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
}

Need a 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
}

Need a 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
}

Need a 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
}

Need a hint?

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