Address(this) and _approve

Support Contracts
1

I am facing a bug with openzepplin ERC20 contracts. In my code, I implement a burn tax and implement it this way:

--- SNIP--- 
function _afterTokenTransfer(
        ReflectionValues memory reflectionValues,
        TokenValues memory tokenValues
    ) private {
        if (_burnTax != 0) {
            _tokenBalances[address(this)] += tokenValues.burnFee;
            _reflectionBalances[address(this)] += reflectionValues.burnFee;
            // This _approve is required for some reasons...
            _approve(address(this), _msgSender(), tokenValues.burnFee);
            burnFrom(address(this), tokenValues.burnFee);
        }
--- SNIP--- 

function burnFrom(address account, uint256 amount) public virtual {
        uint256 currentAllowance = allowance(account, _msgSender());
        require(currentAllowance >= amount, "ERC20: burn amount exceeds allowance");
        _approve(account, _msgSender(), currentAllowance - amount);
        _burn(account, amount);
    }
}
--- SNIP ---

But this code snippets triggers this require exception:

function _approve(address owner, address spender, uint256 amount) internal virtual {
    require(owner != address(0), "ERC20: approve from the zero address");
    require(spender != address(0), "ERC20: approve to the zero address");

    _allowances[owner][spender] = amount;
    emit Approval(owner, spender, amount);
}

As far as I know, address(this) is different from address(0)… Spent two days on that already and can’t figure out what is going on… See:

  Contract: Transfers
senderBalance 1000000000000000000000000000000000
recipientBalance 0
    1) should charge taxes on transfers from non-excluded accounts
AssertionError: expected promise to be fulfilled but it was rejected with 'Error: Returned error: VM Exception while processing transaction: revert ERC20: approve to the zero 
address caca -- Reason given: ERC20: approve to the zero address caca.'
    2) "after each" hook: after test for "should charge taxes on transfers from non-excluded accounts"


  0 passing (4s)
  2 failing

  1) Contract: Transfers
       should charge taxes on transfers from non-excluded accounts:

      AssertionError: expected '0' to equal '1000000000000000000000000000000000'
      + expected - actual

      -0
      +1000000000000000000000000000000000

      at Context.<anonymous> (test\ERC20Deflationary.test.js:152:46)
      at processTicksAndRejections (internal/process/task_queues.js:93:5)

  2) Contract: Transfers
       "after each" hook: after test for "should charge taxes on transfers from non-excluded accounts":
     Uncaught RuntimeError: abort(AssertionError: expected promise to be fulfilled but it was rejected with 'Error: Returned error: VM Exception while processing transaction: 
revert ERC20: approve to the zero address -- Reason given: ERC20: approve to the zero address caca.'). Build with -s ASSERTIONS=1 for more info.
      at process.abort (C:\Users\ateyar\.config\truffle\compilers\node_modules\soljson-v0.8.4+commit.c7e474f2.js:1:13012)
      at process.emit (C:\Users\ateyar\AppData\Roaming\npm\node_modules\truffle\build\webpack:\node_modules\source-map-support\source-map-support.js:495:1)
      at processPromiseRejections (internal/process/promises.js:245:33)
      at processTicksAndRejections (internal/process/task_queues.js:94:32)
2

Emmm, seems like _msgSender() returns address(0), a little weird, actually, _msgSender() should returns the address of the current caller rather than address(0), is there a failed transaction hash

3

I figured out what the issue was, the issue was when trying to approve uniswap router when it was not yet initialised that’s why it returned address(0). Solved now!

1 Like