[Ethereum] Could somebody please explain in detail what this Ethereum contract is doing

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I am new to ethereum & blockchain technology and trying to understand somethings(Etheruem, Truffle, DApps etc.) here and there from the internet.
From Ethereum official website,

Create a cryptocurrency contract in Ethereum

I am trying to specifically understand these functions

approve(address _spender, uint _value) returns (bool success) {...}

approveAndCall(address _spender, uint _value, bytes _extraData) returns (bool success) {...}

transferFrom(address _from, address _to, uint _value) returns (bool success) {...}

and lastly this function() {throw;}

This contract code

contract tokenRecipient {
  function receiveApproval(address _from, uint _value, address _token, bytes _extraData);
}

and this declaration mapping (address => mapping (address => uint) ) public allowance;

Best Answer

approve(address _spender, uint _value) returns (bool success) {...}

This function is just being used to make an entry to the allowance array when another contract want to spend some tokens. _ spender is the address of the contract which is going to use it. _value denotes the number of tokens to be spend.

approveAndCall(address _spender, uint _value, bytes _extraData) returns (bool success) {...}

if approve() function returns true, it will invoke the receiveApproval() function of contract tokenRecipient.

transferFrom(address _from, address _to, uint _value) returns (bool success) {...}

This function will be used to transfer tokens from one address to another. Variables are self-explanatory.

function() {throw;}

This is fallback function. According to its functionality, this function will be executed when someone tries to send the ether to the contract. throw; prevents accidental sending of ether.

contract tokenRecipient { function receiveApproval(address _from, uint _value, address _token, bytes _extraData); }

This tells the MyToken contract that the function receiveApproval of contract tokenRecipient can be invoked somewhere in this code.

mapping (address => mapping (address => uint) ) public allowance;

And lastly this line is a way to declare an array by defining the type of key and value. Value further can contain an array. You can assume it a 2D array. This is being used here to keep the record of users asking for the approval of spending the token for the other contracts.

This contract is ERC20-compliant as it implements the approve(...), transfer(..) and transferFrom(...) functions.
For more detail : https://theethereum.wiki/w/index.php/ERC20_Token_Standard#How_Does_A_Token_Contract_Work.3F

Related Solutions

[Ethereum] Token transfer: Intrinsic gas too low

I suspect that your issue is because you have not approved the transfer required to call transferFrom(...).

Here are the relevant functions:

    /* Send coins */
    function transfer(address _to, uint256 _value) {
        if (balanceOf[msg.sender] < _value) throw;           // Check if the sender has enough
        if (balanceOf[_to] + _value < balanceOf[_to]) throw; // Check for overflows
        balanceOf[msg.sender] -= _value;                     // Subtract from the sender
        balanceOf[_to] += _value;                            // Add the same to the recipient
        Transfer(msg.sender, _to, _value);                   // Notify anyone listening that this transfer took place
    }

    /* Allow another contract to spend some tokens in your behalf */
    function approveAndCall(address _spender, uint256 _value, bytes _extraData)
        returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        tokenRecipient spender = tokenRecipient(_spender);
        spender.receiveApproval(msg.sender, _value, this, _extraData);
        return true;
    }

    /* A contract attempts to get the coins */
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
        if (balanceOf[_from] < _value) throw;                 // Check if the sender has enough
        if (balanceOf[_to] + _value < balanceOf[_to]) throw;  // Check for overflows
        if (_value > allowance[_from][msg.sender]) throw;   // Check allowance
        balanceOf[_from] -= _value;                          // Subtract from the sender
        balanceOf[_to] += _value;                            // Add the same to the recipient
        allowance[_from][msg.sender] -= _value;
        Transfer(_from, _to, _value);
        return true;
    }

If you are the current owner of the tokens, you can call transfer(...) to transfer your balance to another address.

However if you want to transfer from another address, you will need to call transferFrom(...) which will check the allowance[...][...] data structure to confirm whether this transfer has been approved.

So, call approveAndCall(...) first to approve your transfer and you should be able to call transferFrom(...) to transfer your tokens.

When you call transferFrom(...) without first calling approveAndCall(...), and error is thrown in the line

if (_value > allowance[_from][msg.sender]) throw;   // Check allowance

This error will appear as an Out Of Gas error in the Ethereum Wallet.

If you are comfortable using geth, see How can the transaction status from a thrown error be detected when gas can be exactly the same as the gasUsed for a successful transaction? for the following code that can be used to check the exact reason for your error:

> var status = debug.traceTransaction("0xd23219e2ea10528b245deb9e47993cae2ffd3ffe9fc27aeb808e94fc4e75d37b")
undefined
> if (status.structLogs.length > 0) {
  console.log(status.structLogs[status.structLogs.length-1].error)
}
"invalid jump destination (PUSH1) 2"

ERC20 Tokens Balance – How to Send Token and Check Balance in Ethereum

If you are using Ethereum wallet, the cleaner way is to `Watch Token

Go to contracts Tab.
Click on Watch Token
Enter details like Token Contract Address, Token Name, Token Symbol, Decimal Places of smallest unit
Click Ok

And you will be able to see balances.

To execute functions of the contract like 'transfer, you need to Watch Contract.

Go to contracts Tab.
Click on Watch Token
Enter details like Contract Name, Contract Address, JSON Interface (Contract ABI)
Click Ok
Now the contract will be listed under contracts tab and you can call any of it's functions

If you are not using Ethereum wallet and want to continue using geth, probably you should write functions for the actions like checkBalance, transferCoin using web3.js.

Best Answer

Related Solutions

[Ethereum] Token transfer: Intrinsic gas too low

ERC20 Tokens Balance – How to Send Token and Check Balance in Ethereum

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