Abstract contracts in Blockchain / Solidity - Mini Project: Build & Apply

Abstract Contracts in Solidity

📖 Scenario: You are building a simple blockchain-based system where different types of accounts share some common features but have their own specific behaviors. To organize your code well, you will use an abstract contract as a blueprint.

🎯 Goal: Create an abstract contract called Account with a function to get the account balance. Then, create a concrete contract called SavingsAccount that inherits from Account and implements the balance function. Finally, display the balance.

📋 What You'll Learn

Create an abstract contract named Account

Add an abstract function getBalance that returns a uint

Create a contract SavingsAccount that inherits from Account

Implement the getBalance function in SavingsAccount

Create a state variable balance in SavingsAccount initialized to 1000

Add a function to return the balance

Write a function to get the balance and print it

💡 Why This Matters

🌍 Real World

Abstract contracts help organize blockchain code by defining common interfaces for different types of accounts or contracts.

💼 Career

Understanding abstract contracts is important for blockchain developers to write clean, reusable, and maintainable smart contracts.

1

Create the abstract contract

Create an abstract contract called Account with an abstract function getBalance that returns a uint.

Blockchain / Solidity

// Create an abstract contract Account with an abstract function getBalance
// Your code here

Hint

Use the abstract keyword before contract. Declare getBalance without a body and mark it virtual.

2

Create the concrete contract with balance

Create a contract called SavingsAccount that inherits from Account. Add a uint state variable called balance and set it to 1000.

Blockchain / Solidity

abstract contract Account {
    function getBalance() public view virtual returns (uint);
}

// Create contract SavingsAccount inheriting Account
// Add uint balance = 1000
// Your code here

Hint

Use is Account to inherit. Declare balance as a state variable with initial value 1000.

3

Implement the abstract function

In SavingsAccount, implement the getBalance function to return the balance variable.

Blockchain / Solidity

abstract contract Account {
    function getBalance() public view virtual returns (uint);
}

contract SavingsAccount is Account {
    uint balance = 1000;

    // Implement getBalance to return balance
    // Your code here
}

Hint

Use override keyword and return the balance variable.

4

Display the balance

Add a public function called showBalance in SavingsAccount that returns the result of getBalance(). Then, write a line to call showBalance() and print the returned balance.

Blockchain / Solidity

abstract contract Account {
    function getBalance() public view virtual returns (uint);
}

contract SavingsAccount is Account {
    uint balance = 1000;

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

    // Add showBalance function that returns getBalance()
    // Your code here
}

// Call showBalance and print the result
// Your code here

Hint

Create a public function that calls getBalance() and returns its value.

Practice

1.

What is the main purpose of an abstract contract in blockchain programming?

easy

A. To define functions without implementation that child contracts must complete

B. To deploy a contract directly on the blockchain

C. To store data permanently without functions

D. To create a user interface for blockchain apps

Solution

1. Step 1: Understand abstract contract role

   Abstract contracts declare functions without full code, acting as templates.

2. Step 2: Identify deployment capability

   They cannot be deployed alone but require child contracts to implement missing parts.

3. Final Answer:

   To define functions without implementation that child contracts must complete -> Option A

4. Quick Check:

   Abstract contract = function templates [OK]

Hint: Abstract contracts declare incomplete functions for children [OK]

Common Mistakes:

• Thinking abstract contracts can be deployed directly
• Confusing abstract contracts with interfaces
• Believing abstract contracts store data only

2.

Which of the following is the correct syntax to declare an abstract contract in Solidity?

?

easy

A. contract MyContract abstract { function foo() public; }

B. abstract contract MyContract { function foo() public virtual; }

C. abstract MyContract contract { function foo() public; }

D. contract MyContract { abstract function foo(); }

Solution

1. Step 1: Check abstract contract declaration

   In Solidity, use 'abstract contract' before the contract name.

2. Step 2: Verify function declaration

   Abstract functions are declared with 'virtual' and no body.

3. Final Answer:

   abstract contract MyContract { function foo() public virtual; } -> Option B

4. Quick Check:

   Correct abstract syntax = abstract contract MyContract { function foo() public virtual; } [OK]

Hint: Use 'abstract contract' and 'virtual' for abstract functions [OK]

Common Mistakes:

• Placing 'abstract' after 'contract'
• Missing 'virtual' keyword on abstract functions
• Trying to define function body in abstract contract

3.

Given the following Solidity code, what will be the output when child.getValue() is called?

abstract contract Parent {
    function getValue() public virtual returns (string memory);
}

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

medium

A. Compilation error due to missing function body in Parent

B. "Hello from Parent"

C. "Hello from Child"

D. Runtime error because Parent is abstract

Solution

1. Step 1: Understand abstract function in Parent

   Parent declares getValue() as abstract (no body), so Child must implement it.

2. Step 2: Check Child's implementation

   Child overrides getValue() and returns "Hello from Child" string.

3. Final Answer:

   "Hello from Child" -> Option C

4. Quick Check:

   Child's override runs = "Hello from Child" [OK]

Hint: Child must implement abstract functions; output from child method [OK]

Common Mistakes:

• Expecting Parent's abstract function to run
• Thinking abstract contracts cause runtime errors
• Confusing compilation errors with runtime errors

4.

Identify the error in this Solidity code snippet:

abstract contract Base {
    function doSomething() public virtual;
}

contract Derived is Base {
    // Missing implementation of doSomething
}

medium

A. Derived contract must implement doSomething() or be declared abstract

B. Base contract should not have virtual functions

C. Derived contract cannot inherit from abstract contracts

D. doSomething() must have a function body in Base

Solution

1. Step 1: Check abstract function in Base

   doSomething() is abstract (no body), so Derived must implement it or be abstract.

2. Step 2: Analyze Derived contract

   Derived does not implement doSomething() and is not abstract, causing error.

3. Final Answer:

   Derived contract must implement doSomething() or be declared abstract -> Option A

4. Quick Check:

   Abstract functions require implementation in non-abstract children [OK]

Hint: Non-abstract child must implement all abstract functions [OK]

Common Mistakes:

• Assuming virtual means implemented
• Forgetting to mark Derived as abstract if no implementation
• Thinking abstract contracts can be instantiated

5.

You want to create a contract system where multiple child contracts share a common function signature but implement it differently. How should you design the parent contract?

hard

A. Use an interface with implemented functions

B. Use a regular contract with fully implemented functions

C. Use a library instead of a contract

D. Use an abstract contract with the function declared as virtual and no body

Solution

1. Step 1: Understand shared function signature need

   Multiple children must share function names but implement differently.

2. Step 2: Choose abstract contract design

   Abstract contracts declare functions without bodies, forcing children to implement them.

3. Step 3: Exclude other options

   Regular contracts have implementations, libraries can't enforce function signatures, interfaces can't have implementations.

4. Final Answer:

   Use an abstract contract with the function declared as virtual and no body -> Option D

5. Quick Check:

   Abstract contract enforces function signature for children [OK]

Hint: Abstract contracts enforce shared function signatures [OK]

Common Mistakes:

• Using regular contracts which fix implementation
• Confusing interfaces with implemented functions
• Trying to use libraries for function enforcement