Blockchain Smart Contracts with Solidity · Lesson

Functions and Visibility Modifiers

Understand how to define functions, manage their visibility (public, private, internal, external), and handle function parameters and return values.

Functions and Visibility Modifiers is a free Blockchain Smart Contracts with Solidity lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Blockchain Smart Contracts with Solidity learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Functions: Your Contract's Actions

Welcome to Lesson 3! Today, we'll dive into functions in Solidity. Think of functions as the actions or operations your smart contract can perform.

They allow you to organize your code, make it reusable, and define how users (or other contracts) can interact with your contract's data and logic.

Defining a Simple Function

Every function needs a name and a body where its code lives. Here's the basic structure:

Keyword function
Function name (e.g., greet)
Parentheses () for parameters
Curly braces {} for the code block

Let's see a very simple example.

Basic Function Example

This contract has a function named getName. It doesn't take any inputs and simply returns a string. We'll add visibility soon!

Try running it to see how a function can return a value.

pragma solidity ^0.8.0;

contract MyFirstFunctions {
    function getName() pure returns (string memory) {
        return "Coddy";
    }
}

Controlling Access: Visibility

Solidity functions, like variables, have visibility modifiers. These define who can call a function and how.

Understanding visibility is crucial for security and proper contract design. There are four types: public, external, internal, and private.

Public Functions: Open for All

A public function is the most accessible. It can be called:

From outside the contract (by users or other contracts).
From inside the contract (by other functions within the same contract).

If you don't specify a visibility, functions are public by default. It's good practice to always specify it explicitly.

Public Function in Action

Here, setValue and getValue are both public. You can call them directly from an external account or another contract.

Run this example and try calling both functions after deploying.

pragma solidity ^0.8.0;

contract PublicExample {
    uint public myValue;

    function setValue(uint _newValue) public {
        myValue = _newValue;
    }

    function getValue() public view returns (uint) {
        return myValue;
    }
}

External Functions: External Calls Only

external functions can only be called from outside the contract. They cannot be called internally by other functions within the same contract.

This is often used for functions that are entry points for external interactions. It's also more gas-efficient for receiving large arrays of data.

External Function Example

Notice how onlyExternal can only be called from outside. If another function inside ExternalExample tried to call it, it would result in a compilation error.

Run this to see the external interaction.

pragma solidity ^0.8.0;

contract ExternalExample {
    uint public count = 0;

    function incrementExternal() external {
        count++;
    }

    // This function can't call incrementExternal() directly.
    // Try adding 'incrementExternal();' inside this function to see the error.
    function getCount() public view returns (uint) {
        return count;
    }
}

Internal & Private: For Internal Use

internal functions can only be called from within the current contract or by contracts that inherit from it.

private functions are the most restricted. They can only be called from within the contract they are defined in. Even inheriting contracts cannot call them.

These are great for helper functions that shouldn't be exposed externally.

Parameters & Return Values

Functions can accept parameters (inputs) and return values (outputs).

Parameters are defined inside the parentheses ().
Return types are specified using the returns (type) keyword.

This allows functions to be dynamic and process different data.

Function Visibility Check

Which of the following function visibility modifiers means a function can ONLY be called from outside the contract, but NOT from another function within the same contract?

Recap: Functions & Visibility

Great job! You've learned the fundamentals of Solidity functions.

Functions organize contract logic.
public: Callable from anywhere.
external: Callable only from outside.
internal: Callable from within & by derived contracts.
private: Callable only from within the defining contract.
Functions can take parameters and return values.

Next, we'll explore more complex data structures like mappings and dynamic arrays!

Frequently asked questions

Is the “Functions and Visibility Modifiers” lesson free?

Yes — the full text of “Functions and Visibility Modifiers” is free to read here on the web, and the Blockchain Smart Contracts with Solidity course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Blockchain Smart Contracts with Solidity course, upgrade to CoddyKit PRO.

What will I learn in “Functions and Visibility Modifiers”?

Understand how to define functions, manage their visibility (public, private, internal, external), and handle function parameters and return values. You practise Blockchain Smart Contracts with Solidity with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Blockchain Smart Contracts with Solidity?

No prior experience is required. Blockchain Smart Contracts with Solidity on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Functions and Visibility Modifiers” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Blockchain Smart Contracts with Solidity lesson?

Yes. Every Blockchain Smart Contracts with Solidity lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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