I'm learning solidity by official docs, one thing I'm not sure is that the address payable concept is only for contract address?
Since I think the account address, as msg.sender, can auto receive token, isn't it?
BTW, if the payable is for contract address, then the following code is useless, right?
address private _owner = 0x617F2E2fD72FD9D5503197092aC168c91465Eab2;
address payable owner = payable(_owner);
address immutable owner2 = payable(msg.sender);
It can be used for both - contract address as well as account address.
address payable type (extension of address type) enables calling the native .transfer() function on the type, to transfer the native tokens (ETH on EThereum, MATIC on Poylgon, ...) to the specified address.
Docs: https://docs.soliditylang.org/en/v0.8.17/types.html#address
So in the example that you shared, _owner is not payable and your code is not able to send it ETH. While owner and owner2 are payable so your code can send them ETH.
Note: Do not confuse the native transfer with ERC-20 transfer function. Those are two separate functions, even though they have the same name - but they accept different arguments and are member functions of different types.
There's one further condition for contract receivers. The receiving contract needs to implement either the receive() or payable fallback() special functions in order to be able to accept the transfer.
Related
When a user sends some Ether to a smart contract/dApp address, is there a way I can access that transaction in my smart contract? Like a built-in method that's invoked, that I can add some code to?
My ideal work flow, if it's possible:
User sends Ether to my dApp address.
A function is invoked in my smart contract code.
Within that function, I can create some logic, like add their address to some state, etc.
Ideally, I want to avoid a user having to invoke a particular public function of the contract, but instead just send Ether to the dApp address.
If the user has to invoke a particular function, I have to start thinking about services like MEW, or I have to build a web2 frontend app that integrates with the MetaMask browser extension or something. This seems like a lot of work, but is this the way it has to be?
There is the receive() special function that gets executed when you send ETH to the contract address and not specify any function to execute (i.e. the data field of the transaction is empty).
You can get the sender address through the msg.sender global variable and the amount through msg.value.
pragma solidity ^0.8;
contract MyContract {
mapping (address => uint256) contributions;
receive() external payable {
contributions[msg.sender] += msg.value;
}
}
I am following a tutorial for a crowdfunding smart contract that accepts a token from users.
I have developed a simple ERC20 token, then I deploy the crowdfunding smart contract giving the address of the ERC20 token as the token accepted from users.
I would like to use the same smart contract with ethers. In other words, I would like people to fund the smart contract with ethers (using ganache and remix, my 10 users have 100 ethers each). Therefore, I need to deploy the smart contract giving the ethereum token address. What is the ether's address?
I am working with remix and ganache under web3 provider.
The native token of any EVM network (in your case Ether) does not have any address.
In Solidity, you can:
Accept ETH with the payable function modifier
Validate the amount sent by the user stored in the msg.value global property. The variable is read-only, the sender chooses how much they send and your contract can only validate that.
Send ETH with the native .transfer() function of address payable (extension of address) type. Do not confuse with the ERC20 custom transfer() function - these are two separate functions even though they have the same name.
pragma solidity ^0.8;
contract MyContract {
address owner = address(0x123);
// `payable` modifier allows the function to accept ETH
function foo() external payable {
// validate that the received amount is 1e18 wei (1 ETH)
require(msg.value == 1e18);
// typecast `address` variable (name `owner`)
// to `address payable` and effectively redirect the received value
// with the native `transfer()` function of the `address payable` type
payable(owner).transfer(msg.value);
}
}
If you need to work with approvals and other ERC-20 features, many contracts use WETH (Wrapped Ether) token that uses tokenomics supposed to maintain the same WETH price as ETH has, instead of using the regular ETH. Its production address depends on the network where its deployed. For example:
Ethereum: 0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2
Polygon: 0x7ceb23fd6bc0add59e62ac25578270cff1b9f619
is there a ERC/IERC20 function or util that ensures received payment is denominated in a specific token?
receive() external payable virtual {
IERC20 token = IERC20(address(0x123...789));
require(
paymentToken == MYtoken,
"Payment Must be MY token"
);
emit PaymentReceived(_msgSender(), msg.value);
The receive() function is for your contract to accept ETH. Or generally - the native token of the network where the contract is deployed (BNB on BSC network, TRX on Tron network, ...).
But if your contract address receives a standard ERC-20 token, it doesn't get notified in any way.
Except for cases, where the token contract specifically calls the receiver to let them know about the transfer (as defined for example in the EIP-777 tokensReceived() hook). But this hook needs to be defined in the token contract in the first place, so that you can make use of it in your receiver contract. If it's not in the token contract, then your contract can't get directly notified about received tokens.
I want to create a smart contract which people can transfer tokens without ether in their wallet.
Suppose A want to transfer ERC20 tokens to B, but he does not have ether in his wallet.
Does third party C can send the transaction for A and therefore pay the gas? Is it possible to create a function in the contract for this usgae?
I have searched online for a soloution and could not find one.
This is a key issue of Ethereum dApp development, but also of tokens. Here is a very old thread on Ethereum Stack Exchange, and also this one.
There are 2 options with their pros and cons:
Use signatures
Every function in your smart contract must have signature parameter.
People who want to interact with the smart contract must sign the function parameters with their account's private key and send it to the smart contract owner (via any communication channel).
The owner then submits the parameters along with the signature to the blockchain, paying for gas. The signature guarantees that the message was approved by the user.
Refund used gas at the end of the transaction. A modifier refundGasCost can be used for this (see below).
But (1) is really hard to apply to token transfers where you just don't know who uses the token and (2) does not really address the issue.
There is a lot happening recently, there is this blog post about How to save your Ethereum Dapp users from paying gas for transactions, and since you ask about tokens, there is an ERC that suggests to Pay transfers in tokens instead of gas, in one transaction which would be nice if you have tokens but no ETH.
I hope this helps.
Exactly this case is already defined in the ERC20 standard. Its this function:
function transferFrom(address from, address to, uint tokens) public returns (bool success);
But before party C could use it and send tokens from A to B, A would have to approve C to do this via the following function, which is also defined in the ERC20 standard:
function approve(address spender, uint tokens) public returns (bool success);
No, in a standard ERC20 token contract, the token holder must initiate at least one transaction (to call transfer() or approve()), and a transaction must always be paid for by its initiator.
This question is a little conceptual, so hopefully this picture will help clear up my misunderstanding.
Image there is a crowdsale smart contract deployed on address 0x2. A User at address 0x01 buys a token.
Here is my understanding of what happens:
The crowdsale contract (# address: 0x2) accepts ether from the user account (# address: 0x1)
The crowdsale contract stores 0x1 as having purchased a token (important: this information is stored in the smart contract #address 0x2)
Now my Question: If 0x1 is a user account (and not a smart contract) there is no code at address 0x1. I thought a user account just consisted of an address + ether associated with the address, how can it also store the fact that 0x1 owns an ERC20 token? For example, I can login to MetaMask and (before clicking the "add token" option) MetaMask can see that I have a token... how is this possible?
Every ERC20 contract has the following function:
function balanceOf(address _owner) public view returns (uint256 balance) {
return balances[_owner];
}
Your wallet just calls this function from the known token contracts with your address. Since it's a view function it doesn't cost any gas.
I recon most ERC20 token get added rather quickly to a wallet like Metamask or MEW. But if your balance doesn't automatically show, you can add the contract address manually (in MEW at least, not sure about Metamask) and it will show up afterwards.
In solidity there are two ways to get the address of the person who sent the transaction
tx.origin
msg.sender
In your example, in the method inside ERC20 Token.sol, the value tx.origin will be 0x1 and msg.sender will be 0x2
So to answer your question, how does the ERC20 token know about 0x2 is: it depends on how the token contract is written and whether it uses tx.origin or msg.sender. I would imagine it uses msg.sender, because that is the more prevalent one.
If it does use msg.sender you can still make the crowdsale contract work by first buying the tokens and then immediatelly transfering the tokens from the crowdsale contract to the caller.
For more information, refer to What's the difference between 'msg.sender' and 'tx.origin'?
how can it also store the fact that 0x1 owns an ERC20 token?
Token transfers, or transfers in accounting in general, are kept in a ledger. In this case, the ledger is ERC-20 smart contract that internally keeps balances who owns and what in its balances mapping. Or, the smart contract manage the storage (EVM SSTORE instructions) where the records of ownership are kept.
Note that some other blockchains, like Telos and EOS (and mayne Solana) might be opposite and there the storage is maintained on the user account (user account has associated RAM and tables for any token user owns).