In Solidity, how does the slot assignation work for storage variables when there's inheritance?

Question

In a simple solidity contract, without inheritance, storage variables get assigned sequentially starting from slot 0.

But in case of inheritance, the inherited variables appear before or afterwards?

Answer 1

The assignment is as follows: first to the inherited variables, from the leftmost contract to the rightmost, finishing with the current contract you're in.

In the following example C inherits from Base and Base2, in that order. The slot assignment will start from Base, following Base2, and finally C.

pragma solidity ^0.5;

contract Base {
    uint256 basevar = 0;
    constructor() public {
        basevar = 1;
    }
}

contract Base2 {
    uint256 basevar2 = 2;
    constructor() public {
        basevar2 = 1;
    }
}

contract C is Base, Base2 {
    uint256 cvar = 0;

    function getCvar() public view returns(uint256) {
        return cvar;
    }

    function getBasevar () public view returns(uint256) {
        return basevar;
    }

}

basevar gets assigned slot 0, basevar2 gets assigned slot 1, and cvar gets assigned slot 2.