SoulboundToken

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

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

/**
 * @title SoulboundToken
 * @dev An ERC721 Token that cannot be transferred once minted.
 * It represents an identity, certification, or reputation attached to a specific address.
 */
contract SoulboundToken is ERC721, Ownable, ERC721Burnable {
    uint256 private _nextTokenId;

    constructor(address initialOwner)
        ERC721("MySoulboundToken", "SBT")
        Ownable(initialOwner)
    {}

    /**
     * @notice Mints a new Soulbound Token to a specific address.
     * @dev Only the contract owner can mint.
     * @param to The address receiving the SBT.
     */
    function safeMint(address to) public onlyOwner {
        uint256 tokenId = _nextTokenId++;
        _safeMint(to, tokenId);
    }

    /**
     * @notice Blocks standard transfers to make the token Soulbound.
     * @dev This overrides the OpenZeppelin _update function (standard in v5.0+).
     * It allows Minting (from 0x0) and Burning (to 0x0), but reverts on any other transfer.
     */
    function _update(address to, uint256 tokenId, address auth)
        internal
        override
        returns (address)
    {
        address from = _ownerOf(tokenId);

        // If 'from' is not the zero address (minting) AND 'to' is not the zero address (burning),
        // then it is a transfer. We revert to block it.
        if (from != address(0) && to != address(0)) {
            revert("Soulbound: Transfer is not allowed");
        }

        return super._update(to, tokenId, auth);
    }
}