Virtual and override keywords in Blockchain / Solidity - Mini Project: Build & Apply

Understanding Virtual and Override Keywords in Blockchain Smart Contracts

📖 Scenario: Imagine you are building a simple blockchain smart contract system where you want to create a base contract for a digital wallet and then create a specialized contract that changes how the balance is displayed.

🎯 Goal: You will create a base contract with a virtual function and then override that function in a derived contract to change its behavior.

📋 What You'll Learn

Create a base contract called Wallet with a public balance variable set to 100

Add a virtual function called getBalance that returns the balance

Create a derived contract called SpecialWallet that inherits from Wallet

Override the getBalance function in SpecialWallet to return double the balance

Deploy the SpecialWallet contract and call getBalance to see the overridden result

💡 Why This Matters

🌍 Real World

In blockchain development, contracts often inherit from base contracts to reuse code and customize behavior. Virtual and override keywords help manage this safely.

💼 Career

Understanding inheritance and function overriding is essential for blockchain developers to build secure and maintainable smart contracts.

1

Create the base contract with a balance and virtual function

Create a contract called Wallet with a public uint variable balance set to 100. Add a virtual function called getBalance that returns the balance.

Blockchain / Solidity

contract Wallet {
    // Your code here
}

Need a hint?

Use virtual keyword in the function declaration to allow overriding.

2

Create a derived contract and override the getBalance function

Create a contract called SpecialWallet that inherits from Wallet. Override the getBalance function using the override keyword to return balance * 2.

Blockchain / Solidity

contract Wallet {
    uint public balance = 100;
    
    function getBalance() public view virtual returns (uint) {
        return balance;
    }
}

contract SpecialWallet is Wallet {
    // Your code here
}

Need a hint?

Use override keyword in the derived contract's function to replace the base function.

3

Deploy the SpecialWallet contract and call getBalance

Write a function called testGetBalance in SpecialWallet that calls getBalance and returns its value.

Blockchain / Solidity

contract Wallet {
    uint public balance = 100;
    
    function getBalance() public view virtual returns (uint) {
        return balance;
    }
}

contract SpecialWallet is Wallet {
    function getBalance() public view override returns (uint) {
        return balance * 2;
    }

    function testGetBalance() public view returns (uint) {
        // Your code here
    }
}

Need a hint?

Call the overridden getBalance function inside testGetBalance.

4

Print the result of testGetBalance

Add a public function called printBalance in SpecialWallet that returns the result of testGetBalance. This simulates printing the overridden balance.

Blockchain / Solidity

contract Wallet {
    uint public balance = 100;
    
    function getBalance() public view virtual returns (uint) {
        return balance;
    }
}

contract SpecialWallet is Wallet {
    function getBalance() public view override returns (uint) {
        return balance * 2;
    }

    function testGetBalance() public view returns (uint) {
        return getBalance();
    }

    function printBalance() public view returns (uint) {
        // Your code here
    }
}

Need a hint?

Return the value from testGetBalance inside printBalance.