Mastering Blockchain Programming with Solidity

Avoid changing the variable declaration order

The order of the state variables present in the contract should not be changed. If you want to add a new state variable, add it at the end of all the state variables:

//Bad Practicecontract SampleContract {    string public y;  // Variable order changed    uint256 public x; }

In the preceding code, the variable order has changed. Previously, the uint256 public x variable was defined first in the contract, and then it was the string public y variable. However, in the new code, the order has changed—now, the string public y variable is defined first. This should be avoided.