Function modifiers in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function modifier in Solidity?

A function modifier is a special code block that can be used to change the behavior of functions. It helps to add checks or logic before or after the function runs, like a gatekeeper.

beginner

How do you apply a modifier to a function in Solidity?

You write the modifier name after the function declaration, like this: <code>function myFunc() public myModifier { ... }</code>. This means the modifier code runs when the function is called.

intermediate

What does the _; symbol mean inside a modifier?

The _; is a placeholder that tells Solidity where to run the original function's code inside the modifier. Code before _; runs before the function, and code after runs after.

beginner

Why use function modifiers in smart contracts?

Modifiers help keep code clean and safe by reusing common checks like access control or input validation. This avoids repeating the same code in many functions.

intermediate

Can modifiers take arguments? Give an example.

Yes, modifiers can take arguments to customize their behavior. For example:
modifier onlyOwner(address _addr) { require(msg.sender == _addr); _; }
This modifier checks if the caller is the owner passed as argument.

What is the main purpose of a function modifier in Solidity?

A To create new functions automatically

B To store data in the blockchain

C To change how a function behaves by adding reusable code before or after it

D To compile the smart contract

Function modifiers add reusable code that runs before or after a function, changing its behavior.

In a modifier, what does the _; symbol do?

A It marks where the original function's code will run

B It ends the modifier

C It declares a variable

D It calls another function

The _; symbol is a placeholder for the original function's code inside the modifier.

How do you apply a modifier named onlyOwner to a function withdraw?

A modifier withdraw() onlyOwner { ... }

B function withdraw() public onlyOwner { ... }

C onlyOwner function withdraw() { ... }

D function onlyOwner withdraw() { ... }

You add the modifier name after the function declaration to apply it.

Can function modifiers take parameters in Solidity?

A Yes, to customize their checks

B No, modifiers cannot have parameters

C Only if the function is private

D Only for constructors

Modifiers can take parameters to customize their behavior, like checking different addresses.

Why are function modifiers useful in smart contract development?

A They encrypt data

B They speed up the blockchain

C They create new tokens

D They help reuse code and improve security by adding checks

Modifiers help reuse common code like access control, making contracts safer and cleaner.

Explain what a function modifier is and how it changes a function's behavior in Solidity.

Think of a modifier as a helper that wraps around a function.

Describe how you would use a function modifier to restrict access to only the contract owner.

Use require to check who calls the function.

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