Virtual and override keywords in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a virtual function in a smart contract.

Blockchain / Solidity

contract Base {
    function greet() public view [1] returns (string memory) {
        return "Hello from Base";
    }
}

Drag options to blanks, or click blank then click option'

Aexternal

Bvirtual

Coverride

Dpure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'override' instead of 'virtual' when declaring the base function.

Omitting the keyword entirely, which prevents overriding.

The virtual keyword marks a function that can be overridden in a derived contract.

2fill in blank

medium

Complete the code to override the virtual function in the derived contract.

Blockchain / Solidity

contract Derived is Base {
    function greet() public view [1] returns (string memory) {
        return "Hello from Derived";
    }
}

Drag options to blanks, or click blank then click option'

Aoverride

Bpure

Cexternal

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' instead of 'override' when overriding a function.

Forgetting to mark the base function as virtual.

The override keyword is used to replace the behavior of a virtual function in a child contract.

3fill in blank

hard

Fix the error in the derived contract by adding the correct keyword to the overridden function.

Blockchain / Solidity

contract Derived is Base {
    function greet() public view [1] returns (string memory) {
        return "Hello from Derived";
    }
}

Drag options to blanks, or click blank then click option'

Aoverride

Bvirtual

Cexternal

Dpayable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' instead of 'override' in the derived contract.

Omitting the keyword entirely, causing a compilation error.

The overridden function must use the override keyword to compile correctly.

4fill in blank

hard

Fill both blanks to declare a virtual function and override it correctly.

Blockchain / Solidity

contract Base {
    function calculate() public view [1] returns (uint) {
        return 1;
    }
}

contract Derived is Base {
    function calculate() public view [2] returns (uint) {
        return 2;
    }
}

Drag options to blanks, or click blank then click option'

Avirtual

Bexternal

Coverride

Dpure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'override' in the base function instead of 'virtual'.

Forgetting to use 'override' in the derived function.

The base function uses virtual to allow overriding, and the derived function uses override to replace it.

5fill in blank

hard

Fill all three blanks to declare a virtual function, override it, and call the base function inside the override.

Blockchain / Solidity

contract Base {
    function info() public view [3] returns (string memory) {
        return "Base info";
    }
}

contract Derived is Base {
    function info() public view [1] returns (string memory) {
        return string(abi.encodePacked(Base.[2](), " and Derived info"));
    }
}

Drag options to blanks, or click blank then click option'

Aoverride

Binfo

Cvirtual

Dpure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' instead of 'override' in the derived function.

Calling a wrong function name instead of the base function.

Not marking the base function as virtual.

The derived function uses override to replace the base function, calls the base function by its name info, and the base function is marked virtual.

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