Visibility modifiers (public, private, internal, external) in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Visibility modifiers (public, private, internal, external)

O(n)

Understanding Time Complexity

When working with blockchain smart contracts, functions have visibility modifiers that control who can call them.

We want to understand how these modifiers affect the number of operations when functions are called.

Scenario Under Consideration

Analyze the time complexity of calling functions with different visibility modifiers.

contract Example {
    uint data;

    function setDataPublic(uint x) public {
        data = x;
    }

    function setDataPrivate(uint x) private {
        data = x;
    }

    function callPrivate(uint x) public {
        setDataPrivate(x);
    }
}
    

This contract has functions with public and private visibility and shows calling a private function internally.

Identify Repeating Operations

Look for repeated function calls or loops that affect execution time.

• Primary operation: Calling functions with different visibility.
• How many times: Each function call happens once per external or internal call.

How Execution Grows With Input

The number of operations grows directly with how many times functions are called.

Input Size (n)	Approx. Operations
10 calls	10 function executions
100 calls	100 function executions
1000 calls	1000 function executions

Pattern observation: Each call adds a fixed amount of work, so total work grows linearly with calls.

Final Time Complexity

Time Complexity: O(n)

This means the total work grows in direct proportion to the number of function calls made.

Common Mistake

[X] Wrong: "Private functions run faster or cost less time than public ones because they are hidden."

[OK] Correct: Visibility controls who can call the function, but the time to execute the function itself is about what the code does, not its visibility.

Interview Connect

Understanding how visibility affects function calls helps you reason about contract design and performance in real blockchain projects.

Self-Check

"What if a public function calls an external function multiple times inside a loop? How would the time complexity change?"

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