A fundamental guideline when writing classes, is to relieve the user of the class from dealing with the internal state by exposing a strict interface. In C++, the copy-semantics of a class is part of the interface, and shall therefore also be as strict as necessary.
Classes should either behave as deep-copied or should fail to compile when copied. Copying a class should not have side effects where the resulting copied class can modify the original class. This may sound obvious, but there are many circumstances when, for example, a class requires a heap-allocated object accessed by a pointer member variable of some sort, for example std::shared_ptr, as follows:
class Engine {
public: auto set_oil_amount(float v) { oil_ ...
