Extending ERC20PresetMinterPauser

Support Contracts
#1

Need help with the following code that throws an error!

pragma solidity ^0.6.0;

import "@openzeppelin/contracts/presets/ERC20PresetMinterPauser.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20Capped.sol";

contract MyToken is ERC20PresetMinterPauser, ERC20Capped {
    constructor() public ERC20PresetMinterPauser("MyToken","MNT") ERC20Capped(452552412) {
        _setupDecimals(8);
    }
}

Throws an error when compiling:

TypeError: Derived contract must override function "_beforeTokenTransfer". Two or more base classes define function with same name and parameter types.
1 Like
#2

Hi @DeusNexus,

Welcome to the community forum :wave:

Thanks for posting here.

As per the error we need to override function _beforeTokenTransfer as both ERC20PresetMinterPauser and ERC20Capped define it. See your updated code below:

MyToken.sol

// contracts/MyToken.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "@openzeppelin/contracts/presets/ERC20PresetMinterPauser.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20Capped.sol";

contract MyToken is ERC20PresetMinterPauser, ERC20Capped {
    constructor()
        public
        ERC20PresetMinterPauser("MyToken", "MNT")
        ERC20Capped(452552412)
    {
        _setupDecimals(8);
    }

    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 amount
    ) internal virtual override(ERC20PresetMinterPauser, ERC20Capped) {
        super._beforeTokenTransfer(from, to, amount);
    }
}

To learn more about hooks see: https://docs.openzeppelin.com/contracts/3.x/extending-contracts#rules_of_hooks

Please note, your cap is set to 452552412. With decimals of 8 this is only 4.52552412 MNT.
See: https://docs.openzeppelin.com/contracts/3.x/erc20#a-note-on-decimals

#3

Awesome that cleared up a lot, the inheritance is kind of confusing when you first dive into this, many thanks!

Is there any documentation where I can read more about approve, decreaseAllowance, increaseAllowance, allowance. I want to use burnTokens but not sure how to properly use this in combination with allowence.

1 Like
#4

Hi @DeusNexus,

I recommend having a look at the API documentation:

Also see: Example on how to use ERC20 token in another contract

In addition you may also want to read through the EIP: https://eips.ethereum.org/EIPS/eip-20

I also suggest reading: Points to consider when creating a fungible token (ERC20, ERC777)

1 Like
#5

Hi @DeusNexus,

Do you need any more information?