Function to Retrieve Tokens from Contract fails

Question

In case people send ERC20 tokens to my contract, I want to be able to retrieve them. Hence the following function:

/**
   * @dev Allows owner to transfer ERC20 tokens to vault
   * @param _token the contract address of the ERC20 contract
   */
  function retrieveTokens(address _token) public onlyOwner {
    ERC20 erctoken = ERC20(_token);
    erctoken.transfer(Vault, erctoken.balanceOf(this));
  }

On Ropsten testnet, I successfully sent some tokens to the contract. Then I run the above, but it fails. (It used very little gas and did not run out. I entered a gas limit of 6000000, which was much more than needed.)

In remix.ethereum.org (with Injected Web3), I get the following error:

transact to browser/Test.sol:TestSale.retrieveTokens errored: Gas required exceeds limit: 4000000. An important gas estimation might also be the sign of a problem in the contract code. Please check loops and be sure you did not sent value to a non payable function (that's also the reason of strong gas estimation).

In remix.ethereum.org (with Javascript VM), I'm getting the following error:

from:0xca3...a733c, to:broswer/Test.sol:TestSale.retrieveTokens(address) 0x643...62e55, value:0 wei, data:0xac4...95963, 0 logs, hash:0xe76...c0094

transact to browser/Test.sol:TestSale.retrieveTokens errored: VM error: invalid opcode. The constructor should be payable if you send value. The execution might have thrown. Debug the transaction to get more information.

How do I fix this? I'm not sending any value when I call this transaction.

I copied the code from TenX and modified it for my Solidity smart contracts. You can see all of my code at https://gist.github.com/anonymous/3497e6b4e09953a009a4fdbacd11946a

Answer 1

Being payable is the important part to allow you to transfer, the owner check is secondary. Use:

function retrieveTokens(address _token) public payable {
   require(msg.sender == owner);
   ERC20 erctoken = ERC20(_token);
   erctoken.transfer(Vault, erctoken.balanceOf(this));
}

That should take care of it.

Answer 2

As suspected, two issues jump out from the code posted in the gist in the comments.

First, line 1 pragma solidity ^0.4.11. This is not the case. In line 100, you use the modifier pure that was introduced with solidity 0.4.17. Therefore, it doesn't compile with a lower version and 0.4.11 misleads about the required dialect.

Second, line 297 has directional quotes (leaning left and right) and the compiler doesn't want that. It's a syntax error. Use " on both sides.

The error message is misleading in that it's suggesting a common issue with failure to deploy (make it payable) but in this case the failure has to do with failing to compile it at all.

Since compiler version can be a source of issues, in practice, it's handy to specify the exact version (without ^) and make sure it's consistently used. For example, if switching between Remix, Truffle, and solc, you'll get head-scratchers unless everything is on the same version level. You might find, for example, that truffle is forcing you to 0.4.15, so you would want to set your code to check for 0.4.15 and set Remix to load that version. Install solc at that level if using it.

Hope it helps.