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
}

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
}

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
    }
}

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
    }
}

Hint

Return the value from testGetBalance inside printBalance.

Practice

1. What does the virtual keyword do in a blockchain smart contract?

easy

A. Makes a function private

B. Prevents a function from being changed

C. Deletes a function from the contract

D. Allows a function to be changed in child contracts

Solution

1. Step 1: Understand the role of virtual

   The virtual keyword marks a function so child contracts can change it.

2. Step 2: Compare with other options

   Options B, C, and D describe behaviors that virtual does not do.

3. Final Answer:

   Allows a function to be changed in child contracts -> Option D

4. Quick Check:

   virtual = allows override [OK]

Hint: Virtual means function can be changed later [OK]

Common Mistakes:

• Thinking virtual makes function private
• Confusing virtual with override
• Believing virtual deletes functions

2. Which of the following is the correct way to override a virtual function named transfer in a child contract?

easy

A. function transfer() public virtual { }

B. function transfer() public override { }

C. function transfer() public final { }

D. function transfer() public abstract { }

Solution

1. Step 1: Identify override syntax

   To replace a virtual function, use override keyword in the child function.

2. Step 2: Check options

   function transfer() public override { } uses override, correct for overriding. function transfer() public virtual { } uses virtual which allows override but does not override itself. Options C and D are incorrect keywords here.

3. Final Answer:

   function transfer() public override { } -> Option B

4. Quick Check:

   Override keyword replaces virtual function [OK]

Hint: Use override keyword to replace virtual function [OK]

Common Mistakes:

• Using virtual instead of override to replace function
• Using final or abstract incorrectly
• Omitting override keyword

3. Consider this Solidity code snippet:

contract Parent {
    function greet() public virtual returns (string memory) {
        return "Hello from Parent";
    }
}

contract Child is Parent {
    function greet() public override returns (string memory) {
        return "Hello from Child";
    }
}

contract Test {
    function testGreet() public returns (string memory) {
        Child c = new Child();
        return c.greet();
    }
}

What will testGreet() return when called?

medium

A. "Hello from Parent"

B. Compilation error

C. "Hello from Child"

D. Runtime error

Solution

1. Step 1: Understand virtual and override behavior

   The greet function in Parent is marked virtual, allowing Child to override it.

2. Step 2: Check which function is called

   When c.greet() is called on a Child instance, the overridden greet in Child runs, returning "Hello from Child".

3. Final Answer:

   "Hello from Child" -> Option C

4. Quick Check:

   Override function runs on child instance [OK]

Hint: Child override runs, not parent virtual function [OK]

Common Mistakes:

• Expecting parent function output
• Thinking override causes error
• Confusing virtual with abstract

4. What is wrong with this Solidity code?

contract Base {
    function foo() public virtual returns (string memory) {
        return "Base";
    }
}

contract Derived is Base {
    function foo() public returns (string memory) {
        return "Derived";
    }
}

medium

A. Missing override keyword in Derived's foo function

B. Base's foo function should not be virtual

C. Derived's foo function should be private

D. No errors, code is correct

Solution

1. Step 1: Check override rules

   Since foo in Base is virtual, the overriding function in Derived must use override keyword.

2. Step 2: Identify missing keyword

   The foo function in Derived lacks override, causing a compilation error.

3. Final Answer:

   Missing override keyword in Derived's foo function -> Option A

4. Quick Check:

   Override keyword required when replacing virtual function [OK]

Hint: Override keyword is mandatory when replacing virtual functions [OK]

Common Mistakes:

• Forgetting override keyword
• Thinking virtual is optional in base
• Making function private incorrectly

5. You have a contract hierarchy:

contract A {
    function calc() public virtual returns (uint) {
        return 1;
    }
}

contract B is A {
    function calc() public virtual override returns (uint) {
        return 2;
    }
}

contract C is B {
    function calc() public override returns (uint) {
        return 3;
    }
}

If you call calc() on a C contract instance, what is the output and why?

hard

A. 3, because C overrides B's function

B. 1, because A's function is virtual

C. 2, because B overrides A and is virtual

D. Compilation error due to multiple overrides

Solution

1. Step 1: Trace overrides in inheritance

   Contract A defines calc as virtual. B overrides it and keeps it virtual. C overrides B's version without virtual.

2. Step 2: Determine which function runs on C instance

   Calling calc() on C runs C's override, returning 3.

3. Final Answer:

   3, because C overrides B's function -> Option A

4. Quick Check:

   Most derived override runs on instance [OK]

Hint: Most derived override runs, even if base is virtual [OK]

Common Mistakes:

• Thinking base virtual function runs
• Confusing virtual with override
• Expecting compilation error from multiple overrides