How to calculate token ID for my own NFT marketplace

Question

I am just creating my own NFT marketplace. I need to create unique token ID for each NFT but the current demo for token ID is like below.

contract ERC721 {
  ...
  mapping(uint256 => address) private _tokenOwners;

  function _mint(address to, uint256 tokenID) {
    _tokenOwner[tokenID] = to;
    ...
  }
}

contract CryptoDog is ERC721 {
    ...
  string[] public cryptoDogz;  

  function mint(string memory nftName) public {

    cryptoDogz.push(ntfName);
    uint256 _tokenID = cryptoDogz.length - 1;
    _mint(msg.sender, _tokenID);
    ...
    
  }
}

I will appreciate it if you provide me with a way to generate uniquely token ID.

Answer 1

You can use a counter to start from ID 1, and increment it when you mint new NFTs. You will get a different and incremental number for each NFT. You can use this code from OpenZeppelin:

// contracts/GameItem.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/utils/Counters.sol";

contract GameItem is ERC721URIStorage {
    using Counters for Counters.Counter;
    Counters.Counter private _tokenIds;

    constructor() ERC721("GameItem", "ITM") {}

    function awardItem(address player, string memory tokenURI)
        public
        returns (uint256)
    {
        uint256 newItemId = _tokenIds.current();
        _mint(player, newItemId);
        _setTokenURI(newItemId, tokenURI);

        _tokenIds.increment();
        return newItemId;
    }
}

The example is using the Counters library from OpenZeppelin.

You can check how it is implemented here.