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

Recall & Review

beginner

What is the ERC-721 standard?

ERC-721 is a standard for creating Non-Fungible Tokens (NFTs) on the Ethereum blockchain. It defines how unique digital assets can be owned and transferred.

beginner

What makes ERC-721 tokens different from ERC-20 tokens?

ERC-721 tokens are unique and non-fungible, meaning each token is different and cannot be exchanged one-to-one. ERC-20 tokens are fungible and identical to each other.

intermediate

Name two key functions defined in the ERC-721 standard.

Two key functions are ownerOf(tokenId) which returns the owner of a specific token, and transferFrom(from, to, tokenId) which transfers ownership of a token.

intermediate

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

tokenURI(tokenId) returns a URL pointing to metadata about the token, such as images or descriptions, helping wallets and apps display the NFT properly.

intermediate

Why is the Approval event important in ERC-721?

The Approval event signals that the owner has allowed another address to manage a specific token, enabling marketplaces or other contracts to transfer tokens on behalf of the owner.

Which of the following best describes an ERC-721 token?

A A decentralized exchange protocol

B A fungible token like a cryptocurrency

C A smart contract programming language

D A unique digital asset on Ethereum

ERC-721 tokens represent unique digital assets, unlike fungible tokens.

What does the ownerOf(tokenId) function return?

A The metadata URL of the token

B The owner address of the token with the given ID

C The total supply of tokens

D The balance of tokens owned by an address

ownerOf returns the address that owns a specific token.

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

A Approval

B Mint

C Transfer

D Burn

The Transfer event is emitted whenever a token changes ownership.

What is the main purpose of the tokenURI function?

A To provide metadata about the token

B To transfer the token

C To approve another address

D To check token balance

tokenURI returns a link to the token's metadata like images or descriptions.

Which of these is NOT a feature of ERC-721 tokens?

A Tokens are interchangeable one-to-one

B Each token is unique

C Supports ownership transfer

D Includes metadata support

ERC-721 tokens are non-fungible, so they are not interchangeable one-to-one.

Explain what ERC-721 tokens are and how they differ from ERC-20 tokens.

Think about how each token is special or identical.

Describe the role of the transferFrom function and the Approval event in ERC-721.

Consider how tokens move and how others get permission.

Practice

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

easy

A. To define a standard for unique non-fungible tokens (NFTs)

B. To create fungible tokens like cryptocurrencies

C. To manage smart contract deployment

D. To encrypt blockchain transactions

Solution

1. Step 1: Understand ERC-721's role

   ERC-721 is designed specifically for unique tokens, called NFTs, which are different from fungible tokens like cryptocurrencies.

2. Step 2: Compare with other token standards

   ERC-20 is for fungible tokens, while ERC-721 is for unique, non-fungible tokens.

3. Final Answer:

   To define a standard for unique non-fungible tokens (NFTs) -> Option A

4. Quick Check:

   ERC-721 = Unique NFTs [OK]

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

Common Mistakes:

• Confusing ERC-721 with ERC-20 fungible tokens
• Thinking ERC-721 manages encryption
• Assuming ERC-721 handles contract deployment

2. Which of the following is the correct function signature to get the owner of an NFT in ERC-721?

easy

A. function getOwner(address token) public returns (uint256);

B. function ownerOf(uint256 tokenId) external view returns (address);

C. function owner(uint256 id) external returns (address);

D. function tokenOwner(uint256 tokenId) public view returns (string);

Solution

1. Step 1: Recall ERC-721 standard function names

   The standard defines ownerOf with a uint256 tokenId parameter returning an address.

2. Step 2: Check function visibility and return type

   The function is external view and returns an address, matching function ownerOf(uint256 tokenId) external view returns (address); exactly.

3. Final Answer:

   function ownerOf(uint256 tokenId) external view returns (address); -> Option B

4. Quick Check:

   ownerOf(tokenId) returns owner address [OK]

Hint: Look for 'ownerOf' with tokenId returning address [OK]

Common Mistakes:

• Using wrong function names like getOwner or tokenOwner
• Incorrect return types like string instead of address
• Missing view or external keywords

3. Given the following Solidity snippet, what will be the output of ownerOf(1) if token 1 is owned by address 0xABC...?

contract MyNFT is ERC721 {
    mapping(uint256 => address) private _owners;
    
    function ownerOf(uint256 tokenId) public view override returns (address) {
        return _owners[tokenId];
    }

    function mint(address to, uint256 tokenId) public {
        _owners[tokenId] = to;
    }
}

medium

A. 0x0000000000000000000000000000000000000000

B. Compilation error

C. 0xABC...

D. Function does not return anything

Solution

1. Step 1: Understand the mint function effect

   The mint function sets the owner of tokenId to the given address in the _owners mapping.

2. Step 2: Check ownerOf return value

   The ownerOf function returns the address stored in _owners[tokenId]. If token 1 was minted to 0xABC..., it returns that address.

3. Final Answer:

   0xABC... -> Option C

4. Quick Check:

   ownerOf(1) = minted owner address [OK]

Hint: ownerOf returns stored owner address from mapping [OK]

Common Mistakes:

• Assuming default zero address if not minted
• Thinking function has no return
• Confusing compilation errors with runtime behavior

4. Identify the error in this ERC-721 transfer function snippet:

function transferFrom(address from, address to, uint256 tokenId) public override {
    require(ownerOf(tokenId) == msg.sender, "Not owner");
    _owners[tokenId] = to;
}

medium

A. The require should check if msg.sender is approved or owner, not just owner

B. The function should not override transferFrom

C. The _owners mapping should not be updated

D. The function is missing a return statement

Solution

1. Step 1: Check transferFrom permission logic

   ERC-721 requires that the caller is either the owner or approved to transfer the token, not just the owner.

2. Step 2: Identify missing approval check

   The code only checks if msg.sender is the owner, missing approval checks, which can block valid transfers.

3. Final Answer:

   The require should check if msg.sender is approved or owner, not just owner -> Option A

4. Quick Check:

   transferFrom needs owner or approved check [OK]

Hint: Check both owner and approved in transferFrom require [OK]

Common Mistakes:

• Only checking owner without approval
• Forgetting to override transferFrom
• Not updating ownership mapping

5. You want to create an NFT contract that allows only the original minter to burn their token. Which ERC-721 function should you override and what check should you add?

hard

A. Override mint and check if msg.sender is approved

B. Override transferFrom and check if msg.sender is the owner

C. Override approve and check if msg.sender is the contract owner

D. Override _burn and check if msg.sender is the original minter

Solution

1. Step 1: Identify burn function in ERC-721

   The _burn internal function handles token destruction in ERC-721 contracts.

2. Step 2: Add ownership check for burning

   To restrict burning to original minters, override _burn and add a check that msg.sender matches the original minter address stored during minting.

3. Final Answer:

   Override _burn and check if msg.sender is the original minter -> Option D

4. Quick Check:

   Burn control = override _burn + minter check [OK]

Hint: Control burn by overriding _burn with minter check [OK]

Common Mistakes:

• Trying to control burn in transferFrom or approve
• Not storing original minter address
• Checking approval instead of minter for burn