one of the most common libraries that was used for a long time was this library called safemath.sol, and in your Solidity journeys you'll probably see it pop up from time to time. Now we're going to go off on a little bit of a tangent

here about safe math in the library safemath, and this is an excerpt from my course last year so you might see stuff like rink B and some other stuff but just focus on the safe math stuff. Just know when we refer to the JavaScript virtual machine, that's talking about the

Remix virtual machine. One of the most common libraries that was used for the longest time was this library called safemath.sol, and you'll probably see it a lot of different places. We're going to go off on a quick little tangent here and teach you about

safe math. So let's close fund me and close price converter. Now let's create a new file called safemath tester. That's all. And let's start with some basic stuff in here. Safe math was all over the place before version 0.8 of

Solidity, and now it's almost in no contracts. What happened? Why is safe math no longer used as much? Well, let's create a sample contract. This is a section that you don't have to follow along if you don't want to code along with me, but if

you want to, you absolutely still can. This is going to be a contract we are going to deploy on a JavaScript virtual machine. We can use any version of Solidity before version 0.8 of Solidity. So for example, we use pragma solidity 0.6.0, and we'll create contract Safe

math tester. So, now if I create a uint8, I set it to public, big number, and I set this to 255. Oops, safe math tester, let's go ahead and compile safe math tester with 0.6.7 regma solidity. The maximum size of

a uint8 is going to be 255. This is going to be the biggest number that we can fit in a uint8, and if I were to deploy this to a JavaScript VM or even a test network, save math tester, let's go ahead and deploy it.

If I hit big number, we're going to get 255. Well, what happens if I create a function called add that sets big number equal to big number plus one? Let's save that, delete that old contract, and deploy. Well, right now big number is 255. What happens when we add one to big

number when 255 is the max size a uint8 can be? Well, let's hit add. Now, let's check what big number is. Big number gets reset to zero. So, what's going on? Well, prior to version 0.8 of solidity, unsigned integers and integers

ran on this concept of being unchecked, which means that if you passed the upper limit of a number, it would just wrap around and start back from the lowest number it could be. So, if I call add a whole bunch more times and hit big number now it's eight. If I were to hit

this add button a ton more times and get it back to 255, it would then continue to wrap over to zero. So, one of the most popular libraries that was out there was this safe math library, which would basically check to make sure that you weren't wrapping around a uint256 or an

n256 basically it was a way to say hey you've reached the max this number can be and now your transaction is going to fail if we switch this to 0.8 of solidity delete the old contract go switch this to 0.8

we'll go ahead and compile it and now we deploy this to JavaScript VM if I hit big number we get 255 but if we hit add it actually fails and we still get 255. Version 0.8 of solidity they added this bit where it automatically

checks to make sure if you're gonna do what's called overflow or underflow on a variable we can actually revert back to the unchecked version by using an unchecked keyword so if we wrap this big number equals big number plus one in this unchecked bracket let's delete our

old contract we'll compile we'll redeploy we hit big numbers 255. Now we hit add we hit big number again it reverted back to zero so that's a little bit more about safe math checked and unchecked so in version 0.76 and below this code that

you see in front of you is going to be the exact same as this code in 0.8 and above with this unchecked keyword now you might be thinking in newer versions of solidity why would I use this unchecked keyword well you'll find out later that this unchecked keyword makes

your code a little bit more gas efficient so if you're absolutely positive that your math is never going to reach the top or bottom limit of a number then it might make sense for you to use the unchecked keyword let's head back over to our fund me contract where

we are now using the price converter library that we just created

Lesson 4 Part 15 | SafeMath - Solidity & Foundry Full Course

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