ERC20

ERC20Interface.sol

pragma solidity ^0.4.24;

contract ERC20Interface {
    function totalSupply() public constant returns (uint);
    function balanceOf(address tokenOwner) public constant returns (uint balance);
    function transfer(address to, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);
    function approve(address spender, uint tokens) public returns (bool success);
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining);

    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

MyToken.sol

pragma solidity ^0.4.24;

contract MyToken {
    // トークン名
    string public name;
    // トークンシンボル
    string public symbol;
    //public 修飾子を指定すると getter が生成される
    // => function totalSupply() returns (uint)
    uint public totalSupply;

    constructor(string _name, string _symbol, uint tokens) public {
        name = _name;
        symbol = _symbol;
        
        balanceOf[msg.sender] += tokens;
        totalSupply += tokens;
    }

    // ハッシュテーブル
    // => function balanceOf(address owner) returns (uint)
    mapping(address => uint) public balanceOf;
    
    mapping(address => mapping(address =>uint)) public allowance;
    
    function transfer(address to, uint tokens) public returns (bool success) {
        // msg.sender: 関数の呼び出し元のアドレス
        return transferFrom(msg.sender, to, tokens);
    }
    
    function transferFrom(address from, address to, uint tokens) public returns (bool success) {
        // 送信元が必要な量のトークンを所持しているか確認する
        require(balanceOf[msg.sender] >= tokens);
        
        // 呼び出し元と送信元が同じでない時、承認されているか確認する
        if (msg.sender != from) {
            require(allowance[from][msg.sender] >= tokens);
            allowance[from][msg.sender] -= tokens;
        }
        
        // トークンを移動する
        balanceOf[from] -= tokens;
        balanceOf[to] += tokens;
        
        // Transfer イベントを発火する
        emit Transfer(from, to, tokens);
        
        return true;
    }
    
    function approve(address spender, uint tokens) public returns (bool success) {
        allowance[msg.sender][spender] += tokens;
        
        emit Approval(msg.sender, spender, tokens);
        return true;
    }

    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);

}

https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md