Problem when accessing variables in Storage geth

Question

I found some contract that looks like this:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.2;
contract IceCream {
    address a;
    address b;
    bool public c = false;
    uint256 public d;

constructor(...){...}
}

I would now expect to get the following values when calling the following on my geth console:

await web3.eth.getStorageAt(address, 0) --> a
await web3.eth.getStorageAt(address, 1) --> b
await web3.eth.getStorageAt(address, 2) --> c
await web3.eth.getStorageAt(address, 3) --> d

Position 0: I find a (Which I also suspect to be there).

Position 1: I find b (Which I also suspect to be there).

Position 2: I find some value 0x0000000000000000000000000000000000000000000000000000000000000a36 which i suppose is the value of the uint256 d. WHY?

Position 3: I find: 0x0000000000000000000000000000000000000000000000000000000000000000. WHY?

I thought the variables in storage are stored in order.

What am I missing here?

Probably it is something really easy and stupid but I just can`t make any sense of this.

Answer 1

Each storage slot has a size of 256 bits (or 32 bytes). If several elementary types can be packed into 32 bytes they will use the same slot.

Let's see what we have here :

• address a : 20 bytes.

It is stored on slot 0 and web3.eth.getStorageAt(address, 0) returns the value of a (0 in your case as it was not instantiated).

• address b : 20 bytes

Since slot 0 has only 12 bytes left, b is stored on slot 1.

• bool public c = false : 1 byte

Booleans only occupy 1 byte. Hence c can be packed with b on slot 1. web3.eth.getStorageAt(address, 1) therefore returns 0x00 + b.

This example is not very instructive as 0x00 + b = b (on 32 bytes), that's why I advise you to deploy the same contract on whatever testnet setting c to true. You will then get 0x01 + b on slot 1.

• uint256 public d : 32 bytes

It uses slot 2 and web3.eth.getStorageAt(address, 2) returns d

• Slot 3 is not used and therefore web3.eth.getStorageAt(address, 3) gives 0.