3.15 Delete Mappings

The next security pitfall is related to the concept of the delete primitive and Solidity and how it applies to mappings. If there is a struct data structure in a smart contract that contains a mapping as one of its fields, then deleting that structure would delete all the fields of the struct, but the mapping field itself would remain intact, so this is one of the Solidity's behaviors that needs to be kept in mind.

That can have unintended consequences, if the developer assumes that the mapping field within the struct also got deleted and reinitialized to its default values.

The best practice is to use an alternative approach such as considering the data structure that is meant to be deleted as being logged to prevent future logic from using the data structure or the mapping fields within that data structure.