Constants and immutables in Blockchain / Solidity - Mini Project: Build & Apply

Using Constants and Immutables in Solidity

📖 Scenario: You are creating a simple smart contract for a blockchain-based voting system. You want to set some fixed values that never change after deployment, such as the election name and the maximum number of votes per person.

🎯 Goal: Build a Solidity contract that uses constant and immutable variables to store fixed data and demonstrate how to read them.

📋 What You'll Learn

Create a constant string variable called ELECTION_NAME with the value "Blockchain Election 2024".

Create an immutable unsigned integer variable called maxVotesPerPerson set via the constructor with the value 3.

Write a public function getElectionInfo that returns both ELECTION_NAME and maxVotesPerPerson.

Deploy the contract and print the election info.

💡 Why This Matters

🌍 Real World

Constants and immutables help save gas and ensure important values do not change after deployment in blockchain smart contracts.

💼 Career

Understanding these keywords is essential for blockchain developers to write efficient and secure smart contracts.

1

Create the contract and constant variable

Create a Solidity contract named Voting. Inside it, declare a public constant string variable called ELECTION_NAME and set it to "Blockchain Election 2024".

Blockchain / Solidity

contract Voting {
    // Declare constant election name here
    
}

Hint

Use string public constant ELECTION_NAME = "Blockchain Election 2024"; inside the contract.

2

Add an immutable variable set by constructor

Inside the Voting contract, declare a public immutable uint variable called maxVotesPerPerson. Add a constructor that sets maxVotesPerPerson to 3.

Blockchain / Solidity

contract Voting {
    string public constant ELECTION_NAME = "Blockchain Election 2024";

    // Declare immutable variable
    uint public immutable maxVotesPerPerson;

    // Add constructor to set maxVotesPerPerson to 3
    
}

Hint

Use constructor() { maxVotesPerPerson = 3; } to set the immutable variable.

3

Create a function to return election info

Add a public view function called getElectionInfo that returns (string memory, uint). It should return ELECTION_NAME and maxVotesPerPerson.

Blockchain / Solidity

contract Voting {
    string public constant ELECTION_NAME = "Blockchain Election 2024";

    uint public immutable maxVotesPerPerson;

    constructor() {
        maxVotesPerPerson = 3;
    }

    // Create getElectionInfo function
    function getElectionInfo() public view returns (string memory, uint) {
        // Your code here
    }
}

Hint

Return both variables as a tuple: return (ELECTION_NAME, maxVotesPerPerson);

4

Print the election info

Write a script or test code that calls getElectionInfo and prints the election name and max votes per person in the format: Election: Blockchain Election 2024, Max Votes: 3.

Blockchain / Solidity

contract Voting {
    string public constant ELECTION_NAME = "Blockchain Election 2024";

    uint public immutable maxVotesPerPerson;

    constructor() {
        maxVotesPerPerson = 3;
    }

    function getElectionInfo() public view returns (string memory, uint) {
        return (ELECTION_NAME, maxVotesPerPerson);
    }
}

// Call getElectionInfo and print the result
// Your code here

Hint

Solidity contracts do not print directly. Imagine calling getElectionInfo and displaying the returned values as shown.

Practice

1. What is the main difference between constant and immutable variables in a blockchain smart contract?

easy

A. Both constant and immutable variables can be changed after deployment.

B. constant variables can be changed anytime, immutable variables cannot be changed.

C. immutable variables are fixed at compile time, constant variables are set during contract creation.

D. constant variables are fixed at compile time, immutable variables are set once during contract creation.

Solution

1. Step 1: Understand constant variables

   constant variables are assigned a value at compile time and cannot be changed later.

2. Step 2: Understand immutable variables

   immutable variables are assigned once during contract creation (deployment) and then remain fixed.

3. Final Answer:

   constant fixed at compile time, immutable set once at creation -> Option D

4. Quick Check:

   constant = compile time, immutable = creation time [OK]

Hint: Constant fixed at compile, immutable set once at deployment [OK]

Common Mistakes:

• Thinking immutable can be changed after deployment
• Confusing when constants are assigned
• Assuming both can be modified anytime

2. Which of the following is the correct way to declare a constant integer named MAX_SUPPLY with value 1000 in Solidity?

easy

A. constant uint MAX_SUPPLY = 1000;

B. uint constant MAX_SUPPLY = 1000;

C. uint MAX_SUPPLY constant = 1000;

D. constant MAX_SUPPLY uint = 1000;

Solution

1. Step 1: Recall Solidity constant syntax

   In Solidity, the type comes first, then the keyword constant, then the variable name.

2. Step 2: Check each option

   uint constant MAX_SUPPLY = 1000; matches the correct syntax: uint constant MAX_SUPPLY = 1000;. Others have wrong order.

3. Final Answer:

   uint constant MAX_SUPPLY = 1000; -> Option B

4. Quick Check:

   Type + constant + name = correct order [OK]

Hint: Type first, then constant keyword, then variable name [OK]

Common Mistakes:

• Placing constant before type
• Putting constant after variable name
• Missing semicolon at end

3. Consider the Solidity contract snippet:

contract Example {
    uint public immutable creationTime;
    constructor() {
        creationTime = block.timestamp;
    }
}

What will creationTime hold after deployment?

medium

A. The timestamp when the contract was deployed

B. The timestamp when the contract was compiled

C. Zero, because it is not initialized

D. It will cause a compilation error

Solution

1. Step 1: Understand immutable assignment

   immutable variables can be assigned once during contract creation, here in the constructor.

2. Step 2: Analyze the assignment

   creationTime = block.timestamp; sets it to the current block timestamp at deployment.

3. Final Answer:

   The timestamp when the contract was deployed -> Option A

4. Quick Check:

   Immutable set at deployment time [OK]

Hint: Immutable set once in constructor at deployment [OK]

Common Mistakes:

• Thinking it holds compile time value
• Assuming default zero value
• Confusing immutable with constant

4. What is wrong with this Solidity code snippet?

contract Test {
    uint public immutable value;
    function setValue(uint _val) public {
        value = _val;
    }
}

medium

A. You cannot assign to an immutable variable outside the constructor.

B. The variable value should be declared as constant.

C. The function setValue should be marked as view.

D. There is no error; the code is correct.

Solution

1. Step 1: Recall rules for immutable variables

   immutable variables can only be assigned once during contract creation, typically in the constructor.

2. Step 2: Check assignment location

   Here, value is assigned inside a public function, which is not allowed.

3. Final Answer:

   Cannot assign immutable outside constructor -> Option A

4. Quick Check:

   Immutable assigned only once in constructor [OK]

Hint: Immutable only set in constructor, not in other functions [OK]

Common Mistakes:

• Trying to assign immutable in regular functions
• Confusing constant and immutable rules
• Thinking public functions can set immutable

5. You want to store a fixed token symbol in your smart contract that never changes and a deployment timestamp that is set once at deployment. Which declarations are correct?

contract Token {
    // Declare symbol
    // Declare deploymentTime
    constructor() {
        deploymentTime = block.timestamp;
    }
}

hard

A. string public constant symbol;
uint public immutable deploymentTime = block.timestamp;

B. string public immutable symbol = "TKN";
uint public constant deploymentTime = block.timestamp;

C. string public constant symbol = "TKN";
uint public immutable deploymentTime;

D. string public immutable symbol = "TKN";
uint public immutable deploymentTime = block.timestamp;

Solution

1. Step 1: Declare fixed symbol as constant

   Since symbol never changes and is known at compile time, declare as constant with value assigned inline.

2. Step 2: Declare deploymentTime as immutable

   Deployment timestamp is set once at deployment, so declare as immutable and assign in constructor.

3. Final Answer:

   symbol constant with value, deploymentTime immutable assigned in constructor -> Option C

4. Quick Check:

   Constant fixed value, immutable set once at deployment [OK]

Hint: Constant for fixed value, immutable for deployment-time set [OK]

Common Mistakes:

• Assigning immutable variables inline without constructor
• Declaring constant without value
• Confusing when to use constant vs immutable