Can i receive/use an already existing ERC 20 token in a new smart contract?

Question

If i already have an ERC20 tokens created and has basic functionalities of a token sat at contract address A, can i start to develop a new smart contract which can make use of the ERC20 token at contract address A.

Answer 1

Yes. This should be no problem to implement this, as erc-20 tokens can be owned by smart contracts as well as external accounts. In general, there's no restriction on which accounts can own a token.

You can use the code below as a reference:

pragma solidity 0.4.25;

// Import OpenZeppelin's ERC20 interface defenition
import "https://github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC20/IERC20.sol";
import "https://github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";

// Simple ERC20 token that mints the given initialSupply to the deployer
// For testing purposes
contract MyToken is ERC20 {
    string public name = "MyToken";
    string public symbol = "MYT";
    uint256 public decimals = 18;

    constructor(uint256 _intialSupply) public {
        _mint(msg.sender, _intialSupply);
    }
}

// Contract that uses any ERC20 token
contract UsingERC20 {
    IERC20 public associatedToken;

    // Constructor. Pass it the token you want this contract to work with
    constructor(IERC20 _token) public {
        associatedToken = _token;
    }

    function doSomethingThatRequiresERC20tokens() public {
        // The key here is to use ERC20's transferFrom method.
        // For this to work, the given address has to have enough balance,
        // and it has to allow this contract to transfer tokens from their account.
        // This can be done using ERC20's approve method/

        // If transferFrom fails, the transaction reverts. So if the transaction
        // does not revert, we know that the transer succeeded.

        // Using msg.sender here, the caller of this function.
        // Could be any address you like, though.
        // This transfers 100 tokens from msg.sender to this contract.
        associatedToken.transferFrom(msg.sender, address(this), 100);

        // Ok, now the tokens are transferred successfully, let's do some cool stuff!
        emit YayIReceivedTokens(100, msg.sender, associatedToken.balanceOf(address(this)));
    }

    event YayIReceivedTokens(uint256 amount, address fromAccount, uint256 totalBalance);
}