_You can follow along with the video course from here._ ### Introduction In this lesson, we will explore `SafeMath`, a widely used library before Solidity version 0.8, and understand why its usage has now decreased. ### Integer Overflow `SafeMath.sol` was a staple in Solidity contracts before version 0.8. After this version, its usage has significantly dropped. Let's begin by creating a new file called `SafeMathTester.sol` and adding a function `add` that increments the `bigNumber` state variable. ```solidity // SafeMathTester.sol pragma solidity ^0.6.0; contract SafeMathTester { uint8 public bigNumber = 255; function add() public { bigNumber = bigNumber + 1; } } ``` Notice we are using compiler version `0.6.0`. The `bigNumber` is a `uint8` variable with a maximum value of `255`. If we call the `add` function, it will return `0` instead of the expected `256`. Before Solidity version **0.8.0**, signed and unsigned integers were **unchecked**, meaning that if they exceeded the maximum value the variable type could hold, they would reset to the lower limit. This pattern is known as **integer overflow** and the `SafeMath` library was designed to prevent it. ### SafeMath `SafeMath.sol` provided a mechanism to revert transactions when the maximum limit of a `uint256` data type was reached. It was a typical security measure across contracts to avoid erroneous calculations and potential exploits. ```solidity function add(uint a, uint b) public pure returns (uint) { uint c = a + b; require(c >= a, "SafeMath: addition overflow"); return c; } ``` ### Solidity 0.8.0 With the introduction of Solidity version 0.8, automatic checks for overflows and underflows were implemented, making `SafeMath` redundant for these checks. If `SafeMathTester.sol` is deployed with Solidity `0.8.0`, invoking the `add` function will cause a transaction to fail, when, in older versions, it would have reset to zero. For scenarios where mathematical operations are known not to exceed a variable's limit, Solidity introduced the `unchecked` construct to make code more _gas-efficient_. Wrapping the addition operation with `unchecked` will _ignore the overflow and underflow checks_: if the `bigNumber` exceeds the limit, it will wrap its value to zero. ```solidity uint8 public bigNumber = 255; function add() public { unchecked { bigNumber = bigNumber + 1; } } ``` > 🔥 **CAUTION**:br > It's important to use unchecked blocks with caution as they reintroduce the possibility of overflows and underflows. ### Conclusion The evolution of Solidity and `SafeMath.sol` highlights the continuous advancements in Ethereum smart contract development. Although recent updates have made `SafeMath.sol` less essential, it remains a significant part of Ethereum's history. Understanding its role provides valuable insight into the progress and maturation of Solidity. ### 🧑‍💻 Test yourself 1. 📕 Why was the `SafeMath` library widely used before version 0.8? 2. 📕 Explain the meaning of integer overflow and integer underflow. Make an example using `uint16`. 3. 📕 What happened after Solidity version 0.8? 4. 📕 What is the unchecked construct? 5. 🧑‍💻 Modify the `SafeMathTester` contract by using the SafeMath library to prevent integer overflow.