In what order are destructors called, and why does it matter?

Objects are destroyed in the reverse order of their construction, last constructed, first destroyed (LIFO, like a stack). This is guaranteed by the standard and it's essential for correct resource unwinding when resources have dependencies. For example, if you construct a ClockGate and then an SpiDevice that uses that clock, reverse destruction means the SpiDevice shuts down before its clock is gated, correct by construction. If destruction happened in construction order, you'd gate the clock while the device still needed it. The same principle makes nested RAII guards unwind safely. For members of a class, they're destroyed in reverse declaration order after the destructor body runs.