ERC-721 implementation in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is ERC-721 in blockchain?

ERC-721 is a standard for creating Non-Fungible Tokens (NFTs) on the Ethereum blockchain. Each token is unique and cannot be replaced by another.

beginner

What function in ERC-721 returns the owner of a specific token?

The function ownerOf(tokenId) returns the address of the owner of the token with the given tokenId.

intermediate

Why does ERC-721 require a transferFrom function?

The transferFrom function allows the owner or an approved address to transfer ownership of a token to another address, enabling token trading and movement.

intermediate

What is the purpose of the approve function in ERC-721?

The approve function lets the token owner authorize another address to transfer a specific token on their behalf.

intermediate

Name two events defined in the ERC-721 standard and their purpose.

1. Transfer: Emitted when ownership of a token changes.
2. Approval: Emitted when a token owner approves another address to manage a token.

Which function in ERC-721 returns the total number of tokens owned by an address?

A ownerOf

B balanceOf

C totalSupply

D tokenURI

The balanceOf function returns how many tokens an address owns.

What does the tokenURI function provide in an ERC-721 contract?

A The owner address of a token

B The total number of tokens

C The metadata URL for a token

D The approval status of a token

The tokenURI function returns a URL pointing to metadata describing the token.

Which event is emitted when a token is transferred in ERC-721?

A Transfer

B TokenMoved

C OwnershipChanged

D Approval

The Transfer event signals that a token's ownership has changed.

What is the main difference between ERC-20 and ERC-721 tokens?

A ERC-20 tokens are only for Ethereum, ERC-721 tokens are for Bitcoin

B ERC-20 tokens are unique, ERC-721 tokens are fungible

C ERC-20 tokens cannot be transferred, ERC-721 tokens can

D ERC-20 tokens are fungible, ERC-721 tokens are unique

ERC-20 tokens are fungible (all tokens are the same), while ERC-721 tokens are unique and non-fungible.

Which function allows an approved address to transfer a token on behalf of the owner?

A transferFrom

B safeTransferFrom

C approve

D setApprovalForAll

The transferFrom function lets an approved address transfer a token.

Explain the key functions and events that an ERC-721 contract must implement.

Think about how tokens are owned, transferred, and approved.

Describe how ERC-721 tokens differ from ERC-20 tokens and why this matters.

Consider what makes each token type special and their typical uses.

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