How to set a max supply of an ERC-721 token?

Question

I am using the OpenZeppelin framework and I've tried to find solutions on how to put a max supply of an ERC-721. The only answers I've found seems unnecessary complex and I haven't been able to make it work. It seems like there should be a simple solution to what seems like a simple problem. This is my current code:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.2;

import "@openzeppelin/[email protected]/token/ERC721/ERC721.sol";
import "@openzeppelin/[email protected]/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/[email protected]/token/ERC721/extensions/ERC721Burnable.sol";
import "@openzeppelin/[email protected]/access/Ownable.sol";

contract MyContract is ERC721, ERC721URIStorage, ERC721Burnable, Ownable {
    constructor() ERC721("MyContract", "MCT") {}

    function safeMint(address to, uint256 tokenId, string memory uri)
        public
        onlyOwner
    {
        _safeMint(to, tokenId);
        _setTokenURI(tokenId, uri);
    }

    // The following functions are overrides required by Solidity.

    function _burn(uint256 tokenId) internal override(ERC721, ERC721URIStorage) {
        super._burn(tokenId);
    }

    function tokenURI(uint256 tokenId)
        public
        view
        override(ERC721, ERC721URIStorage)
        returns (string memory)
    {
        return super.tokenURI(tokenId);
    }
}

It's a simple copy paste from the wizard on OpenZeppelin. The owner of the contract will be able to mint and burn tokens after deployment. But I still need to put a max supply to ensure scarcity of the NFTs.

Edit: It seems like I found a solution. It was pretty simple, just make the contract ERC721Enumerable and add require (totalSupply() < maxSupply); to the mint function. Maybe there's some problem with this but so far it seems to work when I test it:

    // SPDX-License-Identifier: MIT
pragma solidity ^0.8.2;

import "@openzeppelin/[email protected]/token/ERC721/ERC721.sol";
import "@openzeppelin/[email protected]/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/[email protected]/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/[email protected]/token/ERC721/extensions/ERC721Burnable.sol";
import "@openzeppelin/[email protected]/access/Ownable.sol";

contract MyToken is ERC721, ERC721Enumerable, ERC721URIStorage, ERC721Burnable, Ownable {
    constructor() ERC721("MyToken", "MTK") {}
    uint256 public constant maxSupply = 2;

    function safeMint(address to, uint256 tokenId, string memory uri)
        public
        onlyOwner
    {
        require (totalSupply() < maxSupply);
        _safeMint(to, tokenId);
        _setTokenURI(tokenId, uri);
    }

    // The following functions are overrides required by Solidity.

    function _beforeTokenTransfer(address from, address to, uint256 tokenId)
        internal
        override(ERC721, ERC721Enumerable)
    {
        super._beforeTokenTransfer(from, to, tokenId);
    }

    function _burn(uint256 tokenId) internal override(ERC721, ERC721URIStorage) {
        super._burn(tokenId);
    }

    function tokenURI(uint256 tokenId)
        public
        view
        override(ERC721, ERC721URIStorage)
        returns (string memory)
    {
        return super.tokenURI(tokenId);
    }

    function supportsInterface(bytes4 interfaceId)
        public
        view
        override(ERC721, ERC721Enumerable)
        returns (bool)
    {
        return super.supportsInterface(interfaceId);
    }
}

Answer 1

In my contracts, I use a require statement to check if the new mint count + totalSupply() does not exceed the MAX_SUPPLY that I arbitrarily set.