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I am trying to understand how to use the ERC721 token.I want to change the function mint with parameter of type uint256 to parameter of type string and create a unique string, it's possible to replace uint256 tokenid to string ?.

Exemple:

UniqueString("test"); //good

And the next time will call the function with the same string("test") we will got error .

function UniqueString(string tokenid) public{

  _mint(msg.sender,tokenid);    
}

My question is how to create a unique string with the erc721 token. Thanks you in advance

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As per the documentation for the ERC721 standard, if you want your token to be ERC721 compliant, then the tokenId must be a uint256.

Every NFT is identified by a unique uint256 ID inside the ERC-721 smart contract. This identifying number SHALL NOT change for the life of the contract. The pair (contract address, uint256 tokenId) will then be a globally unique and fully-qualified identifier for a specific asset on an Ethereum chain. While some ERC-721 smart contracts may find it convenient to start with ID 0 and simply increment by one for each new NFT, callers SHALL NOT assume that ID numbers have any specific pattern to them, and MUST treat the ID as a "black box".

However, if you want a tokenId based on a string, you could make the tokenId the hash of the string identifier. As noted in the ERC721 documentation:

The choice of uint256 allows a wide variety of applications because UUIDs and sha3 hashes are directly convertible to uint256.

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