Visibility modifiers (public, private, internal, external) in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the public visibility modifier mean in blockchain smart contracts?

A public function or variable can be accessed both internally within the contract and externally by other contracts or users.

beginner

Explain the private visibility modifier.

A private function or variable is only accessible inside the contract where it is defined. It cannot be accessed by derived contracts or externally.

intermediate

What is the difference between internal and private visibility?

Internal functions or variables can be accessed inside the contract and by contracts that inherit from it, while private ones are only accessible inside the defining contract.

intermediate

When should you use the external visibility modifier?

External functions are meant to be called from other contracts or transactions, but not from within the same contract. They are efficient for functions called only externally.

advanced

Can a public function call an external function internally?

No, a public function cannot call an external function internally using a simple function call. External functions require a message call (like a transaction). However, a public function can call an external function using this.<functionName>().

Which visibility modifier allows access only within the contract it is defined in?

A external

B public

C private

D internal

Private members are accessible only inside the contract they are defined in.

Which visibility modifier allows access from derived contracts?

A internal

B private

C external

D public

Internal members can be accessed inside the contract and by contracts that inherit from it.

Which visibility modifier is designed to be called only from outside the contract?

A internal

B private

C public

D external

External functions are meant to be called from other contracts or transactions, not internally.

Can a public function be called from outside the contract?

A Yes

B No

C Only if inherited

D Only if marked external

Public functions are accessible both internally and externally.

Which visibility modifier is the most restrictive?

A public

B private

C internal

D external

Private is the most restrictive, accessible only inside the defining contract.

Describe the four visibility modifiers used in blockchain smart contracts and give a simple example of when to use each.

Think about who can call the function or access the variable.

Explain why you might choose 'internal' over 'private' for a function in a smart contract.

Consider contract inheritance and extending functionality.

Practice

1. Which visibility modifier allows a function to be called both inside and outside the contract, including by other contracts and users?

easy

A. public

B. private

C. internal

D. external

Solution

1. Step 1: Understand visibility modifiers

   Public functions can be accessed anywhere, inside or outside the contract.

2. Step 2: Compare with other modifiers

   Private is only inside the contract, internal is inside and inherited contracts, external only callable from outside.

3. Final Answer:

   public -> Option A

4. Quick Check:

   public = open access [OK]

Hint: Public means open to all inside and outside calls [OK]

Common Mistakes:

• Confusing private with public access
• Thinking internal allows outside calls
• Assuming external works inside contract

2. Which of the following is the correct way to declare a function that can only be called from outside the contract in Solidity?

easy

A. function foo() private {}

B. function foo() external {}

C. function foo() internal {}

D. function foo() public {}

Solution

1. Step 1: Identify external function syntax

   External functions are declared with the keyword external after the function name.

2. Step 2: Check other options

   Private and internal restrict access inside the contract or inheritance, public allows all calls.

3. Final Answer:

   function foo() external {} -> Option B

4. Quick Check:

   external = outside calls only [OK]

Hint: External functions use keyword 'external' after function name [OK]

Common Mistakes:

• Using private instead of external
• Confusing public with external
• Missing parentheses or braces

3. Consider the following Solidity contract snippet:

contract Test {
    uint private data = 10;
    function getData() public view returns (uint) {
        return data;
    }
    function setData(uint _data) external {
        data = _data;
    }
}

What will happen if another contract tries to call data directly?

medium

A. It will succeed and return 10

B. It will fail because data is external

C. It will fail because data is private

D. It will succeed because data is public by default

Solution

1. Step 1: Check variable visibility

   The variable data is declared private, so it cannot be accessed outside the contract.

2. Step 2: Understand access from other contracts

   Other contracts cannot access private variables directly, only public or external functions.

3. Final Answer:

   It will fail because data is private -> Option C

4. Quick Check:

   private = no outside access [OK]

Hint: Private variables can't be accessed outside their contract [OK]

Common Mistakes:

• Assuming private variables are public by default
• Confusing external with private
• Thinking public functions expose private variables directly

4. Given this Solidity code snippet:

contract Sample {
    uint internal count = 0;
    function increment() private {
        count += 1;
    }
    function callIncrement() public {
        increment();
    }
}

Which of the following fixes the error preventing increment() from being called?

medium

A. No fix needed, code is correct

B. Change increment() visibility to internal

C. Change increment() visibility to external

D. Change increment() visibility to public

Solution

1. Step 1: Check function call context

   The function increment() is private and called inside the same contract by callIncrement().

2. Step 2: Understand private function access

   Private functions can be called inside the same contract, so this call is valid.

3. Final Answer:

   No fix needed, code is correct -> Option A

4. Quick Check:

   private functions callable inside contract [OK]

Hint: Private functions work inside same contract only [OK]

Common Mistakes:

• Thinking private functions can't be called inside contract
• Confusing internal with private
• Changing visibility unnecessarily

5. You want to create a Solidity contract where a function can only be called by this contract or contracts that inherit from it, but not by external users. Which visibility modifier should you use for this function?

hard

A. public

B. external

C. private

D. internal

Solution

1. Step 1: Understand access needs

   The function should be callable inside this contract and by derived contracts, but not externally.

2. Step 2: Match visibility modifier

   Internal visibility allows access inside the contract and by inheriting contracts only.

3. Final Answer:

   internal -> Option D

4. Quick Check:

   internal = contract + inheritance only [OK]

Hint: Internal means accessible inside and by child contracts only [OK]

Common Mistakes:

• Choosing public which allows external calls
• Choosing private which blocks inheritance access
• Confusing external with internal