How to have the contract check if the msg.sender currently owns a specific token. (erc721)

Question

Assuming I have a contract to mint my own NFTs: (I'm thinking ERC-721, but not necessarily).

I want to create another smart contract that before doing anything, verifies that the address interacting with it (msg.sender) currently owns an NFT minted by that specific contract. Even better, if it could read an attribute, like the NFT id.

Is that even possible? I did some research on it, but apparently the address doesn't actually stores the balances/tokens, only eth. But I'm guessing it must be a way, given how those NFTs gallery works. If it's possible, how could I do it?

Answer 1

If contracts is an ERC721 then you can query how many tokens a user owns with balanceOf

uint count = token.balanceOf(user);
if (count > 0) {
    // User owns at least 1 index
}

If the tokens implements the optional ERC721Enumerable interface you can use tokenOfOwnerByIndex to retrieve the tokenId of owned tokens by a user.

try token.tokenOfOwnerByIndex(user, 0) returns (uint tokenId) {
    // First token owned by user
} catch (bytes memory) {
    // No tokens owned by user
}

For other details see the EIP 721, or EIP 1155.