在这段代码中如何使用SafeMath of OpenZeppelin？

Question

以下是简单的代码：

pragma solidity ^0.4.4;

contract Token {

    function totalSupply() constant returns (uint256 supply) {}
    function balanceOf(address _owner) constant returns (uint256 balance) {}
    function transfer(address _to, uint256 _value) returns (bool success) {}
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {}
    function approve(address _spender, uint256 _value) returns (bool success) {}
    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {}
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

}

contract StandardToken is Token {

    function transfer(address _to, uint256 _value) returns (bool success) {

        if (balances[msg.sender] >= _value && _value > 0) {
            balances[msg.sender] -= _value;
            balances[_to] += _value;
            Transfer(msg.sender, _to, _value);
            return true;
        } else { return false; }
    }

    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {

        if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && _value > 0) {
            balances[_to] += _value;
            balances[_from] -= _value;
            allowed[_from][msg.sender] -= _value;
            Transfer(_from, _to, _value);
            return true;
        } else { return false; }
    }

    function balanceOf(address _owner) constant returns (uint256 balance) {
        return balances[_owner];
    }

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

    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
      return allowed[_owner][_spender];
    }

    mapping (address => uint256) balances;
    mapping (address => mapping (address => uint256)) allowed;
    uint256 public totalSupply;
}

contract TestCoin is StandardToken { // CHANGE THIS. Update the contract name.


    string public name;                   // Token Name
    uint8 public decimals;                // How many decimals to show. To be standard complicant keep it 18
    string public symbol;                 // An identifier: eg SBX, XPR etc..
    string public version = 'H1.0'; 
    uint256 public unitsOneEthCanBuy;     // How many units of your coin can be bought by 1 ETH?
    uint256 public totalEthInWei;         // WEI is the smallest unit of ETH (the equivalent of cent in USD or satoshi in BTC). We'll store the total ETH raised via our ICO here.  
    address public fundsWallet;           // Where should the raised ETH go?


    function TestCoin() {
        balances[msg.sender] = 1000000000000000000000;               // Give the creator all initial tokens. This is set to 1000 for example. If you want your initial tokens to be X and your decimal is 5, set this value to X * 100000. (CHANGE THIS)
        totalSupply = 1000000000000000000000;                        // Update total supply (1000 for example) (CHANGE THIS)
        name = "TestCoin";                                   // Set the name for display purposes (CHANGE THIS)
        decimals = 18;                                               // Amount of decimals for display purposes (CHANGE THIS)
        symbol = "TCN";                                             // Set the symbol for display purposes (CHANGE THIS)
        unitsOneEthCanBuy = 10;                                      // Set the price of your token for the ICO (CHANGE THIS)
        fundsWallet = msg.sender;                                    // The owner of the contract gets ETH
    }

    function() payable{
        totalEthInWei = totalEthInWei + msg.value;
        uint256 amount = msg.value * unitsOneEthCanBuy;
        if (balances[fundsWallet] < amount) {
            return;
        }

        balances[fundsWallet] = balances[fundsWallet] - amount;
        balances[msg.sender] = balances[msg.sender] + amount;

        Transfer(fundsWallet, msg.sender, amount); // Broadcast a message to the blockchain

        //Transfer ether to fundsWallet
        fundsWallet.transfer(msg.value);                               
    }

    /* Approves and then calls the receiving contract */
    function approveAndCall(address _spender, uint256 _value, bytes _extraData) returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);

        if(!_spender.call(bytes4(bytes32(sha3("receiveApproval(address,uint256,address,bytes)"))), msg.sender, _value, this, _extraData)) { throw; }
        return true;
    }
}

你对每一个数学函数都使用SafeMath库吗？

例如，balances[msg.sender] -= _value;将是balances[msg.sender]=sub(balances[msg.sender], _value)？

Answer 1

是的，因为safeMath将使您的代码使用它们的断言更安全，

sub = Substracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).

 function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }

balances[msg.sender] = balances[msg.sender].sub(_value);

也有

mul = Multiplies two numbers, throws on overflow.
div = Integer division of two numbers, truncating the quotient.
add =  Adds two numbers, throws on overflow.

所以最好像OpenZeppelin那样在任何地方使用它。