added comment and approval fixes

Author: vinay035

contracts/SRTToken.sol

@@ -34,6 +34,11 @@ contract StandardToken is ERC20 {
     mapping (address => uint256) public balances;
     mapping (address => mapping (address => uint256)) public allowed;
 
+    /**
+    * @dev transfer token for a specified address
+    * @param _to The address to transfer to.
+    * @param _value The amount to be transferred.
+    */
     function transfer(address _to, uint256 _value) returns (bool success) {
         require(balances[msg.sender] >= _value && balances[_to] + _value > balances[_to]);
         balances[msg.sender] = balances[msg.sender].sub(_value);
@@ -42,6 +47,12 @@ contract StandardToken is ERC20 {
         return true;
     }
 
+    /**
+    * @dev Transfer tokens from one address to another
+    * @param _from address The address which you want to send tokens from
+    * @param _to address The address which you want to transfer to
+    * @param _value uint256 the amout of tokens to be transfered
+    */
     function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
         require(balances[_from] >= _value && allowed[_from][msg.sender] >= _value && balances[_to] + _value > balances[_to]);
         balances[_to] = balances[_to].add(_value);
@@ -51,16 +62,37 @@ contract StandardToken is ERC20 {
         return true;
     }
 
+    /**
+    * @dev Gets the balance of the specified address.
+    * @param _owner The address to query the the balance of.
+    * @return An uint256 representing the amount owned by the passed address.
+    */
     function balanceOf(address _owner) constant returns (uint256 balance) {
         return balances[_owner];
     }
 
+    /**
+    * @dev Aprove the passed address to spend the specified amount of tokens on behalf of msg.sender.
+    * @param _spender The address which will spend the funds.
+    * @param _value The amount of tokens to be spent.
+    */
     function approve(address _spender, uint256 _value) returns (bool success) {
+        // To change the approve amount you first have to reduce the addresses`
+        //  allowance to zero by calling `approve(_spender, 0)` if it is not
+        //  already 0 to mitigate the race condition described here:
+        //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
+        require((_value == 0) || (allowed[msg.sender][_spender] == 0));
         allowed[msg.sender][_spender] = _value;
         Approval(msg.sender, _spender, _value);
         return true;
     }
 
+    /**
+    * @dev Function to check the amount of tokens that an owner allowed to a spender.
+    * @param _owner address The address which owns the funds.
+    * @param _spender address The address which will spend the funds.
+    * @return A uint256 specifing the amount of tokens still available for the spender.
+    */
     function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
       return allowed[_owner][_spender];
     }