transferFrom function doesn't work

Question

I have made a simple ERC20 token smart contract, but my transferFrom function does not work. I keep getting "Note: The constructor should be payable if you send value." even if I declared it as payable.

function transferFrom(address from, address to, uint256 value) payable public returns(bool success) {
  require(
    allowed[from][msg.sender] >= value
    && balances[from] >= value
    && value >= 0
  );
  balances[from] -= value;
  balances[to] += value;
  allowed[from][msg.sender] -= value;
  Transfer(from, to, value);
  return true;
}

When do you get this error? You probably need at least three transactions to make it work: one to deploy contract, another to approve some value, and the third one to execute transferFrom. — Mikhail Vladimirov, Apr 15 '19 at 13:53

score 0 · Answer 1 · answered Jul 28 '19 at 00:31

If you are using transfer, your function is initiating a form of payment. Payable is to designate which function responds to a payment of ONLY ETHER when getting ether to the contract.

You need to remove the transfer call from your function.

Transfer(from, to, value);

It'll work then. Then issue a different payable function if you wish.

score -1 · Answer 2 · answered Feb 18 '18 at 18:23

I guess your new to solidity. There might be two issues might be calling function and requirements. First let me explain payable function. When you declare a function as payable that means transation will send ethers to that function.

How to call payable function,

contractObj.sampleFunction.sendTransaction(ar1, arg2,{from: <FROM>, value:<Eth_in_wei>});

If your not calling function with eth transaction it will throw an error.Note: The constructor should be payable if you send value

And coming to your qus:

transferFrom() function will check user A has given permission for auto debit for B address with X number of tokens or not? If yes then it will deduct tokens from A and credited to B address

First user A has to call approve() function with B address and number of tokens. In approve method your directly allocating X number of tokens to B.

So you have to call approve() then, after some time or 3rd party contract will call transferFrom() method with address A, address B and Y tokes.

When you want to save storage variables in solidity you need to call sendTransaction() with {from: , gas: }, If function is payable then value key will be part.

Please refer below link's for ref:

payable() function In solidity

https://theethereum.wiki/w/index.php/ERC20_Token_Standard

https://medium.com/@jgm.orinoco/understanding-erc-20-token-contracts-a809a7310aa5

transferFrom function doesn't work

2 Answers2