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Creating a FundMe smart contract in Remix IDE (notes from Freecodecamp)

Creating a FundMe smart contract in Remix IDE (notes from Freecodecamp)

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Murat Can Yüksel
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Table of contents

These are my notes for Freecodecamp's we3/blockchain course Lesson 4: Remix FundMe. Again, all the credit goes to Patrick Collins and Freecodecamp's wonderful community. I'm just sharing my notes.

You can find my notes for the Lesson 3: Storage Factory here => muratcanyuksel.hashnode.dev/creating-a-stor..

You can find the original video tutorial here=> youtube.com/watch?v=gyMwXuJrbJQ&ab_chan..

You can find the source code to this tutorial in Patrick's GitHub => https://github.com/PatrickAlphaC/fund-me-fcc

In this lesson, we're going to work with two contracts primarily: FundMe.sol and PriceConverter.sol. The idea is to create a smart contract that can be sent money in the form of native blockchain token (whether it be ethereum, avalanche, polygon, phantom or whatever else), that which can hold them until the owner of the contract withdraws them. Also, we want to keep track of who sent how much money. While doing that, we want to work with US dollars, but there's a catch in here: Blockchains are deterministic systems, closed boxes if I understood correctly, and they cannot get real-time value of ETH in USD. So, we need to use a third party service to convert the USD to ETH: Oracles- Chainlink in our case. Down the road, we'll be covering liraries, some weird Solidity math, and some advanced concepts to make our contracts more professional and more gas efficient.

I know it sounds a lot, and probably it is, Patrick even suggests that some parts of this tutorial might be difficult to grasp, but we'll not dishearten ourselves, because we know that even if we don't get every single detail in this tutorial, we'll eventually understand them more and more as we continue writing smart contracts. So, buckle up and let's get started with our FundMe.sol contract!

FundMe.sol smart contract

Now, we want this contract to do a couple of things. It should

  • Get funds from users
  • Withdraw funds
  • Set a minimum funding value in USD

In this contract, we have two main functions: fund and withdraw. Along the way, we'll create helper functions that support these two functions.

Let's start with fund function. Now, we want people or entities to send some money to this function, but we also want to set requirement for the minimum amount of money sent. So, first of all, this function should be public so it can be interacted outside of this contract, and it also should be payable so that it can receive funds. Check the snippet out:

    function fund() public payable{
        require(msg.value>1e18, "Didn't send enough!");
    }

Now, what is going on inside of this function? First off, we're using the require syntax which means that "execute what follows in this function only if the following conditions are met, if not, revert and give an error message" (which is Didn't send enough! in this case). The msg.value is the amount of money that was sent to this function. The 1e18 is the smallest amount of money that can be sent to this function.

msg.value is the amount of money sent by the person or entity who called the fund function. We want this msg.value to be greater than 1e18, which means 1 ETH. It actually means 1*10**18 that is 1000000000000000000 wei and that is 1 ETH. No worries, we're going to deal with more manageable numbers soon.

If the amount paid is less than 1 ETH, it will "revert" the transaction and give an error message. But what that revert thing means? Well, it means that "undo any action before, and send remaining gas back". So it DOES spend some gas. For instance, if there's an action before the require statement, it will do that action, spend the necessary amount of gas for that action, and when it faces the require statement and if it couldn't pass the requirement, it will send what remains from the initial gas cost (probably estimated, or agreed IDK). So it will spend gas for any computation before the require statement and can't return them, but the cost of the ones after the require statement will be returned.

But we don't want to deal with these kind of things all the time. We would maybe like our users to send us some amount of ether in terms of usd? In order to do that, we're going to use oracles, or in our case, Chainlink's Price Feeds.

A blockchain oracle is any device that interacts with the off-chain world to provide external data or computation to smart contracts.

Now if you go to the following link => https://docs.chain.link/docs/get-the-latest-price you'll see an example smart contract that makes use of Chainlink's AggregatorV3Interface aggregator contract. We're going to use that contract.

If we wanted to inspect this AggregatorV3Interface aggregator contract, we could go to the following link and check it out => https://github.com/smartcontractkit/chainlink/blob/develop/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol As you can see, it has some functions like version, getRoundData, latestRoundData and so on. The thing is, we can import this aggrgator contract in our smart contract and actually use those functions in our smart contract.

In order to interact with an outside contract, we need the contract ABI (application binary interface) and the address. And we need the address for goerli network because the others are deprecated now.

We need two public functions, getPrice and getConversionRate.

The ABI here is the aggregator contract AggregatorV3Interface, but we also need the address. To get the address that we'll make use of in our call, we'll go to the following link => https://docs.chain.link/docs/ethereum-addresses/, find the section for Goerli Testnet and copy the address related to ETH/USD because that's what we want to do: We want to convert between ether and usd. The address we're looking for is the following => 0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e

Now, let's chill a bit and write some code, things will be clear by the end of this section.

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;
import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

contract FundMe{
    function fund() public payable{
        require(msg.value>1e18, "Didn't send enough!");
    }

    function getPrice () public{
        //the function that we take the price in terms of USD
        //ABI
        //Adress
        AggregatorV3Interface priceFeed= AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
        (,int256 price,,,)= priceFeed.latestRoundData();
    }

    function getConversionRate () public{
        //
    }


   // function withdraw{}
}

As you can see, after the usual License identifier and indicating the Solidity compiler, we're importing something with the line import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";. That's our aggregator contract AggregatorV3Interface, now since we've imported it, we can use its functions inside our contract! And that's what we're doing in our getPrice function.

We are creating a variable named priceFeed with the type of AggregatorV3Interface we've just imported, and set it to the AggregatorV3Interface contract called with göerli ETH/USD address. But, what about the (,int256 price,,,)= priceFeed.latestRoundData(); part? It has too many commas, right? That's not a mistake.

Now, if we examine the snippet using AggregatorV3Interface in this page => https://docs.chain.link/docs/get-the-latest-price/ we notice that their priceFeed variable returns the following when it calls latestRoundData() function:

     /*uint80 roundID*/,
            int price,
            /*uint startedAt*/,
            /*uint timeStamp*/,
            /*uint80 answeredInRound*/

But we do not need all of them. We don't need roundID, startedAt, timeStamp and answeredInRound, we only need int price. But if we omitted them, the Solidity compiler would give an error. It's like we're saying that okay okay we know, we realize and notice that you return those things too, but since we're not going to use them, we're going to pass them as blanks.

Nb! In the documentation, it's int price, we changed it to int256 price to make it flexible.

Now, this part is confusing a bit. When returning the price, we first need to convert it from int256 to uint256 (a practice that's called typecasting). Now, for some reason, Solidity doesn't work well with decimals, so we need to convert them. Eth in terms of Wei has 18 decimals. But, the USD price returned has 8 decimals (in the tutorial it's 3000.00000000). So, to convert them we write as such: return uint256(price * 1e10), 1e10 meaning 1**10. This is the latest version of our getPrice function.


    function getPrice () public view returns(uint256){
        //the function that we take the price in terms of USD
        //ABI
        //Adress
        AggregatorV3Interface priceFeed= AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
        (,int256 price,,,)= priceFeed.latestRoundData();
        //ETH in terms of USD
        //3000.00000000
        return uint256(price * 1e10)
    }

Please be aware that things have changed since the tutorial was recorded. Now ETH worths less than that, but I'm not changing anything as this part was quite mind boggling in my opinion (not a CS major you see).

Now let's check the getConversionRate function.

getConversionRate function

Check out this snippet:


    function getConversionRate (uint256 ethAmount) public view returns (uint256){
        //this is the function that takes the ethAmount and returns the amount in terms of USD
        uint ethPrice= getPrice();
        uint256 ethAmountInUSD= (ethPrice * ethAmount) / 1e18;
        return ethAmountInUSD;
    }

This function takes some value in eth form, and spits it out in usd form.

uint ethPrice calls the previous getPrice function, and then we multiply it by the ethAmount. Then we divide it to 1e18 to get rid of the decimals so that it'll be converted to usd form. We return ethAmountInUSD.

Now, since we can convert from eth to usd, we can go all the way back to the top and our first function, fund() and change it accordingly so that it would be a function that really checks for the amount of usd sent:


contract fundMe{
uint256 public minimumUsd= 50 * 1e18;
function fund() public payable{
require(getConversionRate(msg.value)>= minimumUsd, "Didn't send enough!");
}
//...
//...
}

Now, you realize that minimumUsd variable equals to 50 * 1e18. The reason for it that, getConversionRate returns the number with 18 zeroes after the decimal point, so we upgrade the minimumUsd to 50 * 1e18.

Basic Solidity arrays & structs

Now we want to keep track of all the people who sent us money. To do that, we'll reate a dynamic array of addresses called funders like so => address[] public funders;

And in our fund function we'll push the msg.sender, i.e. the address of the person or entity who's calling the function, to the funders array of addresses like so=> funders.push(msg.sender);

Now, the latest version of our fund function is as follows:

function fund() public payable{
    require(getConversionRate(msg.value)>= minimumUsd, "Didn't send enough!");
    funders.push(msg.sender);
}

To reiterate: msg.value stands for how much ethereum or how much native blockchain currency is sent msg.sender is the addresss of whoever calls the fund function

Now that we have our funders, we might want to check how much money they're sending by using mappings.

First off, we create this: mapping(address => uint256) public addressToAmountFunded;. Then, we add addressToAmountFunded[msg.sender] = msg.value; to our fund function to assign the amount of money sent to the address who sent it in our addressToAmountFunded mapping, so now our fund function alongside with the variables coming before it looks like this :


    uint256 public minimumUsd= 50 * 1e18;
    address[] public funders;
    mapping(address => uint256) public addressToAmountFunded;

    function fund() public payable{
    require(getConversionRate(msg.value)>= minimumUsd, "Didn't send enough!");
    funders.push(msg.sender);
    addressToAmountFunded[msg.sender] = msg.value;

    }

Now, to the next part of the lesson.

Libraries

So far we've created some cool stuff, but it seems like our FundMe.sol contract is gtting a bit cluttered. We did indeed say we'd have two main functions and some helpers around them. Those two function we had in mind were fund() and withdraw(). So maybe we'd like to move all the other helper functions to somewhere else that we can then import into our FundMe.sol smart contract and use as we wish. In order to do that, we'll create what's called a library in Solidity.

Now, libraries are like contracts. But they enable us to give functions to uint256, like msg.value. So that we can call, say getConversionRate on msg.value for instance, like so => msg.value.getConversionRate();

Also, libraries can't have state variables, can't send ether, and all the functions in the library are going to be internal.

To start, we create a new solidity file named PriceConverter.sol.

Now we cut the following functions from FundMe.sol and paste them into PriceConverter.sol: getPrice, getVersion, and getConversionRate. We're going to do the same for our AggregatorV3Interface import since we're not using it anymore in FundMe.sol. So we remove/add this => import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol"; Lastly, we'll make all the functions inside our library internal.

Now, as I've mentioned above, we're going to make this library's functions usable for uint256. In order to do that, we'll import this library into our contract, and then add using PriceConverter for uint256; inside our FundMe contract. Now, we can change the require statement from this require(getConversionRate(msg.value)>= minimumUsd, "Didn't send enough!"); to this => require(msg.value.getConversionRate() >= minimumUsd, "Didn't send enough!");

How this works is, after creating our library, the functions which normally expect a variable passed into them (such as function getConversionRate (uint256 ethAmount) internal view returns (uint256) is expecting an uint256 ethAmount) when used in a smart contract (such as our FundMe.sol) will regard msg.value as their first variable. "So, msg.value will be considered as the first parameter for any of these library functions."

If we wanted to pass a 2nd variable to that function (getConversionRate in our case), like, if was like this => function getConversionRate (uint256 ethAmount, uint256 somethingElse) internal view returns (uint256) that uint256 somethingElse would be passed like a normal function call. Like so: msg.value.getConversionRate(123); where 123 is the 2nd parameter.

But we're not gonna do that.

Now, this is our PriceConverter.sol file so far:

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

library PriceConverter{
        function getPrice () internal view returns(uint256){
        //the function that we take the price in terms of USD
        //ABI
        //Adress
        AggregatorV3Interface priceFeed= AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
        (,int256 price,,,)= priceFeed.latestRoundData();
        //ETH in terms of USD
        //3000.00000000
        return uint256(price * 1e10);
    }

    function getConversionRate (uint256 ethAmount) internal view returns (uint256){
        //this is the function that takes the ethAmount and returns the amount in terms of USD
        uint ethPrice= getPrice();
        uint256 ethAmountInUSD= (ethPrice * ethAmount) / 1e18;
        return ethAmountInUSD;
    }

    function getVersion() internal view returns (uint256){
        // an AggregatorV3Interface variable called priceFeed
        AggregatorV3Interface priceFeed= AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
        return priceFeed.version();
    }
}

And this is our FundMe.sol file:

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;
import "./PriceConverter.sol";
contract FundMe{
    //importing the PriceConverter library
    using PriceConverter for uint256;

    uint256 public minimumUsd= 50 * 1e18;
    address[] public funders;
    mapping(address => uint256) public addressToAmountFunded;

    function fund() public payable{
        //getConversionRate function became available for type uin256 (msg.value is type uint256)
         require(msg.value.getConversionRate() >= minimumUsd, "Didn't send enough!");
        funders.push(msg.sender);
        addressToAmountFunded[msg.sender] = msg.value;

    }



   // function withdraw{}
}

SafeMath, Overflow Checking, and the "unchecked" keyword

This section is not directly related to the project at hand, but a handy piece of knowledge that Patrick presents to us so that we won't be surprised when we see such a thing.

Now, OpenZeppelin's SafeMath contract was paramount before the solidity version 0.8, and now it's almost nonexistent. So, in order to test it, we create a SafeMathTester.sol file and use any version BELOW 0.8.0.

Now, before solidity version 0.8.0, numbers were "unchecked". What does that mean? Consider the following code:

//SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

contract SafeMathTester{
    uint8 public bigNumber = 255; // unchecked

    function add() public{
        bigNumber= bigNumber + 1;
    }
}

Now, first off, uint8 public bigNumber = 255; because 256 is the biggest number an uint8 variable can contain. So, if we deployed this contract using a compiler between 0.6 and 0.8, and tried to add 1 to it, it would overflow, i.e. revert back to 0 instead of 256 . SafeMath seems to be dealing with this unexpected problem. After 0.8.0, numbers are checked, so this problem doesn't occur. Although, we can use the unchecked keyword to make solidity follow the same behavior. Check this out:

//SPDX-License-Identifier: MIT
// upgraded the compiler version
pragma solidity ^0.8.0;

contract SafeMathTester{
    uint8 public bigNumber = 255; // unchecked

    function add() public{
        //unchecked keyword makes the number go to 0 if it's bigger than 255 now
        unchecked{bigNumber= bigNumber + 1;}
    }
}

The two snippets above do the very same thing, in different compiler versions.

So what's the use of this unchecked keyword? Well, it turns out it is more gas efficient. So, if you sure that your math won't round and cause problems, you can use this technique to make your contract more gas efficient.

Basic Solidity For Loop

Now we'll write the withdraw function. This function will loop through the funders array and set the amount of money sent by an individual number to 0 in addressToAmountFunded mapping. It is no different from the for loop in Javascript.

This is the withdraw function:

    function withdraw() public{

        for (uint256 funderIndex= 0; funderIndex < funders.length; funderIndex++){
        //create a funder address variable that's equal to the current index in the funders array
            address funder= funders[funderIndex];
        //set the amount of money sent by the funder found in addressToAmountFunded mapping to 0
            addressToAmountFunded[funder]= 0;
        }
    }

Now we need to do 2 more things: Reset the funders array, and actually withdraw the funds.

Resetting an array

We reset the funders array by writing this piece of code => funders= new address[](0);

We reset the array funders in the withdraw function because we were keeping track of who sent us money, and now since we've withdrawn the money, we don't need to keep track of them. And to think about it, when after withdrawing, if someone else sends us funds, we'd like to keep track of them and distinguish them from the ones who sent us money before we've withdrew the funds.

Now, withdraw function with the above line added to it:

    function withdraw() public{

        for (uint256 funderIndex= 0; funderIndex < funders.length; funderIndex++){
            address funder= funders[funderIndex];
            addressToAmountFunded[funder]= 0;
        }
        //reset the array
        funders= new address[](0);
        //actually withdraw the funds
    }

Our next step is to actually withdraw the funds, i.e. send all the money in this contract to an address. But, how to send money from a contract?

Sending ETH from a contract

There are 3 different ways: transfer, send, and call.

For reference, this link is quite useful => https://solidity-by-example.org/sending-ether/

transfer

We can write the following line => `payable(msg.sender).transfer(address(this).balance)

Normally msg.sender is of type address. What we're doing when we add payable in front of it here is we're typecasting like we did with ints and uints earlier. We are doing this because in Solidity native tokens can only be sent to payable addresses. With this little tweak, our address can be sent money.

this keyword here refers to the whole contract, i.e. FundMe.sol.

The thing with transfer method is that its gas cost is capped at 2300 gas, if it requires more gas cost, then it reverts the function and throws an error.

send

send method is also capped at 2300 gas limit, but instead of an error, it returns a boolean. So, we write it as follows to revert it in case of an error (because it gives a boolean, it wouldn't revert by itself)


        bool sendSuccess= payable(msg.sender).send(address(this).balance);
        require(sendSuccess, "Couldn't send the funds");

call

Call is one of the lower level commands in Solidity. It's really powerful. We can use it to virtually call any function in Ethereum- without even having to have the ABI.

Check this snippet out:

(bool callSuccess, bytes memory dataReturned)= payable(msg.sender).call{value: address(this).balance}("");

Now, let's explain that. the parenthesis after .balance is empty. Normally, when we call a function, we put any function information or any information about the function we want to call in some other contract. We actually don't want to call a function, so we're going to leave this one blank. And to tell Solidity that we leave it blank, we write is as such ("").

The line call.{value: address(this).balance} works as such: You know there's this Value part in Remix IDE where we enter the amount we want to pay, it works like that. So it calls, or say, enters the amount of money in the address of this contract.

This call function actually returns 2 variables. And when a function returns two variables, we can show that by placing them into parenthesis on the left-hand side. The first variable that's returned is bool callSuccess and the second is bytes dataReturned. The second one is the data returned if we were calling a function. And bytes objects are arrays, that's why we wrote them in memory in the beginning like so bytes memory dataReturned.

But, since we're not calling any function with our call method, we do not need this second variable. We used this syntax before in getPrice function. We just delete the 2nd parameter but leave the comma intact to tell Solidity "yea we know there's a second variable but we don't need it". We also add a require statement. So we write it like this in the end:

//notice that the comma is there
(bool callSuccess,)= payable(msg.sender).call{value: address(this).balance}("");
require(callSuccess, "Call failed");

For the most part, call is recommended. Most, I say.

Here's the last version of withdraw function :

    function withdraw() public{

        for (uint256 funderIndex= 0; funderIndex < funders.length; funderIndex++){
            address funder= funders[funderIndex];
            addressToAmountFunded[funder]= 0;
        }
        //reset the array
        funders= new address[](0);
        //actually withdraw the funds
        (bool callSuccess,)= payable(msg.sender).call{value: address(this).balance}("");
        require(callSuccess, "Call failed");
    }

Basic solidity constructor

There's a BIG problem with our contract right now. That is, with the latest withdraw function we wrote, ANYBODY can withdraw the funds. We do not want that. To make sure only the person who deployed the contract can withdraw the funds we're going to make use of constructors.

So we want to make sure that whomever deploys this contract will be the owner of the contract, and only the owner can withdraw funds.

Constructors get called immediately with the contract being deployed.

Now, in order to make sure only the owner can call certain functions, such as the withdraw function, we'll create an address variable of owner and set it to msg.sender in our constructor. Check these out=>

    address public owner;

    constructor(){
        owner= msg.sender;
    }

But, in order for these to work, we need require statements in our functions, or better, a modifier. Before writing the modifier, let's see how we'd write the require statement: require(msg.sender == owner, "You are not the owner!");. This line goes inside the withdraw function, in the 1st line of that function. And Patrick here points out the difference between = and ==. So, a single equal sign (=) means it is setting something to something. Whereas a double (==) means it is checking something against something.

Now let's write our modifier named onlyOwner=>

    modifier onlyOwner{
        require(msg.sender == owner, "You are not the owner!");
        _;
    }

The underscore means that "execute the rest of the code in the function that this modifier was attached to ". Now we need to attach this modifier into the function(s) we need to have this modifier. In our case, it is the withdraw function and we do it like this:

     function withdraw() public onlyOwner{
//...
//...
}

advanced solidity: immutable & constant

In this section, we're going to make this contract a bit more professional. More gas efficient for instance. We'll start by 2 keywords: constant and immutable.

Now, our uint256 public minimumUsd= 50 * 1e18; is fired once the contract is deployed, like the constructor, and never changes. We can make this variable more gas efficient by adding the constant keyword like so:

uint256 public constant minimumUsd= 50 * 1e18;

With constant keyword added, this minimumUsd variable does not take a storage spot, and is much easier to read.

Now, constant have a naming convention. Instead of writing the variable name with camelCase, we generally write them in pascal_case and with majuscule letters. So, minimumUsd becomes MINIMUM_USD

We can use the immutable keyword for variables we set for one time, but outside the same line they're declared, opposed to our MINIMUM_USD for instace, we set the owner and use it in the constructor, so it's sued in 2 lines. The declaration convention with them is they start with i_, like, for our owner variable, it becomes i_owner like so => address public immutable i_owner

advanced solidity custom errors

As of 0.8.4 version of solidity ,we can write custom errors for our reverts. This saves us lots of gas too. To work with them, we first define our errors OUTSIDE of the contract like so =>

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;
import "./PriceConverter.sol";

//realize that this error thingy is outside of the FundMe contract
error NotOwner();

contract FundMe{
    //...

And in our contract, wherever we wish to use instead of the require statement, say, we want to use it in one of the modifiers, we do it such=>

    modifier onlyOwner{
        //comment this old way out
        // require(msg.sender == i_owner, "You are not the owner!");
        if(msg.sender != i_owner){ revert NotOwner(); }
        _;
    }

advanced solidity receive & fallback

Okay, problem: What if people send money to our contract without calling the fund function? Yes, they can do that, and we won't be able to track who sent us what in that case. Solidity has 2 special functions for situations like that: receive and fallback.

This link is useful => https://docs.soliditylang.org/en/latest/contracts.html?highlight=special#special-functions

Now, say that we create a separate file called FallbackExample.sol and populate it with the following code:

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

contract FallbackExample{
    uint256 public result;

    receive() external payable{
        result= 1;
    }
}

NB! Notice that special functions such as the receive function do not have the "function" keyword in front of them. Similar examples are: fallback, constructor...

Now, in Remix IDE, to send this contract money, all we need to do is to deploy it (we don't need to use Injected for this one, we can just use Javascript EVM for it) and after deployment, at the bottom we'll see Low level interactions that has some CALLDATA input field and then a Transact button. We can define the amount of money we want to send like usual with the Value field above, and when we hit transact, as the receive function suggests, the value of result should turn into 1 from 0.

What if we don't leave the CALLDATA section blank and add some data into it? Like say we entered 0x00 into the CALLDATA field. We'd get the following error "Fallback" function is not defined What happens here is that Solidity says "oh, since you're sending some data you're not looking for the receive, you're looking for some function, so let me find that function for you. Mmm, I don't see any function that matches to 0x00 so I'm gonna look for your fallback function." Of course, since we don't have it, we get the above error.

Now, let's add the fallback function like so:

    fallback() external payable{
        result= 2;
    }

After adding the code above, if we'd done what we did above, i.e. send some transaction with data in it, so say, entering 0x00 into the CALLDATA field and hitting the Transact button, instead of getting an error, the transaction would pass successfully. What happens here, since our contract is being called without a VALID function, Solidity realizes that we're doing this, i.e. calling the contract without a valid function, a function that it cannot find mostly because it doesn't exist, it returns and fires the fallback function, which turns the result into 2.

Here's a nice chart explaiing what to expect from Solidity in such cases:

    // Explainer from: https://solidity-by-example.org/fallback/
    // Ether is sent to contract
    //      is msg.data empty?
    //          /   \
    //         yes  no
    //         /     \
    //    receive()?  fallback()
    //     /   \
    //   yes   no
    //  /        \
    //receive()  fallback()

To finish it up, let's add those receive() and fallback() functions to our contract, so that if there's something wrong going on, these functions can in turn call the fund function so that our contract can function as we wished.


    fallback() external payable {
        fund();
    }

    receive() external payable {
        fund();
    }

Now, if somebody accidentally sends us money without calling our fund function, they'll get routed to the fund function automatically.

This costs a bit more gas though.

Latest version of our contracts

FundMe.sol

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;
import "./PriceConverter.sol";

//realize that this error thingy is outside of the FundMe contract
error NotOwner();

contract FundMe{

    using PriceConverter for uint256;

    uint256 public constant MINIMUM_USD= 50 * 1e18;

    address[] public funders;

    mapping(address => uint256) public addressToAmountFunded;

    address public immutable i_owner;

    constructor(){
        i_owner= msg.sender;
    }


    function fund() public payable{
         require(msg.value.getConversionRate() >= MINIMUM_USD, "Didn't send enough!");
        funders.push(msg.sender);
        addressToAmountFunded[msg.sender] = msg.value;

    }

    function withdraw() public onlyOwner{
        for (uint256 funderIndex=0; funderIndex < funders.length; funderIndex++){
            address funder = funders[funderIndex];
            addressToAmountFunded[funder] = 0;
        }
        //reset the array
        funders= new address[](0);
        //actually withdraw the funds
        (bool callSuccess,)= payable(msg.sender).call{value: address(this).balance}("");
        require(callSuccess, "Call failed");
    }

    modifier onlyOwner{
        // require(msg.sender == i_owner, "You are not the owner!");
        if(msg.sender != i_owner){ revert NotOwner(); }
        _;
    }


    fallback() external payable {
        fund();
    }

    receive() external payable {
        fund();
    }

}

PriceConverter.sol

//SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

library PriceConverter{
        function getPrice () internal view returns(uint256){
        //the function that we take the price in terms of USD
        //ABI
        //Adress
        AggregatorV3Interface priceFeed= AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
        (,int256 price,,,)= priceFeed.latestRoundData();
        //ETH in terms of USD
        //3000.00000000
        return uint256(price * 1e10);
    }

    function getConversionRate (uint256 ethAmount) internal view returns (uint256){
        //this is the function that takes the ethAmount and returns the amount in terms of USD
        uint ethPrice= getPrice();
        uint256 ethAmountInUSD= (ethPrice * ethAmount) / 1e18;
        return ethAmountInUSD;
    }

    function getVersion() internal view returns (uint256){
        // an AggregatorV3Interface variable called priceFeed
        AggregatorV3Interface priceFeed= AggregatorV3Interface(0xD4a33860578De61DBAbDc8BFdb98FD742fA7028e);
        return priceFeed.version();
    }
}
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