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
}

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
}

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
}

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
}

Hint

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

Practice

1. What is the main purpose of a function modifier in blockchain smart contracts?

easy

A. To change the return type of a function

B. To add extra rules or checks around a function

C. To rename a function

D. To delete a function from the contract

Solution

1. Step 1: Understand function modifiers

   Function modifiers are used to add extra rules or steps before or after a function runs.

2. Step 2: Compare options

   Only To add extra rules or checks around a function correctly describes adding extra rules or checks around a function.

3. Final Answer:

   To add extra rules or checks around a function -> Option B

4. Quick Check:

   Function modifiers add rules [OK]

Hint: Modifiers add rules or checks around functions [OK]

Common Mistakes:

• Thinking modifiers rename or delete functions
• Confusing modifiers with return type changes
• Believing modifiers execute functions themselves

2. Which of the following is the correct syntax to define a function modifier in Solidity?

easy

A. modifier onlyOwner() => { require(msg.sender == owner); _; }

B. modifier onlyOwner() : { require(msg.sender == owner); _; }

C. modifier onlyOwner() -> { require(msg.sender == owner); _; }

D. modifier onlyOwner() { require(msg.sender == owner); _; }

Solution

1. Step 1: Recall Solidity modifier syntax

   Modifiers use the keyword 'modifier', name, parentheses, and curly braces with '_;' inside.

2. Step 2: Identify correct syntax

   modifier onlyOwner() { require(msg.sender == owner); _; } matches the correct Solidity syntax with braces and '_;'. Others use invalid symbols like '=>', '->', or ':'.

3. Final Answer:

   modifier onlyOwner() { require(msg.sender == owner); _; } -> Option D

4. Quick Check:

   Modifier syntax uses braces and '_;' [OK]

Hint: Modifiers use braces { } and _; inside [OK]

Common Mistakes:

• Using arrows (=>, ->) instead of braces
• Omitting the '_;' statement
• Using colon ':' instead of braces

3. What will be the output of the following Solidity code snippet?

contract Test {
    address owner;
    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");
        _;
    }
    function setOwner(address _owner) public {
        owner = _owner;
    }
    function secureAction() public onlyOwner {
        // action code
    }
}

If msg.sender is not the owner and calls secureAction(), what happens?

medium

A. The function runs normally

B. The owner address changes

C. The transaction reverts with error 'Not owner'

D. The function ignores the modifier

Solution

1. Step 1: Understand the modifier behavior

   The modifier checks if msg.sender equals owner; if not, it calls require which reverts with "Not owner".

2. Step 2: Analyze the call with wrong sender

   If msg.sender is not owner, require fails and the transaction reverts with the error message.

3. Final Answer:

   The transaction reverts with error 'Not owner' -> Option C

4. Quick Check:

   require fails if sender not owner [OK]

Hint: require reverts if condition false inside modifier [OK]

Common Mistakes:

• Assuming function runs even if require fails
• Thinking owner changes on secureAction call
• Ignoring the require statement effect

4. Identify the error in this Solidity code using a function modifier:

contract Sample {
    address owner;
    modifier onlyOwner() {
        require(msg.sender == owner);
        _
    }
    function changeOwner(address newOwner) public onlyOwner {
        owner = newOwner;
    }
}

medium

A. Missing semicolon after '_' in modifier

B. Incorrect function visibility for changeOwner

C. owner variable not initialized

D. Modifier name should be 'OnlyOwner' with capital O

Solution

1. Step 1: Check modifier syntax

   The modifier has '_', but it misses the semicolon '_;' which is required to mark function execution point.

2. Step 2: Verify other parts

   Function visibility 'public' is correct, owner can be uninitialized but not a syntax error, and modifier names are case-sensitive but not required to be capitalized.

3. Final Answer:

   Missing semicolon after '_' in modifier -> Option A

4. Quick Check:

   Modifier must have '_;' not just '_' [OK]

Hint: Modifier must end '_' with semicolon ';' [OK]

Common Mistakes:

• Forgetting semicolon after '_' in modifier
• Thinking function visibility causes error here
• Assuming variable initialization causes syntax error

5. You want to create a modifier that allows a function to run only if the contract is not paused. The contract has a boolean paused. Which modifier code correctly implements this logic?

hard

A. modifier whenNotPaused() { require(!paused, "Paused"); _; }

B. modifier whenNotPaused() { if(paused) { revert("Paused"); } }

C. modifier whenNotPaused() { require(paused, "Paused"); _; }

D. modifier whenNotPaused() { if(!paused) { _; } }

Solution

1. Step 1: Understand the paused logic

   The function should run only if paused is false, so require(!paused) ensures this.

2. Step 2: Check modifier completeness

   modifier whenNotPaused() { require(!paused, "Paused"); _; } uses require with negation and includes '_;' to run the function. modifier whenNotPaused() { if(paused) { revert("Paused"); } } lacks '_;', so function never runs. modifier whenNotPaused() { require(paused, "Paused"); _; } requires paused to be true, which is opposite. modifier whenNotPaused() { if(!paused) { _; } } runs function only if not paused but lacks else handling and is less clear.

3. Final Answer:

   modifier whenNotPaused() { require(!paused, "Paused"); _; } -> Option A

4. Quick Check:

   Require !paused with '_;' runs function [OK]

Hint: Use require(!paused) and include '_;' to run function [OK]

Common Mistakes:

• Reversing paused condition in require
• Omitting '_;' causing function not to run
• Using if without else or revert