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There doesn't seem any obligation in the ERC20 standard to provide the number of decimals anywhere, yet etherscan and all other services can automatically determine the number of decimals of a token. How is that done?

Is there an automated way (through RPC to geth) to get the number of decimals of a token defined by some contract?

The Quantum Physicist
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1 Answers1

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For an ERC20 token, number of decimals are defined in the contract deployed on the address of that token.

Example:

pragma solidity ^0.4.11;
import 'zeppelin/contracts/math/SafeMath.sol';
import 'zeppelin/contracts/token/ERC20.sol';
contract ExampleCoin is ERC20 {
  using SafeMath for uint256;
  string public symbol = "EXAMPLE";
  string public name = "ExampleCoin";
  uint8 public decimals = 18;
  mapping (address => uint256) balances;
  mapping (address => mapping (address => uint256)) allowed;
  function ExampleCoin() public {
    balances[msg.sender] = 1000 * (10 ** uint256(decimals));
    totalSupply = 1000 * (10 ** uint256(decimals));
  }
}

Every ERC20 contract follows same set of standards. All of them have a public decimal value (Thank you open Zeppelin)

All you need is to read that decimal value from the contract's address.

What is even more exciting, you can use generic ABI to connect to any ERC20 contract address and fetch its decimal value. See here

I hope this helps!

Karan
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  • What confuses me here is that we're assuming that there's a member variable called "decimals" that has the number of decimals... but this is not in the ERC20 standard! How safe is it to make this assumption? – The Quantum Physicist Jul 04 '18 at 19:58
  • please have a read on ERC20 standards its there along with name and ticker: https://theethereum.wiki/w/index.php/ERC20_Token_Standard – Karan Jul 04 '18 at 20:00
  • As you see there "Some of the tokens include further information describing the token contract", not all tokens implement "decimals" member variable. It seems not to be obligatory. But then what you're saying is that I have to assume that this value is there. – The Quantum Physicist Jul 04 '18 at 20:09
  • Yes, I haven't come across a erc20 token that does not use it because it is quite necessary to conduct an ICO exchange for ethereum . Even in ethereum 10 ^ 18 Wei = 1 Ether. You can always keep a simple check and show the user if it doesn't exist for the contract but then again I will emphasise on the fact that is not the usual standard ice are conducted on. – Karan Jul 04 '18 at 20:12
  • Last question, if you don't mind. Is it possible to use RPC with the generic ABI like you mentioned? Because from what I see, it uses web3 interface in the link you provided, and I only use json rpc. – The Quantum Physicist Jul 04 '18 at 20:16
  • Sure you can look into eth_call: https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_call – Karan Jul 04 '18 at 20:27
  • I use already eth_call to call functions like balanceOf and transfer. But this is not for member variables, now is it? If I understand correctly, eth_call is for functions, and decimals is not a callable, right? – The Quantum Physicist Jul 04 '18 at 20:29
  • you can access methods and public variables of an ABI – Karan Jul 04 '18 at 20:35
  • If I understand you correctly, you're saying that using eth_call on a public member variable using its name as a signature will return its value. To do this, I'll hash decimals with keccak-256 (like I do with the others, except that the others are functions where signatures are like balanceOf(address)) and put that directly in eth_call. Hopefully that'll retrieve that value. – The Quantum Physicist Jul 04 '18 at 20:43
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    Getters for public variables are automatically created by the compiler. so you need to call decimal() (see the link I attached in the answer) – Karan Jul 04 '18 at 20:50
  • Oh, thanks for that piece of info! I saw the link, but it's so confusing for me because I can't distinguish web3 stuff from standard ethereum stuff. I'll use the signature decimals() then. – The Quantum Physicist Jul 04 '18 at 20:55
  • and mention decimals in ABI like the link did – Karan Jul 05 '18 at 06:09