Token.sol

// SPDX-License-Identifier: MIT
pragma solidity >= 0.8.0;


contract TokenStandardInterface {
    // Events
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    // Data
    string public _name;    // "SuchanToken"
    string private _symbol;  // "SCT"
    uint8 private _decimals;  // 18
    uint private _totalSupply;  // 발행량: 42,000

    mapping(address => uint) private _balances;
    mapping(address => mapping(address => uint)) private _allowances;

    function name() public view returns (string memory) {
        return _name;
    }

    function symbol() public view returns (string memory) {
        return _symbol;
    }

    function decimals() public view returns (uint8) {
        return _decimals; 
    }

    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }
    
    function balanceOf(address _owner) public view returns (uint256 balance) {
        return _balances[_owner];
    }

    function transfer(address _to, uint256 _value) public returns (bool success) {        
        _transfer(msg.sender, _to, _value);
        return true;
    }
    
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_allowances[_from][msg.sender] >= _value);
        _allowances[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    function _transfer(address _from, address _to, uint _value) public returns (bool success) {
        require(_from != address(0));
        require(_to != address(0));
        require(_balances[_from] >= _value); 
        _balances[_from] -= _value;
        _balances[_to] += _value;
        emit Transfer(_from, _to, _value);
        return true;
    }

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

    function allowance(address _owner, address _spender) public view returns (uint256 remaining) {
        return _allowances[_owner][_spender];
    }
}