Using an Array to Delete Mappings

Question

BEFORE YOU DOWNVOTE

I know questions on deleting mappings have come up thousands of times but I'd like to pose a question... can you delete it if it's in an array?

the following code allows the caller to delete a nested list of addresses

pragma solidity ^0.8.0;

contract Arrs {
    address[][] private _addy;

    constructor() {
        _addy.push();
        _addy[0].push();
    }

    function def() public {
        _addy[0][0] = msg.sender;
    }

    function del() public {
        delete _addy; // or _addy.pop()
        _addy.push();
        _addy[0].push();
    }

    function value() public view returns (address) {
        return (_addy[0][0]);
    }
}

running value() initially gives you the zero address, def() sets it to your address, del() resets it. It works great.

Now I KNOW that you cannot delete entire mapping arrays, however, I am not getting any errors or warnings in Remix/other IDEs and no errors or reverts when running the functions on a deployed contract on Goerli when I try and delete an array that contains the mapping.

contract Maps {
    mapping(uint256 => address)[] private _map;

    constructor() {
        _map.push();
    }

    function def() public {
        _map[0][0] = msg.sender;
    }

    function del() public {
        delete _map;
        _map.push();
    }

    function value() public view returns (address) {
        return (_map[0][0]);
    }
}

running value() initially gives you the zero address, def() sets it to your address, but del() is where it gets interesting.

In debugging on Remix, it seems that delete _map actually sets the _map array length to 0. However, when _map.push() is called, it somehow recovers the old mapping so when value() is called, is returns your address, not the 0 address.

I was wondering why this was and if there was a way to get around it or if I'll have to keep with the status quo and iterate through the mapping via a second array keeping track of the keys.

Answer 1

Hi Chainstack developer advocate here

Execelent observation. DL,DR: Delete doesn't really work as you think.

From Solidity's documentation:

delete a assigns the initial value for the type to a. I.e. for integers it is equivalent to a = 0, but it can also be used on arrays, where it assigns a dynamic array of length zero or a static array of the same length with all elements set to their initial value. delete a[x] deletes the item at index x of the array and leaves all other elements and the length of the array untouched. This especially means that it leaves a gap in the array. If you plan to remove items, a mapping is probably a better choice.

....

delete has no effect on mappings (as the keys of mappings may be arbitrary and are generally unknown). So if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings. However, individual keys and what they map to can be deleted: If a is a mapping, then delete a[x] will delete the value stored at x.

Hope that helps, happy coding!