare as uint8 a; uint16 b; uint32 c; in this order. orders variables from uint8 (smallest) to uint32 (largest), maximizing packing.

3. Final Answer:

   Declare as uint8 a; uint16 b; uint32 c; in this order. -> Option C

4. Quick Check:

   Smallest to largest order = best packing [OK]

Hint: Order variables smallest to largest for packing [OK]

Common Mistakes:

• Placing largest variables first wastes space
• Using all uint256 wastes storage
• Mixing order without size consideration

3.

Consider this Solidity struct packed into one storage slot:

struct Data {
  uint8 x;
  uint16 y;
  uint8 z;
}
Data d = Data(1, 300, 2);

What is the total storage size used by d?

medium

A. 128 bits (16 bytes)

B. 256 bits (32 bytes)

C. 64 bits (8 bytes)

D. 32 bits (4 bytes)

Solution

1. Step 1: Calculate bits for each variable

   uint8 = 8 bits, uint16 = 16 bits, uint8 = 8 bits; total = 8+16+8 = 32 bits.

2. Step 2: Understand Solidity storage slot size

   Solidity packs variables into 256-bit slots, but here total variables use only 32 bits, so storage used is 32 bits.

3. Final Answer:

   32 bits (4 bytes) -> Option D

4. Quick Check:

   8+16+8 = 32 bits used [OK]

Hint: Sum bits of variables to find packed size [OK]

Common Mistakes:

• Assuming full 256 bits always used
• Adding bytes instead of bits incorrectly
• Confusing variable sizes

4.

Identify the error in this Solidity contract snippet related to variable packing:

contract Example {
  uint256 a;
  uint8 b;
  uint16 c;
}

Why is this not optimized for variable packing?

medium

A. Variables are not ordered from smallest to largest size.

B. uint256 cannot be packed with smaller types.

C. uint8 and uint16 must be declared as uint256.

D. All variables must be declared as uint8 for packing.

Solution

1. Step 1: Check variable order for packing

   Variable packing requires ordering from smallest to largest to fill storage slots efficiently.

2. Step 2: Analyze given order

   Here, uint256 a is first (largest), then smaller uint8 and uint16, which wastes space.

3. Final Answer:

   Variables are not ordered from smallest to largest size. -> Option A

4. Quick Check:

   Order smallest to largest for packing [OK]

Hint: Order variables smallest to largest to fix packing [OK]

Common Mistakes:

• Thinking uint256 can't be packed
• Believing all must be uint8
• Ignoring variable order importance

5.

You want to store these variables in a Solidity contract efficiently:

bool isActive;
uint8 count;
uint256 total;
uint16 code;

Which variable order best uses variable packing to minimize storage slots?

hard

A. uint256 total; uint16 code; uint8 count; bool isActive;

B. bool isActive; uint8 count; uint16 code; uint256 total;

C. uint16 code; uint256 total; bool isActive; uint8 count;

D. uint8 count; bool isActive; uint256 total; uint16 code;

Solution

1. Step 1: Order variables from smallest to largest size

   bool (1 bit), uint8 (8 bits), uint16 (16 bits), uint256 (256 bits) is the correct size order.

2. Step 2: Check options for this order

   bool isActive; uint8 count; uint16 code; uint256 total; lists variables in this order, maximizing packing into fewer storage slots.

3. Final Answer:

   bool isActive; uint8 count; uint16 code; uint256 total; -> Option B

4. Quick Check:

   Smallest to largest order = best packing [OK]

Hint: Place smallest variables first, largest last for packing [OK]

Common Mistakes:

• Placing uint256 first wastes slots
• Ignoring bool size as smallest
• Mixing order without size logic