ethereum - 声明错误 : Undeclared identifier - although it's present in ERC721. sol

Question

我正在写一份关于 Solidity 0.8.3 的契约(Contract)，我收到了 _setTokenURI() 这个奇怪的错误虽然方法是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

功能 _setTokenURI()在 @openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol 中定义( source on GitHub )，但此合约不是由您的代码导入的(包括嵌套导入)。这就是未声明函数的原因。
自 ERC721URIStorage扩展 ERC721 ，您可以扩展您的 NFTB直接来自 ERC721URIStorage .

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

contract NFTB is ERC721URIStorage { // changed parent