DeclarationError: Undeclared identifier - 尽管它存在于 ERC721.sol 中

Question

我正在编写一份关于solidity 0.8.3 的合同，我收到_setTokenURI() 的这个奇怪错误，尽管方法是defined in OpenZeppelin 4.X。

pragma solidity ^0.8.3;

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

contract NFTB is ERC721 {

  using Counters for Counters.Counter;
  Counters.Counter private _tokenIds;
  mapping(string => uint8) hashes;

  constructor() public ERC721("NFTB", "NFTB") {}

  function awardItem(address recipient, string memory hash, string memory metadata) public returns (uint256) {
    require(hashes[hash] != 1);
    hashes[hash] = 1;
    _tokenIds.increment();
    uint256 newItemId = _tokenIds.current();
    _setTokenURI(newItemId, metadata);
    _mint(recipient, newItemId);
    return newItemId;
  } }

Answer 1

函数_setTokenURI() 在@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol (source on GitHub) 中定义，但您的代码不会导入此协定（包括嵌套导入）。这就是函数未声明的原因。

由于ERC721URIStorage 扩展了ERC721，您可以直接从ERC721URIStorage 扩展您的NFTB。

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

contract NFTB is ERC721URIStorage { // changed parent