Factory pattern in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new contract instance using the factory.

Blockchain / Solidity

contract MyFactory {
    function create() public returns (address) {
        [1] newContract = new SimpleContract();
        return address(newContract);
    }
}

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

ASimpleContract

BFactory

CContract

DInstance

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong contract name that does not exist.

Trying to return the contract type instead of its address.

The factory creates a new instance of SimpleContract using the new keyword.

2fill in blank

medium

Complete the code to store the address of the created contract in the factory.

Blockchain / Solidity

contract MyFactory {
    address[] public contracts;

    function create() public {
        SimpleContract newContract = new SimpleContract();
        contracts.[1](address(newContract));
    }
}

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

Ainsert

Bpush

Cremove

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes elements instead of adding.

Trying to assign directly without using array methods.

The push method adds the new contract address to the contracts array.

3fill in blank

hard

Fix the error in the factory function to correctly emit an event after contract creation.

Blockchain / Solidity

contract MyFactory {
    event ContractCreated(address contractAddress);

    function create() public {
        SimpleContract newContract = new SimpleContract();
        emit [1](address(newContract));
    }
}

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

AContractCreatedEvent

BCreatedContract

CContractCreated

DNewContract

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong event name that does not exist.

Misspelling the event name or changing its case.

The event declared is ContractCreated, so it must be emitted with that exact name.

4fill in blank

hard

Fill both blanks to define a factory that creates different contract types based on input.

Blockchain / Solidity

contract Factory {
    function create(uint8 typeId) public returns (address) {
        if (typeId == 1) {
            return address(new ContractA());
        } else if (typeId == 2) {
            return address(new [1]());
        } else {
            revert("Invalid type");
        }
    }
}

contract ContractA {}
contract [2] {}

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

AContractB

BContractC

CContractA

DFactory

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the contract creation and definition.

Trying to create a contract that is not defined.

The factory creates ContractB when typeId is 2, so both blanks must be ContractB.

5fill in blank

hard

Fill all three blanks to implement a factory that creates contracts with an owner and stores them.

Blockchain / Solidity

contract Owned {
    address public owner;
    constructor(address _owner) {
        owner = _owner;
    }
}

contract Factory {
    address[] public contracts;

    function create(address _owner) public returns (address) {
        Owned newContract = new Owned([1]);
        contracts.[2](address(newContract));
        emit Created([3]);
        return address(newContract);
    }

    event Created(address contractAddress);
}

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

A_owner

Bpush

Caddress(newContract)

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Passing wrong argument to the constructor.

Using wrong array method instead of push.

Emitting wrong value in the event.

The constructor needs _owner as argument, the contract address is pushed to the array, and the event emits the new contract's address.

Practice

1.

What is the main purpose of the Factory pattern in blockchain development?

easy

A. To create multiple similar contracts easily and manage their addresses

B. To encrypt data on the blockchain

C. To mine new blocks faster

D. To validate transactions off-chain

Solution

1. Step 1: Understand the Factory pattern role

   The Factory pattern is used to create many similar contracts efficiently.

2. Step 2: Identify its key feature

   It also stores the addresses of these created contracts for easy access later.

3. Final Answer:

   To create multiple similar contracts easily and manage their addresses -> Option A

4. Quick Check:

   Factory pattern = create and manage contracts [OK]

Hint: Factory pattern creates and tracks contracts easily [OK]

Common Mistakes:

• Confusing factory with encryption or mining
• Thinking factory validates transactions
• Assuming factory works off-chain

2.

Which of the following is the correct Solidity syntax to deploy a new contract inside a factory contract?

function create() public returns (address) {
    address newContract = new ?();
    return newContract;
}

easy

A. ContractName

B. contractname

C. new ContractName()

D. ContractName()

Solution

1. Step 1: Recall Solidity contract creation syntax

   To create a new contract instance, use new ContractName().

2. Step 2: Match syntax with code snippet

   The placeholder new ?() expects the contract name without 'new' repeated.

3. Final Answer:

   ContractName -> Option A

4. Quick Check:

   Use 'new ContractName()' but only 'ContractName' inside parentheses [OK]

Hint: Use contract name only inside new keyword parentheses [OK]

Common Mistakes:

• Writing 'new' twice
• Using lowercase contract names
• Omitting parentheses

3.

Consider this Solidity factory contract snippet:

contract Simple {
    uint public value;
    constructor(uint _value) {
        value = _value;
    }
}

contract Factory {
    Simple[] public simples;
    function createSimple(uint _val) public {
        Simple s = new Simple(_val);
        simples.push(s);
    }
    function getValue(uint index) public view returns (uint) {
        return simples[index].value();
    }
}

What will getValue(0) return after calling createSimple(42) once?

medium

A. Address of the contract

B. 0

C. 42

D. Compilation error

Solution

1. Step 1: Understand contract creation and storage

   Calling createSimple(42) creates a new Simple contract with value = 42 and stores it in simples array.

2. Step 2: Check what getValue(0) returns

   It returns the value of the first Simple contract, which is 42.

3. Final Answer:

   42 -> Option C

4. Quick Check:

   Created contract value = 42 [OK]

Hint: Created contract stores value; getValue returns it [OK]

Common Mistakes:

• Confusing contract address with stored value
• Assuming default zero value
• Thinking it returns array length

4.

Identify the error in this factory contract code snippet:

contract Product {
    uint public id;
    constructor(uint _id) {
        id = _id;
    }
}

contract ProductFactory {
    Product[] public products;
    function createProduct(uint _id) public {
        Product p = Product(_id);
        products.push(p);
    }
}

medium

A. Array products should be a mapping

B. Missing new keyword when creating Product

C. Constructor should not have parameters

D. Function createProduct must be view

Solution

1. Step 1: Check contract instantiation syntax

   In Solidity, to create a new contract instance, you must use the new keyword.

2. Step 2: Identify the missing keyword

   The line Product p = Product(_id); misses new, it should be Product p = new Product(_id);.

3. Final Answer:

   Missing new keyword when creating Product -> Option B

4. Quick Check:

   Contract creation requires 'new' keyword [OK]

Hint: Always use 'new' to create contracts in Solidity [OK]

Common Mistakes:

• Forgetting 'new' keyword
• Changing array to mapping unnecessarily
• Marking create function as view incorrectly

5.

You want to build a factory contract that creates multiple token contracts with different initial supplies and keeps track of them. Which approach best applies the factory pattern to save gas and organize your project?

hard

A. Use a single token contract and change its supply dynamically for each user

B. Deploy all token contracts manually and hardcode their addresses in the factory

C. Create token contracts but do not store their addresses anywhere

D. Create each token contract separately and store their addresses in an array inside the factory

Solution

1. Step 1: Understand factory pattern benefits

   The factory pattern helps create many similar contracts and keeps track of them efficiently.

2. Step 2: Evaluate options for managing multiple tokens

   Creating each token contract inside the factory and storing their addresses allows easy management and gas savings.

3. Step 3: Reject other options

   Hardcoding addresses is inflexible, using one contract for all tokens breaks isolation, and not storing addresses loses track.

4. Final Answer:

   Create each token contract separately and store their addresses in an array inside the factory -> Option D

5. Quick Check:

   Factory creates and tracks contracts for organization [OK]

Hint: Factory creates and stores contracts for easy management [OK]

Common Mistakes:

• Hardcoding addresses reduces flexibility
• Using one contract for all tokens causes conflicts
• Not storing addresses loses track of contracts