智能合约Solidity学习（第三课）

@@@ 合约的构造函数

跟C++,java类似，合约的构造函数名跟合约名一样，其会在合约创建时仅被调用一次

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
  address public owner;

  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  function Ownable() public {
    owner = msg.sender;
  }


  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }


  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param newOwner The address to transfer ownership to.
   */
  function transferOwnership(address newOwner) public onlyOwner {
    require(newOwner != address(0));
    OwnershipTransferred(owner, newOwner);
    owner = newOwner;
  }

}

@@@Function Modifier

关键字：modifier

被modifier 修饰的方法不能被直接调用，其通常用于方法头的后面来改变方法的行为。

contract MyContract is Ownable {
  event LaughManiacally(string laughter);

  // Note the usage of `onlyOwner` below:
  function likeABoss() external onlyOwner {
    LaughManiacally("Muahahahaha");
  }
}

@@@代码执行开销

在以太坊上执行任何一个写操作都需要耗费gas, 且代码的复杂度越高，需要的gas就越多，因此开发Dapp要格外注意程序的

高效。

通常uint8 代表着经 unit32更低的存储空间占用，但是通常情况下，以太坊是不区分这两者的，都会统一当成是uint；但是有个例外，那就是在结构体struct里，数据是严格按照变量类型来存放的，而且把相同类型的变量放在相临会消耗更少的gas

If you have multiple uints inside a struct, using a smaller-sized uint when possible will allow Solidity to pack these variables together to take up less storage. For example:

struct NormalStruct {
  uint a;
  uint b;
  uint c;
}

struct MiniMe {
  uint32 a;
  uint32 b;
  uint c;
}

// `mini` will cost less gas than `normal` because of struct packing
NormalStruct normal = NormalStruct(10, 20, 30);
MiniMe mini = MiniMe(10, 20, 30);

@@@ 时间单位

在solidity中，有以下时间单位可用：

变量now，代表unix时间戳（秒数）；

变量seconds

变量minutes

变量hours

变量days

变量weeks

变量years

以上六个变量的值会以秒数来表示，例如

1 minutes 的值为60 （seconds)

uint lastUpdated;

// Set `lastUpdated` to `now`
function updateTimestamp() public {
  lastUpdated = now;
}

// Will return `true` if 5 minutes have passed since `updateTimestamp` was 
// called, `false` if 5 minutes have not passed
function fiveMinutesHavePassed() public view returns (bool) {
  return (now >= (lastUpdated + 5 minutes));
}

智能合约Solidity学习（第三课）

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