Lesson 4 - FundMe.sol

[kunsane]

Can anyone explain what is the need to reset the funders array and addressToAmountFunded mapping to 0 ?

[alymurtazamemon]

@kunsane It is not mandatory to reset these, Patrick just did so so that after every withdrawal we can know the new funders and how much they funded after each withdrawal.

[alymurtazamemon]

@kunsane But the side effect could be that the array gets to increase too large if we use it in any program where too many people will give funding. So for remembering the funders we can use a database instead of storing them in the contract.

[krakxn]

function withdraw() public onlyOwner {
        for (uint256 funderIndex=0; funderIndex < funders.length; funderIndex++){
            address funder = funders[funderIndex];
            addressToAmountFunded[funder] = 0;
        }
        funders = new address[](0);

Setting this mapping addressToAmountFunded[funder] = 0; to zero states that funder has no amount funded in the contract--we need this to be done because it is supposed to withdraw, and will lead to a double-spending problem if not done so.

As for the funders = new address[](0);, it is so because we necessarily are not using the contract as a large-scale one, and only for testing purposes--thus, the clearance of the entire funders array.

[kunsane]

So do we have to do this in every contract where we call a withdraw finction

[alymurtazamemon]

@krakxn I just want to add something here. In our scenario, we are just updating user funds in mapping so we can track who gives how much, and the user is neither directly withdrawing nor user is using the funds, that is why if we do not reset it, it will not affect us.

But the double-spending problem you are talking about matters where the user is directly withdrawing or using the funds, like let's say MetaMask. If we do not remove the amount when the user does withdraw funds then he/she can claim that amount later. But our case is different.

[krakxn]

@krakxn I just want to add something here. In our scenario, we are just updating user funds in mapping so we can track who gives how much, and the user is neither directly withdrawing nor user is using the funds, that is why if we do not reset it, it will not affect us.

But the double-spending problem you are talking about matters where the user is directly withdrawing or using the funds, like let's say MetaMask. If we do not remove the amount when the user does withdraw funds then he/she can claim that amount later. But our case is different.

Yes, and when the withdraw function is called (msg.sender), the amount funded key for that address will have the value zero. So, this is necessary in this case and will affect the functionality (which is addressToAmountFunded).

As for the double-spending, I agree; I will clarify it--if addressToAmountFunded is not reset there, the user, if provided with a function which transfers money, can definitely keep spending infinitely until the contract's balance is zero. Of course, conditionals can be placed to mitigate this. I just wanted to emphasize on the addressToAmountFunded being set to zero, as it is fundamental to actual contracts w/ the same functionality--with the added measures put in place for the actual function/contract which transfers the money (in accordance w/ gas, and hence not redundant/unnecessary checks).