区块链学堂（28）：用Mapping来实现一个简单的代币合约

在上一节里面我们通过

mapping
(bytes32 => uint) public balances;

 实现了一个简单的mapping的hashtable. 但这样的hashTable无法部署在geth上面，也无法完成互相之间的代币转化。

这一节中我们将从0开始，撰写一个简单的合约

Step 1: 创建一个基础合约

pragma solidity 0.4.7;

contract Coin {
address public minter;
mapping (address => uint) public balances;
}

这里我们定义了一个address 作为key, uint做为value的hashTable balances; 我们还定义了一个address的变量minter;

Step 2: 添加一个构造函数Coin()

function Coin() {
minter = msg.sender;
}

这里的代码minter = msg.sender; 代表创建这个合约的账户地址，被赋值给变量minter.

Step 3: 添加一个挖矿合约

function mint(address receiver, uint amount) {
if (msg.sender != minter) throw;
balances[receiver] += amount;
}

这里的核心代码在于，如果调用这个方法的账户，不是minter, 也就是创建合约的账户的话，这个

mint()

将无法被执行。
只有是创建合约的账户，也就是

minter

 才可以执行它

Step 4: 添加一个function send() 也就是从A转移X代币到B账户。代码如下：

function send(address receiver, uint amount) {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
balances[receiver] += amount;
}

这个非常简单的转移货币的代码，也就是说msg.sender减少一定代币，接受者receiver增加一定代币

Step 5: 定义一个事件 Sent()

event Sent(address from, address to, uint amount);

function send(address receiver, uint amount) {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
balances[receiver] += amount;
Sent(msg.sender, receiver, amount);
}

完整代码如下：

pragma solidity 0.4.7;

contract Coin {
address public minter;
mapping (address => uint) public balances;

event Sent(address from, address to, uint amount);

function Coin() {
minter = msg.sender;
}

function mint(address receiver, uint amount) {
if (msg.sender != minter) throw;
balances[receiver] += amount;
}

function send(address receiver, uint amount) {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
balances[receiver] += amount;
Sent(msg.sender, receiver, amount);
}
}

原文：http://www.ethchinese.com/?p=1120

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