how to transfer tokens from user to smart contract using methods in the smart contract?

Question

i have an erc20 token called TokenA, i want the user to be able to call a function from my smart contract that does some calculations and then transfers X amount of TokenA to the smart contract.

the problems is in the openzeppelin implementation if i call tokenA.transfer(address(this),amount) in the smart contract msg.sender would be the address of the smart contract but i want it to be the address of the user, and i know its impossible to change msg.sender so is there a workaround to this or should i change the implementation of openzeppelin and add another variable sender?

Answer 1

I would strongly advise against modifying the OpenZeppelin implementations.

You basically want to introduce a vulnerability where an account could spend funds belonging to another account without relying on explicit authorization.

Do not do that.

To make this work, you should rely on the allowance mechanism already included in OpenZeppelin ERC-20 implementation.

1. Account A approves account B to spend funds on its behalf.
2. Account B spends funds from account A to account C.

The spending in (2) is secured by the allowance mechanism present in the ERC-20 contract, and was explicitely allowed by the owner : Account A.

Alternatively, if you have control over the ERC-20 contract, you could simply inherit from it in your smart contract, no need for allowance then, as the user would be interacting directly with the contract holding the tokens, msg.sender would be the user's account address.

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EDIT : Added code example to demonstrate the non preservation of msg.sender. Following the comments on the other answer.

pragma solidity ^0.8.0;

contract B {
    function testMsgSender(address supposedMsgSender) public view {
        require(supposedMsgSender == msg.sender, "msg.sender is not the right address");
    }
}

contract A {
    B contractInstance;
    
    constructor(address Baddress)
    {
        contractInstance = B(Baddress);
    }
    
    function test() public view {
        contractInstance.testMsgSender(msg.sender);
    }
}

You can deploy this code on Remix.

1. Deploy B
2. Deploy A by giving B's address to the contructor

Call A.test and you will see the following error message :

"msg.sender is not the right address"

Answer 2

In order to transfer tokens to a smart contract, you will use transferFrom() function which takes 3 parameters: the sender(which is msg.sender-address of the user), the receiver(address(this)-address of smart contract), and the amount you want to send. Remember that transferFrom() function has to be called after the approve() function(because you first need to approve the amount of tokens you send to the contract). So it will be:

User => contractInstance.approve(address(this),amount);

In ContractA:

contractInstance.transferFrom(msg.sender, address(this), amount);

contractInstance being the instance of the token contract.

In order to test the approve() function with truffle, you explicitly need to approve the amount of tokens and then call the function from your contract where you have transferFrom(). Later on when you'll build your front end, you can initiate the request from there.