ERC-721 implementation in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity event emission?

Consider the following snippet from an ERC-721 contract. What event is emitted when _mint is called?

Blockchain / Solidity

event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

function _mint(address to, uint256 tokenId) internal {
    require(to != address(0), "ERC721: mint to the zero address");
    require(!_exists(tokenId), "ERC721: token already minted");

    _balances[to] += 1;
    _owners[tokenId] = to;

    emit Transfer(address(0), to, tokenId);
}

A No event is emitted

B Transfer event with from = zero address, to = recipient, tokenId = minted token

C Approval event with owner = zero address, approved = recipient, tokenId = minted token

D Transfer event with from = recipient, to = zero address, tokenId = minted token

💡 Hint

Minting a token means creating it from nothing, so the 'from' address is zero.

When minting a new token, the Transfer event is emitted with the from address set to zero to indicate creation. The to is the recipient address.

❓ Predict Output

intermediate

What is the balance of an address after minting and transferring tokens?

Given this sequence of calls on an ERC-721 contract, what is the balance of 0xABC after all operations?

Blockchain / Solidity

function _mint(address to, uint256 tokenId) internal {
    _balances[to] += 1;
    _owners[tokenId] = to;
}

function _transfer(address from, address to, uint256 tokenId) internal {
    _balances[from] -= 1;
    _balances[to] += 1;
    _owners[tokenId] = to;
}

// Sequence:
// _mint(0xABC, 1);
// _mint(0xABC, 2);
// _transfer(0xABC, 0xDEF, 1);

A 0

B 2

C 1

D 3

💡 Hint

Think about how many tokens 0xABC owns after minting two and transferring one away.

0xABC mints two tokens, so balance is 2. Then transfers one token away, so balance decreases by 1, resulting in 1.

🔧 Debug

advanced

Why does this ERC-721 transfer function revert?

Examine this transfer function snippet. Why does it revert when called?

Blockchain / Solidity

function transferFrom(address from, address to, uint256 tokenId) public {
    require(_owners[tokenId] == from, "Not token owner");
    require(to != address(0), "Transfer to zero address");

    _approve(address(0), tokenId);
    _balances[from] -= 1;
    _balances[to] += 1;
    _owners[tokenId] = to;
}

A It reverts because tokenId is not checked for existence

B It reverts because _approve is called with address(0)

C It reverts because balances are not updated

D It reverts because there is no check that msg.sender is approved or owner

💡 Hint

Think about who is allowed to call transferFrom in ERC-721.

ERC-721 requires that the caller is owner, approved, or operator. This code lacks that check, so unauthorized calls revert.

📝 Syntax

advanced

Which option correctly implements the supportsInterface function for ERC-721?

Choose the correct Solidity code for supportsInterface that returns true for ERC-721 interface ID 0x80ac58cd and calls super otherwise.

A

function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
    return interfaceId == 0x80ac58cd || super.supportsInterface(interfaceId);
}

B

function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
    if (interfaceId == 0x80ac58cd) {
        return false;
    }
    return super.supportsInterface(interfaceId);
}

C

function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
    if (interfaceId == 0x80ac58cd) {
        return true;
    } else {
        return super.supportsInterface(interfaceId);
    }
}

D

function supportsInterface(bytes4 interfaceId) public view override returns (bool) {
    return interfaceId == 0x80ac58cd && super.supportsInterface(interfaceId);
}

💡 Hint

Remember the logical OR operator returns true if either side is true.

The function must return true if the interfaceId matches ERC-721's ID or if the parent contract supports it. Option A correctly uses || operator.

🚀 Application

expert

How many tokens are owned by an address after this sequence of mint, burn, and transfer?

Given this ERC-721 contract snippet and sequence, what is the final balance of 0x123?

Blockchain / Solidity

mapping(address => uint256) private _balances;
mapping(uint256 => address) private _owners;

function _mint(address to, uint256 tokenId) internal {
    _balances[to] += 1;
    _owners[tokenId] = to;
}

function _burn(uint256 tokenId) internal {
    address owner = _owners[tokenId];
    require(owner != address(0), "Token does not exist");
    _balances[owner] -= 1;
    delete _owners[tokenId];
}

function _transfer(address from, address to, uint256 tokenId) internal {
    require(_owners[tokenId] == from, "Not owner");
    _balances[from] -= 1;
    _balances[to] += 1;
    _owners[tokenId] = to;
}

// Sequence:
// _mint(0x123, 10);
// _mint(0x123, 20);
// _transfer(0x123, 0x456, 10);
// _burn(20);

A 0

B 3

C 2

D 1

💡 Hint

Track each token's ownership and balance changes carefully.

Initially, 0x123 owns tokens 10 and 20 (balance 2). Token 10 is transferred to 0x456 (balance 0x123:1). Token 20 is burned (balance 0x123:0). Final balance is 0.

Practice

1.

What is the main purpose of the ERC-721 standard in blockchain?

easy

A. To create fungible tokens like cryptocurrencies

B. To manage smart contract ownership

C. To create unique, non-fungible tokens (NFTs)

D. To enable fast transaction processing

Solution

1. Step 1: Understand ERC-721 token type

   ERC-721 defines a standard for unique tokens, unlike fungible tokens which are identical.

2. Step 2: Identify the purpose of ERC-721

   It is used to create NFTs, which are unique digital assets on the blockchain.

3. Final Answer:

   To create unique, non-fungible tokens (NFTs) -> Option C

4. Quick Check:

   ERC-721 = Unique NFTs [OK]

Hint: ERC-721 is for unique tokens, not identical ones [OK]

Common Mistakes:

• Confusing ERC-721 with ERC-20 fungible tokens
• Thinking ERC-721 manages contract ownership
• Assuming ERC-721 speeds up transactions

2.

Which of the following is the correct way to inherit OpenZeppelin's ERC721 contract in Solidity?

contract MyNFT is ? {}

easy

A. SafeMath

B. ERC721

C. Ownable

D. ERC20

Solution

1. Step 1: Identify the correct base contract for NFTs

   ERC721 is the OpenZeppelin contract for NFTs, ERC20 is for fungible tokens.

2. Step 2: Confirm inheritance syntax

   Using is ERC721 correctly inherits the NFT functionality.

3. Final Answer:

   ERC721 -> Option B

4. Quick Check:

   Inherit ERC721 for NFTs [OK]

Hint: Use ERC721 to inherit NFT features [OK]

Common Mistakes:

• Using ERC20 instead of ERC721
• Confusing Ownable with ERC721
• Trying to inherit SafeMath for NFTs

3.

What will be the output of the following Solidity function when called?

function mintToken(address to, uint256 tokenId) public {
    _mint(to, tokenId);
}

// Assume _mint works as expected and tokenId = 1, to = 0x123...

What happens after calling mintToken(0x123..., 1)?

medium

A. A new NFT with tokenId 1 is assigned to address 0x123...

B. The tokenId 1 is burned from address 0x123...

C. The function throws a syntax error

D. The tokenId 1 is transferred from 0x123... to the caller

Solution

1. Step 1: Understand the _mint function

   _mint creates a new token with the given tokenId and assigns it to the 'to' address.

2. Step 2: Analyze the function call

   Calling mintToken with address 0x123... and tokenId 1 creates that NFT for 0x123....

3. Final Answer:

   A new NFT with tokenId 1 is assigned to address 0x123... -> Option A

4. Quick Check:

   mintToken creates NFT for 'to' address [OK]

Hint: _mint assigns new token to given address [OK]

Common Mistakes:

• Thinking _mint burns tokens
• Assuming _mint transfers tokens
• Expecting syntax errors from correct code

4.

Identify the error in this Solidity snippet for minting an ERC-721 token:

function mint(address to, uint256 tokenId) public {
    _mint(tokenId, to);
}

medium

A. Function must be named mintToken, not mint

B. _mint cannot be called inside a public function

C. tokenId must be a string, not uint256

D. _mint parameters are reversed; should be (to, tokenId)

Solution

1. Step 1: Check _mint function parameter order

   OpenZeppelin's _mint expects (address to, uint256 tokenId).

2. Step 2: Compare with given code

   The code uses _mint(tokenId, to), reversing the parameters.

3. Final Answer:

   _mint parameters are reversed; should be (to, tokenId) -> Option D

4. Quick Check:

   _mint(to, tokenId) correct order [OK]

Hint: _mint needs (address, tokenId) order [OK]

Common Mistakes:

• Swapping parameters of _mint
• Thinking _mint can't be public
• Confusing tokenId type

5.

You want to create an ERC-721 contract that only allows the contract owner to mint new tokens. Which of these Solidity code snippets correctly enforces this?

contract MyNFT is ERC721, Ownable {
    function mint(address to, uint256 tokenId) public {
        ?
        _mint(to, tokenId);
    }
}

hard

A. require(msg.sender == owner(), "Not owner");

B. require(to == owner(), "Not owner");

C. require(tokenId > 0, "Invalid tokenId");

D. require(msg.sender != owner(), "Owner cannot mint");

Solution

1. Step 1: Understand ownership check

   Only the contract owner should mint, so check if msg.sender equals owner().

2. Step 2: Analyze each require statement

   require(msg.sender == owner(), "Not owner"); correctly checks sender is owner; others check wrong conditions.

3. Final Answer:

   require(msg.sender == owner(), "Not owner"); -> Option A

4. Quick Check:

   Owner check uses msg.sender == owner() [OK]

Hint: Check msg.sender equals owner() before minting [OK]

Common Mistakes:

• Checking 'to' address instead of sender
• Allowing anyone to mint without checks
• Using wrong require conditions