Visibility modifiers (public, private, internal, external) in Blockchain / Solidity

Introduction

Visibility modifiers control who can use or see parts of your code. They help keep your program safe and organized.

When you want a function to be used by anyone, like a public service.

When you want a function to be used only inside the same contract to keep it private.

When you want a function to be used inside the contract and contracts that inherit it.

When you want a function to be called only from outside the contract, not from inside.

Syntax

Blockchain / Solidity

function functionName() public { }
function functionName() private { }
function functionName() internal { }
function functionName() external { }

public: Anyone can call this function.

private: Only the current contract can call this function.

internal: Current contract and contracts that inherit it can call this function.

external: Only outside callers can call this function, not internal calls.

Examples

This function can be called by anyone, even outside the contract.

Blockchain / Solidity

function greet() public {
    // Anyone can call this
}

This function is hidden from other contracts and users.

Blockchain / Solidity

function secret() private {
    // Only this contract can call this
}

This function is for internal use and inheritance.

Blockchain / Solidity

function helper() internal {
    // This and child contracts can call this
}

This function cannot be called inside the contract itself.

Blockchain / Solidity

function externalCall() external {
    // Only external calls allowed
}

Sample Program

This example shows how different visibility modifiers control access to variables and functions. Private items are only for the contract itself. Internal items can be used by child contracts. Public items are open to everyone.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract VisibilityExample {
    string private secretMessage = "Hello Private!";
    string internal internalMessage = "Hello Internal!";
    string public publicMessage = "Hello Public!";

    function getSecret() private view returns (string memory) {
        return secretMessage;
    }

    function getInternal() internal view returns (string memory) {
        return internalMessage;
    }

    function getPublic() public view returns (string memory) {
        return publicMessage;
    }

    function callInternal() public view returns (string memory) {
        return getInternal();
    }

    function tryGetSecret() public view returns (string memory) {
        // Cannot call getSecret() here because it is private
        // return getSecret(); // This would cause an error
        return "Cannot access private function from here";
    }
}

contract ChildContract is VisibilityExample {
    function childAccess() public view returns (string memory) {
        // Can access internal but not private
        return internalMessage;
    }
}

Important Notes

Private functions and variables are not visible outside the contract, even to child contracts.

Internal functions and variables can be accessed by the contract and any contract that inherits it.

External functions can only be called from outside the contract, not from inside.

Summary

Public means anyone can use it.

Private means only this contract can use it.

Internal means this contract and child contracts can use it.

External means only outside calls can use it.

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