Do C++ abstract classes need to obey the rule of five?

Question

When implementing an abstract class like this:

class Base
{
public:
    virtual ~Base() = default;
    virtual void foo() = 0;
};

Does this interface have to obey the rule of five i.e. do I have to add a copy constructor, copy assignment operator, move constructor and move assignment operator?

I'd figure that an instace of type Base can not be instantiated due to the pure virtual member function and thus providing default implementations for the other special member functions might serve no real purpose.

Is there any use-case/example that would require me to provide the other special member functions?

Answer 1

"abstract" is irrelevant here. A class needs its own copy constructor, copy assignment operator, etc. if it has data that won't be properly copied by the default versions. Full stop. The presence or absence of pure virtual functions does not change this. Your example doesn't have any data, so doesn't have an issue here.

Answer 2

Actually it is the contrary. I would consider deleting copying and assignment of a class that is supposed to be only an interface class to avoid slicing. Consider

class Base {
public:
    virtual ~Base() {}
};

class D1 : public Base {
    int i;
public:
    ~D1() override {}
};

class D2 : public Base {
    int i;
    double d;
public:
    ~D2() override {}
};

you could write something like this

vector<Base> vec;
D1 d;
  D2 e;
  vec.push_back(d);
  vec.push_back(e);

. You would try to squeeze an object of size D2 into a much smaller object of type base. By deleting copy and assignment you do prevent the user or yourself doing that.