Function modifiers in Blockchain / Solidity - Mini Project: Build & Apply

Function Modifiers in Solidity

📖 Scenario: You are creating a simple smart contract for a shared piggy bank. Only the owner can add money or withdraw it. To keep things safe, you will use function modifiers to check who is calling the functions.

🎯 Goal: Build a Solidity contract that uses a modifier to restrict access to certain functions so only the owner can call them.

📋 What You'll Learn

Create a contract named PiggyBank

Add a state variable owner to store the address of the contract creator

Create a modifier named onlyOwner that allows function execution only if the caller is the owner

Use the onlyOwner modifier on deposit and withdraw functions

Add a balance state variable to track the piggy bank balance

Implement deposit and withdraw functions that update the balance

💡 Why This Matters

🌍 Real World

Function modifiers are used in smart contracts to control who can call certain functions, protecting assets and enforcing rules automatically.

💼 Career

Understanding function modifiers is essential for blockchain developers to write secure and maintainable smart contracts.

1

Create the PiggyBank contract and owner variable

Create a contract named PiggyBank and inside it, declare a public address variable called owner. Set owner to msg.sender inside the constructor.

Blockchain / Solidity

contract PiggyBank {
    // Your code here
}

Need a hint?

The constructor runs once when the contract is created. Use msg.sender to get the creator's address.

2

Add the onlyOwner modifier

Inside the PiggyBank contract, create a modifier named onlyOwner that uses require(msg.sender == owner) to allow function execution only if the caller is the owner. Use _; to continue function execution.

Blockchain / Solidity

contract PiggyBank {
    address public owner;

    constructor() {
        owner = msg.sender;
    }

    // Create onlyOwner modifier here
    // Your code here
}

Need a hint?

A modifier lets you add checks before running a function. Use require to stop if the caller is not the owner.

3

Add balance variable and deposit function with onlyOwner modifier

Add a public unsigned integer variable called balance to track the piggy bank balance. Then create a public function deposit that takes a uint amount parameter. Use the onlyOwner modifier on deposit. Inside the function, add amount to balance.

Blockchain / Solidity

contract PiggyBank {
    address public owner;

    constructor() {
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "Not the owner");
        _;
    }

    // Add balance variable and deposit function here
    // Your code here
}

Need a hint?

Use uint for numbers without decimals. The onlyOwner modifier goes after the function visibility.

4

Add withdraw function with onlyOwner modifier and print balance

Add a public function withdraw that takes a uint amount parameter. Use the onlyOwner modifier on withdraw. Inside the function, subtract amount from balance only if balance is greater or equal to amount using require. Then add a public view function getBalance that returns the current balance.

Blockchain / Solidity

contract PiggyBank {
    address public owner;
    uint public balance;

    constructor() {
        owner = msg.sender;
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "Not the owner");
        _;
    }

    function deposit(uint amount) public onlyOwner {
        balance += amount;
    }

    // Add withdraw and getBalance functions here
    // Your code here
}

Need a hint?

Use require to check if there is enough balance before withdrawing. The view keyword means the function does not change state.