Variable packing in Blockchain / Solidity

Introduction

Variable packing helps save space by putting small pieces of data close together in one place. This makes programs use less memory and run faster.

When you want to store many small values efficiently in a smart contract.

When you want to reduce the cost of storing data on the blockchain.

When you want to optimize your contract to use less gas.

When you have multiple small variables that can fit together in one storage slot.

When you want to improve performance by reducing storage reads and writes.

Syntax

Blockchain / Solidity

contract Example {
    uint128 a;
    uint128 b;
    uint256 c;
}

Variables declared one after another with smaller sizes can be packed into a single 256-bit storage slot.

Packing works best when variables are declared in order from smallest to largest size.

Examples

Two 128-bit variables packed into one 256-bit slot.

Blockchain / Solidity

contract Packed {
    uint128 a;
    uint128 b;
}

Variables not packed because uint256 uses a full slot first.

Blockchain / Solidity

contract NotPacked {
    uint256 a;
    uint128 b;
}

Two 64-bit and one 128-bit variables packed together in one slot.

Blockchain / Solidity

contract Mixed {
    uint64 a;
    uint64 b;
    uint128 c;
}

Sample Program

This contract packs two 128-bit variables a and b into one storage slot, saving space. The function getSum returns their sum plus c.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract VariablePacking {
    uint128 public a = 1;
    uint128 public b = 2;
    uint256 public c = 3;

    function getSum() public view returns (uint256) {
        return a + b + c;
    }
}

Important Notes

Variable packing reduces gas costs by using fewer storage slots.

Always declare smaller variables together before larger ones to maximize packing.

Packing only works for state variables stored on the blockchain, not for local variables.

Summary

Variable packing saves space by storing multiple small variables in one slot.

It helps reduce gas costs and improve contract efficiency.

Order your variables from smallest to largest to get the best packing.

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