How to use a struct in an upgradable contract

[shanefontaine]

Is it possible to upgrade a struct within an upgradable contract without altering the storage slots of variables stored after the struct?

For example:

contract MyContract{
    uint256 a;
    struct Random {
        uint256 b;
        address c;
        bytes d;
    }
    uint256 e;
}

Would it be possible to upgrade the contract to add an address f inside the struct without altering e?

I believe this would work if the struct was listed last in MyContract, but any subsequent upgrade after adding a non-struct variable would then make the struct un-upgradable. Is this statement correct?

Is it not advised to use a struct in an upgradable contract?

Edit:

The example listed in the following Zeppelin blog post does in fact use a struct, but it appears as though this particular struct is not meant to be upgraded at any point.

EVM package deployment with ZeppelinOS—Part I: Introduction and Creating a...

[abcoathup]

The answer is it depends where you are actually using your struct.

ZeppelinOS proxy pattern and structs

Agree with @scammi here, the best way to be sure is to actually test it. That said, adding fields to a struct will heavily depend on where you are actually using such structs. If you have a contract variable of the struct type, then you most likely won’t be able to add any fields (unless the variable is the last one on your contract). If the struct is contained in a mapping, then you probably will, since structs will be stored sparsely in storage. Feel free to share your code here if that hel…

It’s always a good practice to write tests to check the upgrade process itself.

For laying out the variables in storage, Solidity will first linearize the contract hierarchy, and then start laying out the variables starting from the most base contract, onto the most derived one.

Have a look at the following for more details:

Writing upgradeable contracts · ZeppelinOS

ZeppelinOS Upgrades Pattern · ZeppelinOS

[abcoathup]

Housekeeping: moved to #support:zeppelinos category and tagging my reply as the solution.