Difference between the usage of the keyworks `memory` and `storage` for solidity

README.md

Description

This gist shows the difference between the usage of the keywords storage and memory in Solidity.

Calls

setValueMemory

Note that by calling setValueMemory then s is not modified at all. So for example, if we initialize the contract, then s.value1 and s.value2 will be 0. If we pass a 1 to this function and then we call the view getStruct, it returns the old struct with the values 0 and 0 unchanged

setValueStorage

By using storage within the call to setValueStorage we repeat the process above mentioned, but when we call the view getStruct instead of getting 0 and 0 for the members of this struct, we get 1 for value1. So it's modified.

Conclusion

You can see that the keyword storage works like a pointer to s here

MemoryStorageTest.sol

pragma solidity ^0.8.0;

contract MemoryStorageTest {
    struct StorageStruct {
        uint256 value1;
        uint256 value2;
    }

    StorageStruct s;

    function setValueMemory(uint256 _val) external {
        StorageStruct memory a = s;
        a.value1 = _val;
    }

    function setValueStorage(uint256 _val) external {
        StorageStruct storage a = s;
        a.value1 = _val;
    }

    function getStruct() external view returns (StorageStruct memory) {
        return s;
    }
}