How to write a burn function in smart contract for NFT?

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I am learning how to write a smart contract for NFT collections and the following is the example function given by the tutorial I read:

    function _burn(uint256 tokenId) internal virtual override {
        super._burn(tokenId);
        if (bytes(_tokenURIs[tokenId]).length != 0) {
            delete _tokenURIs[tokenId];
        }
    }

I recognise that this function will not remove the token from the blockchain entirely. Instead, it will remove the URI of the token (regardless of who owns it). As a result, the token will still in the collection and be displayed on trading platforms, but the metadata will be gone (but it may take time to be in effect as platforms are not refreshing metadata frequently).

I wonder if this is the right practice for burn function. It would be greatly helpful for me if someone can provide me an example of how burn function is achieved on other NFT smart contracts.

nft smartcontracts solidity
1 Answers

Here is the easiest way to add burn function to an NFT.

  • GO to Openzepplin Wizard
  • Click ERC721
  • Give your token a name and symbol.
  • Click on mintable and burnable, and you would get mintable and burnable NFT token contract.

Here is a sample:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Burnable.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract MyToken is ERC721, ERC721Burnable, Ownable {
    constructor() ERC721("MyToken", "MTK") {}

    function safeMint(address to, uint256 tokenId) public onlyOwner {
        _safeMint(to, tokenId);
    }
}

The corresponding OZ wizard interface would look like this:

enter image description here

You would get the following public burn function:

enter image description here

From Openzepplin ERC721 burnable contract

Update

You can make the contract both enumarable and burnable: enter image description here

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Burnable.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract MyToken is ERC721, ERC721Enumerable, ERC721Burnable, Ownable {
    constructor() ERC721("MyToken", "MTK") {}

    function safeMint(address to, uint256 tokenId) public onlyOwner {
        _safeMint(to, tokenId);
    }

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