Mapping deletion

Severity: Med ID: mapping-deletion

Flags delete applied to a value whose type contains a mapping.

What it does

Reports delete x when x is a struct or array whose type holds a mapping, directly or through a nested struct or array. Deleting a whole mapping is not valid Solidity, but deleting a container of one compiles and silently leaves the mapping's entries in place.

Why is this bad?

delete resets a storage value to its default by zeroing each member, but it cannot iterate a mapping's keys. The mapping's existing entries stay in storage, so the value is only partially cleared. This is a common source of accounting and access-control bugs: the struct looks reset while stale balances or flags remain reachable.

Example

Bad

struct Account {
    uint256 total;
    mapping(address => uint256) balances;
}

mapping(uint256 => Account) accounts;

function reset(uint256 id) external {
    delete accounts[id]; // `total` is zeroed, but `balances` keeps every entry
}

Good

function reset(uint256 id, address[] calldata holders) external {
    Account storage acc = accounts[id];
    for (uint256 i; i < holders.length; ++i) {
        delete acc.balances[holders[i]];
    }
    acc.total = 0;
}