Need help understanding — web3.eth.getStorageAt

Question

Let's say I have three state variables in my contract

contract A {
    uint private x = 255;
    uint[] y; 
    uint z;
}

As I understand it, x will have index 0, y index 1 and z index 2? Is it that simple?

How many indexes there are?

Are there special indexes?

Does index point to a single storage cell holding one variable(irrespective of its size) or does it point to a cell with a limit(like 32 bytes)?

I'm just trying to access private state variables at contract addresses and have no idea how. I believe it's done through this app method. Thank you!

score 6 · Accepted Answer · edited Apr 13 '17 at 13:01

An index will only retrieve a single storage slot which is 32 bytes.

y is a dynamically sized array, so it will not be at index 1.

The information you are looking for is:

https://solidity.readthedocs.io/en/latest/miscellaneous.html#layout-of-state-variables-in-storage

Statically-sized variables (everything except mapping and dynamically-sized array types) are laid out contiguously in storage starting from position 0.

...

Due to their unpredictable size, mapping and dynamically-sized array types use a sha3 computation to find the starting position of the value or the array data. These starting positions are always full stack slots.

(stack should be storage in the docs.)

Related: How do I get the storage indices/keys?

Need help understanding — web3.eth.getStorageAt

1 Answers1