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ERC721は非代替トークンのNFT規格の一つ

pragma solidity ^0.8.20;

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

contract MyToken is ERC721, Ownable {
    constructor(address initialOwner)
        ERC721("HFToken", "HogeFuga") // MyToken..トークン名 MTK..トークン名の略称
        Ownable(initialOwner)
    {}

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

# Solidityのオブジェクト
## msg オブジェクト
– msg.sender: address
– msg.value: eth value
– msg.gas
– msg.data

## txオブジェクト
– tx.gasprice
– tx.origin: transaction address

## blockオブジェクト
– block.coinbase
– block.difficulty
– block.gaslimit
– block.number
– block.timestamp

## addressオブジェクト
– address.balance
– address.transfer
– address.send

## 関数
addmod, mulmod, keccak256, sha3, sha256, erecover, selfdestruct, this

## その他
– external: コントラクトの外部
– internal: コントラクトの内部
– view: データの閲覧のみ
– pure: 読み取りも行わない
– event: ブロックチェーン上で何かが生じた時、web上やアプリに伝えることができる仕組み

## ECR20

pragma solidity ^0.8.17;

interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes calldata _extraData) external; }

contract TokenERC20 {
    string public name;
    string public symbol;
    uint8 public decimals = 18;
    uint256 public totalSupply;

    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed _owner, address indexed _sender, uint256 _value);
    event Burn(address indexed from, uint256 value);

    constructor(
        uint256 initialSupply,
        string memory tokenName,
        string memory tokenSymbol
    ) public {
        totalSupply = initialSupply * 10 ** uint256(decimals);
        balanceOf[msg.sender] = totalSupply;
        name = tokenName;
        symbol = tokenSymbol;
    }

    function _transfer(address _from, address _to, uint _value) internal {
        require(_to != address(0));
        require(balanceOf[_from] = _value);
        require(balanceOf[_to] + _value = balanceOf[_to]);
        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        balanceOf[_from] -= _value;
        balanceOf[_to] += _value;
        emit Transfer(_from, _to, _value);
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }

    function transfer(address _to, uint256 _value) public returns (bool success) {
        _transfer(msg.sender, _to, _value);
        return true;
    }

    function approve(address _spender, uint256 _value) public
        returns (bool success) {
            allowance[msg.sender][_spender] = _value;
            emit Approval(msg.sender, _spender, _value);
            return true;
        }

    function approveAndCall(address _spender, uint256 _value, bytes memory _extraData)
        public 
        returns (bool success) {
            tokenRecipient spender = tokenRecipient(_spender);
            if (approve(_spender, _value)) {
                spender.receiveApproval(msg.sender, _value, address(this), _extraData);
                return true;
            }
        }

    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] = _value);
        balanceOf[msg.sender] -= _value;
        totalSupply -= _value;
        emit Burn(msg.sender, _value);
        return true;
    }

    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(balanceOf[_from] = _value);
        require(_value = allowance[_from][msg.sender]);
        balanceOf[_from] -= _value;
        allowance[_from][msg.sender] -= _value;
        totalSupply -= _value;
        emit Burn(_from, _value);
        return true;
    }
}

from solidity:
TypeError: No matching declaration found after argument-dependent lookup.
–> ecr20.sol:31:8:
|

ん？　何故だ…