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Make approve method compliant with ERC20 #446

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Make approve method compliant with ERC20 #446

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d2f8761
Make approve method compliant with ERC20
jakub-wojciechowski Sep 14, 2017
79c3626
Warning in approve method documentation.
jakub-wojciechowski Sep 15, 2017
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2 changes: 1 addition & 1 deletion contracts/token/ERC20.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,6 @@ import './ERC20Basic.sol';
contract ERC20 is ERC20Basic {
function allowance(address owner, address spender) constant returns (uint256);
function transferFrom(address from, address to, uint256 value) returns (bool);
function approve(address spender, uint256 value) returns (bool);
function approve(address spender, uint256 value) returns (bool success);
event Approval(address indexed owner, address indexed spender, uint256 value);
}
14 changes: 6 additions & 8 deletions contracts/token/StandardToken.sol
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Expand Up @@ -40,17 +40,15 @@ contract StandardToken is ERC20, BasicToken {

/**
* @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
*
* Beware that changing an allowance with this method brings the risk that someone may use both the old
* and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
* race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
* https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
* @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) {

// 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));

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