Understanding ERC721 _setTokenURI after minting

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1

Hello!
I am an engineering student and recently I started with the wonderful world of SC. I am currently trying to learn the implementation of ERC721 and how it works and there is a thing I do not understand and I would appreciate help.
(I am taking as an example a code of an nft project from etherscan).
This project had a mint process and a reveal done a week later. I understand how the minting is done (with the function _safeMint and the modifications one may add) but I do not understand how the token URI is set or how can it be accessed and changed in the future for the reveal as I do not see any DataStructre which stores the uri data or updates it in the contract.

Full code: https://etherscan.io/address/0xd8537b0ea5b21ca39af493525a3c54b6fa8f32b3#code

Functions referencing URI:

    function _baseURI() internal view virtual override returns (string memory) {
        return baseTokenURI;
    }

    function setBaseURI(string memory baseURI) public onlyOwner {
        baseTokenURI = baseURI;
        emit BaseURIChanged(baseURI);
    }

Mint function:

    function mint(uint256 amountOfNfts) external payable whenPublicSaleStarted {
        updatePrice();

        //Requirements here....

        for (uint256 i = 0; i < amountOfNfts; i++) {
            uint256 tokenId = numNftsMinted + 1;

            numNftsMinted += 1;
            _totalClaimed[msg.sender] += 1;
            _safeMint(msg.sender, tokenId);
        }
     //  emits here
}

And as you can see there is a _safeMint but not a _tokenURI function and I would like to unerstand how this is possible.
Thank you very much!

2

The contract inherits ERC721Enumerable which itself inherits ERC721. If you look at that contract, you will find:

    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

        string memory baseURI = _baseURI();
        return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString())) : "";
    }

Here the token id is concatenated with the base URI. Thus the token URI doesn't need to be stored in a data structure, as it's generated dynamically.

3

Thank you!
Just one more question regarding the answer:
The function tokenURI is not called in the contract (at least inside the contract), is it called outside (f.e. in brownie) to update each token metadata?
Thank you!

4

Yes, it's called from outside the blockchain. Not exactly to "update" token metadata, just to fetch/query the metadata when it needs to be shown. Generally though services (e.g. OpenSea) will query it once and keep it in their cache.

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