## What do overflow and underflow mean?

\
In simple words, overflow is a situation when uint (unsigned integer) reaches its byte size. Then the next element added will return the first variable element.

\
Let’s say we have a uint8, which can only have 8 bits. That means the largest number we can store is binary 11111111 (in decimal 2^8 - 1 = 255)

\
Look at the code below.

\
```javascript
uint8 balance = 255;
balance++;
```

\
If you execute the code above the "balance" will be 0. This is a simple example of overflow. If you add 1 to binary 11111111, it resets back to 00000000.

\
In the case of underflow, if you subtract 1 from a uint8 that is 0, it will change the value to 255.

Now I show you a simple implementation of underflow in Solidity.

 \n Let's create a simple smart contract called "ChangeBalance.sol". We're going to use Solidity 0.7 for this example because Solidity 0.8 was secured.

\
```javascript
pragma solidity 0.7.0;

contract ChangeBalance {
    uint8 public balance;

    function decrease() public {
        balance--;
    }

    function increase() public {
        balance++;
    }
}
```

\
 ![](https://cdn.hackernoon.com/images/2VftJjxuwsZ19AsybJmsSIp6DX53-mb02gfo.jpeg)

\
Let's compile and deploy the smart contract. As you can see the initial balance is 0.

\
 ![](https://cdn.hackernoon.com/images/2VftJjxuwsZ19AsybJmsSIp6DX53-fh12g1c.jpeg)

\
If you press the "decrease" button, then the "balance" is decremented by one. Let's see what happens next.

\
 ![](https://cdn.hackernoon.com/images/2VftJjxuwsZ19AsybJmsSIp6DX53-da22gk0.jpeg)

As you can see the updated balance is now 255.

## How to avoid overflow and underflow in Solidity?

\
The easiest way is to use at least a 0.8 version of the Solidity compiler. In Solidity 0.8, the compiler will automatically take care of checking for overflows and underflows.

\
Let me show you how it works in practice. To do that I will change the compiler version and deploy a new contract.

\
```javascript
pragma solidity 0.8.0;

contract ChangeBalance {
    uint8 public balance;

    function decrease() public {
        balance--;
    }

    function increase() public {
        balance++;
    }
}
```

\
The initial balance is the same, i.e. 0.

\
 ![](https://cdn.hackernoon.com/images/2VftJjxuwsZ19AsybJmsSIp6DX53-aq42g3i.jpeg)

\
Let's see what happened when we hit "decrease". Now you will get an error in the Remix console.

\
 ![](https://cdn.hackernoon.com/images/2VftJjxuwsZ19AsybJmsSIp6DX53-x852g2j.jpeg)


---

*Also Published [Here](https://dev.to/kamilpolak/integer-overflow-and-underflow-in-solidity-3j7h)*

Ethereum

2022 - HackerNoon Contributor of the Year - Ethereum

2022 - HackerNoon Contributor of the Year - Programming

2022 - HackerNoon Contributor of the Year - Security

2022 - HackerNoon Contributor of the Year - Smart Contracts

2022 - HackerNoon Contributor of the Year - Solidity

Do not forget to follow me 

Nominated for 2022 - HackerNoon Contributor of the Year - Ethereum

Nominated for 2022 - HackerNoon Contributor of the Year - Smart Contracts

Nominated for 2022 - HackerNoon Contributor of the Year - Solidity

Nominated for 2022 - Software Developer of the Year

Nominated for 2022 - HackerNoon Contributor of the Year - Programming

Nominated for 2022 - HackerNoon Contributor of the Year - Security

Too Long; Didn't Read

Build JavaScript blockchain apps easily with Lisk

Hack Solidity: Integer Overflow and Underflow

Hack Solidity: Integer Overflow and Underflow

Too Long; Didn't Read

Coin Mentioned

@kamilpolak

RELATED STORIES