ERC-721 implementation in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the contract inherits from the OpenZeppelin ERC721 standard.

Blockchain / Solidity

contract MyNFT is [1] {
    constructor() ERC721("MyNFT", "MNFT") {}
}

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AERC721

BERC20

COwnable

DIERC20

Attempts:

3 left

💡 Hint

Common Mistakes

Using ERC20 instead of ERC721

Not inheriting any contract

The contract must inherit from ERC721 to implement the NFT standard.

2fill in blank

medium

Complete the function to mint a new token to the given address.

Blockchain / Solidity

function mintNFT(address to, uint256 tokenId) public {
    _[1](to, tokenId);
}

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Amint

BsafeMint

CtransferFrom

DsafeTransferFrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using mint instead of _safeMint

Using transfer functions to mint

_safeMint safely mints a new token and checks if the receiver can handle NFTs.

3fill in blank

hard

Fix the error in the function that returns the token URI.

Blockchain / Solidity

function tokenURI(uint256 tokenId) public view override returns (string memory) {
    require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");
    return string(abi.encodePacked(_baseURI(), [1]));
}

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AStrings.toString(tokenId)

BtoString(tokenId)

CtokenId

Duint2str(tokenId)

Attempts:

3 left

💡 Hint

Common Mistakes

Using tokenId directly (uint) in string concatenation

Using undefined conversion functions

The Strings.toString function converts the tokenId to a string for URI concatenation.

4fill in blank

hard

Fill both blanks to define a mapping and a function to check token ownership.

Blockchain / Solidity

mapping(uint256 => address) private [1];

function ownerOf(uint256 tokenId) public view override returns (address) {
    address owner = [2][tokenId];
    require(owner != address(0), "ERC721: owner query for nonexistent token");
    return owner;
}

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A_owners

Bowners

DtokenOwners

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for mapping and access

Using public instead of private mapping

The standard ERC721 implementation uses a private mapping named _owners to track token owners.

5fill in blank

hard

Fill all three blanks to implement a function that safely transfers a token.

Blockchain / Solidity

function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public override {
    require(_isApprovedOrOwner(msg.sender, tokenId), "ERC721: caller is not token owner nor approved");
    [1](from, to, tokenId, data);
}

function [2](address from, address to, uint256 tokenId, bytes memory data) internal {
    _transfer(from, to, tokenId);
    require(_checkOnERC721Received(from, to, tokenId, data), "ERC721: transfer to non ERC721Receiver implementer");
}

function [3](address from, address to, uint256 tokenId) internal {
    // actual transfer logic here
}

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A_safeTransfer

C_transfer

D_transferFrom

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up function names

Calling transfer instead of safeTransfer

The public function calls _safeTransfer, which calls _transfer to move the token.