For data structures, an invariant is something that must hold true at the end of all its public APIs. For example a doubly-linked list may have to break invaria

For data structures, an invariant is something that must hold true at the end of all its public APIs. For example a doubly-linked list may have to break invariants around its pointers while in the middle of a splice or insert. But at the end of those public routines they must all be valid and conformant to the type’s requirements.