Variable packing in Blockchain / Solidity - Mini Project: Build & Apply

Variable Packing in Solidity

📖 Scenario: You are building a smart contract to store user information efficiently on the Ethereum blockchain. Since storage costs gas, you want to pack multiple small variables into a single storage slot.

🎯 Goal: Create a Solidity contract that uses variable packing to store a user's age, a boolean flag for membership, and a small number representing user level in a single storage slot.

📋 What You'll Learn

Create a contract named UserInfo

Declare three state variables: uint8 age, bool isMember, and uint8 level

Use variable packing by declaring the variables in the correct order

Write a function setUserInfo to set all three variables

Write a function getUserInfo that returns all three variables

💡 Why This Matters

🌍 Real World

Variable packing helps reduce gas costs in Ethereum smart contracts by storing multiple small variables in a single 32-byte storage slot.

💼 Career

Understanding variable packing is important for blockchain developers to write efficient and cost-effective smart contracts.

1

Create the contract and declare packed variables

Create a Solidity contract named UserInfo. Inside it, declare three state variables in this order: uint8 age, bool isMember, and uint8 level. This order helps Solidity pack them into one storage slot.

Blockchain / Solidity

contract UserInfo {
    // Declare variables here
    // Your code here
}

Hint

Declare the variables in the order: uint8 age, then bool isMember, then uint8 level to enable packing.

2

Add a function to set user info

Inside the UserInfo contract, write a public function named setUserInfo that takes three parameters: uint8 _age, bool _isMember, and uint8 _level. Assign these parameters to the state variables age, isMember, and level respectively.

Blockchain / Solidity

contract UserInfo {
    uint8 age;
    bool isMember;
    uint8 level;

    // Write setUserInfo function here
    // Your code here
}

Hint

Define a public function setUserInfo with three parameters and assign them to the state variables.

3

Add a function to get user info

Inside the UserInfo contract, write a public view function named getUserInfo that returns three values: uint8 for age, bool for isMember, and uint8 for level. Return the state variables in the order age, isMember, level.

Blockchain / Solidity

contract UserInfo {
    uint8 age;
    bool isMember;
    uint8 level;

    function setUserInfo(uint8 _age, bool _isMember, uint8 _level) public {
        age = _age;
        isMember = _isMember;
        level = _level;
    }

    // Write getUserInfo function here
    // Your code here
}

Hint

Define a public view function getUserInfo that returns the three state variables in order.

4

Test the contract by setting and getting values

Write a simple test in Solidity by adding a public function named testUserInfo that calls setUserInfo with age = 30, isMember = true, and level = 5, then calls getUserInfo and returns the results.

Blockchain / Solidity

contract UserInfo {
    uint8 age;
    bool isMember;
    uint8 level;

    function setUserInfo(uint8 _age, bool _isMember, uint8 _level) public {
        age = _age;
        isMember = _isMember;
        level = _level;
    }

    function getUserInfo() public view returns (uint8, bool, uint8) {
        return (age, isMember, level);
    }

    // Write testUserInfo function here
    // Your code here
}

Hint

Create a function testUserInfo that sets values and returns them to verify packing works.

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