Before the days of std::move, auto_ptr's copy modeled an ownership-transfer semantic in a more efficient way than could be accomplished by shared_ptr (no refcount churn). It wasn't broken; one must have simply known when it was appropriate to use.

Consider a helper function that returns a heap-allocated object and think whether auto_ptr or shared_ptr better modeled the function's intent of giving ownership of the object to the caller.

This was just a workaround for the lack of move constructors.

The real problem with auto_ptr is that if anything emits a call to auto_ptr destructor at the point where the object pointed to is incomplete (forward-declared), the destructor code won't run, only memory would be freed.