Using Solidity, how can I execute the transferFrom function of an ERC20 Token?

Question

I am trying to accept ERC20 Tokens as payment in a contract. As I understand it the sender will have to use the approve function to give my contract an allowance first. So far so good. What I do not understand is how does my contract call the transferFrom function to credit itself.

Specifically I want to test if the 'allowance' approved for my contract is equal to a value and if the remaining balance of the sender is equal or greater than my contract's allowance. Conditionally to that I want to execute the 'transferFrom' function of the ERC20 Token.

I am trying to do the following:

if (
         && _Token.call(bytes4(sha3("allowance(owner, this)"))) = 100
         && _Token.call(bytes4(sha3("balanceOf(owner)"))) >= 100
         )
         {
         _Token.call(bytes4(sha3("transferFrom(owner, this, 100)")));

But the Myst compiler says Expected primary expression.

 && _Token.call(bytes4(sha3("allowance(owner, this)"))) = 100

Many thanks.

Answer 1

Your code fragment has a _Token variable, which you're treating as a generic address (using call() on it). If you know that this address is an ERC20 token, you can use Solidity to cast that variable to the appropriate type, and then it's a more intuitive function call, rather than figuring out signature hashes:

contract ERC20Token {
  function transferFrom(address from, address to, uint value);
}

contract MyContract {
  function myFunction(address tokenAddr) {
    ERC20Token tok = ERC20Token(tokenAddr);
    tok.transferFrom(_owner, _recipient, 100);
  }
}

Answer 2

First, you need to be aware that calls only return a true or false in solidity, as they basically just invoke a transaction with a function call to another contract. There is, at least to my knowledge, no way to have data returned like that, in solidity. The only thing you can do is have such a function call trigger another function, which in it's turn sets some var in your calling contract, which you then in turn can use.

Second, in your call parameter, you have to provide a function identifier, which is a hash of the function name and it's datatypes. so assuming your target function takes an integer and an address, you go by

bool success;
success = _Token.call(bytes4(sha3 ("allowance(uint256,address)") ), someInteger, someAddress)
if (!success) { throw; // or something else}

and for the hash string.... don't forget to pack things tightly!

Good luck!