Contract inheritance in Blockchain / Solidity

Introduction

Contract inheritance lets you create new contracts based on existing ones. This helps you reuse code and add new features easily.

You want to build a new contract that shares common features with another contract.

You need to add extra functions to a contract without changing the original code.

You want to organize your contracts better by separating common logic into a base contract.

You want to upgrade or extend a contract's behavior safely.

You want to avoid repeating the same code in multiple contracts.

Syntax

Blockchain / Solidity

contract ChildContract is ParentContract {
    // new or overridden code here
}

Use the keyword is to inherit from one or more parent contracts.

You can override functions from the parent contract by redefining them in the child contract.

Examples

The Child contract inherits the greet function from Parent.

Blockchain / Solidity

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

contract Child is Parent {
    // inherits greet() from Parent
}

The Child contract overrides the greet function to change its behavior.

Blockchain / Solidity

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

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

Contract C inherits from two contracts A and B, gaining both functions.

Blockchain / Solidity

contract A {
    function foo() public pure returns (string memory) {
        return "A";
    }
}

contract B {
    function bar() public pure returns (string memory) {
        return "B";
    }
}

contract C is A, B {
    // inherits foo() from A and bar() from B
}

Sample Program

This program shows a base contract Animal with a function sound. Two contracts Dog and Cat inherit from Animal and override sound to return their own sounds. The Test contract creates instances and calls their sounds.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Animal {
    function sound() public pure virtual returns (string memory) {
        return "Some sound";
    }
}

contract Dog is Animal {
    function sound() public pure override returns (string memory) {
        return "Bark";
    }
}

contract Cat is Animal {
    function sound() public pure override returns (string memory) {
        return "Meow";
    }
}

contract Test {
    function testSounds() public pure returns (string memory, string memory) {
        Dog dog = new Dog();
        Cat cat = new Cat();
        return (dog.sound(), cat.sound());
    }
}

Important Notes

Remember to mark functions in the parent contract as virtual if you want to allow overriding.

Use override keyword in the child contract when you change a function from the parent.

Multiple inheritance is allowed, but be careful with function name conflicts.

Summary

Contract inheritance helps reuse and extend code in blockchain contracts.

Use is keyword to inherit from one or more contracts.

Override functions with virtual and override keywords to customize behavior.

Practice

1. What keyword is used in Solidity to inherit from a parent contract?

easy

A. extends

B. inherits

C. is

D. implements

Solution

1. Step 1: Understand Solidity inheritance syntax

   In Solidity, the keyword is is used to inherit from a parent contract.

2. Step 2: Compare with other options

   Keywords like extends, inherits, and implements are not used in Solidity for inheritance.

3. Final Answer:

   is -> Option C

4. Quick Check:

   Inheritance keyword = is [OK]

Hint: Remember: Solidity uses 'is' for inheritance [OK]

Common Mistakes:

• Confusing Solidity with other languages like Java using 'extends'
• Using 'implements' which is for interfaces
• Trying 'inherits' which is not a Solidity keyword

2. Which of the following is the correct way to declare a function in a parent contract that can be overridden in Solidity?

easy

A. function foo() public virtual {}

B. function foo() public abstract {}

C. function foo() public override {}

D. function foo() public final {}

Solution

1. Step 1: Identify the keyword to allow overriding

   In Solidity, to allow a function to be overridden in child contracts, it must be marked virtual.

2. Step 2: Check other options for correctness

   override is used in child contracts, abstract is for abstract contracts, and final is not a Solidity keyword.

3. Final Answer:

   function foo() public virtual {} -> Option A

4. Quick Check:

   Allow override = virtual [OK]

Hint: Use 'virtual' in parent to allow overrides [OK]

Common Mistakes:

• Using 'override' in parent instead of child
• Confusing 'abstract' with 'virtual'
• Using non-existent 'final' keyword

3. What will be the output of the following Solidity code when calling greet() on contract Child?

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";
    }
}

medium

A. Runtime error

B. "Hello from Parent"

C. Compilation error due to missing override

D. "Hello from Child"

Solution

1. Step 1: Understand function overriding

   The Child contract overrides the greet() function from Parent using override keyword.

2. Step 2: Determine which function runs on Child

   Calling greet() on Child runs the overridden function returning "Hello from Child".

3. Final Answer:

   "Hello from Child" -> Option D

4. Quick Check:

   Override function output = "Hello from Child" [OK]

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

Common Mistakes:

• Thinking parent function runs instead of child
• Missing 'override' keyword causing compile error
• Expecting runtime error without cause

4. Identify the error in the following Solidity code snippet:

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. Missing 'virtual' keyword in Base's foo function

C. Derived contract should not inherit Base

D. Function foo should be private

Solution

1. Step 1: Check overriding rules in Solidity

   When overriding a function marked virtual in the parent, the child function must use override keyword.

2. Step 2: Identify missing keyword

   The foo() function in Derived lacks the override keyword, causing a compilation error.

3. Final Answer:

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

4. Quick Check:

   Override missing = compile error [OK]

Hint: Override functions must use 'override' keyword [OK]

Common Mistakes:

• Forgetting 'override' keyword in child function
• Thinking 'virtual' is needed in child function
• Assuming inheritance itself causes error

5. Given the following contracts, what will be the output of calling getMessage() on contract GrandChild?

contract Parent {
    function getMessage() public virtual returns (string memory) {
        return "Parent";
    }
}

contract Child is Parent {
    function getMessage() public virtual override returns (string memory) {
        return "Child";
    }
}

contract GrandChild is Child {
    function getMessage() public override returns (string memory) {
        return super.getMessage();
    }
}

hard

A. "Parent"

B. "Child"

C. "GrandChild"

D. Compilation error due to missing virtual

Solution

1. Step 1: Understand super keyword in Solidity inheritance

   The GrandChild contract calls super.getMessage(), which refers to the immediate parent Child implementation.

2. Step 2: Identify which function runs

   The Child contract's getMessage() returns "Child", so GrandChild returns "Child" as well.

3. Final Answer:

   "Child" -> Option B

4. Quick Check:

   super calls immediate parent = "Child" [OK]

Hint: super calls immediate parent function [OK]

Common Mistakes:

• Thinking super calls grandparent function
• Expecting 'GrandChild' string without override
• Missing 'virtual' in intermediate contract causing error