Rule of Zero confusion?

Question

So I have been reading about the Rule of Zero.

Simplified version: I do not understand the purpose of this rule. The rule of three and five are sort of "rule of thumbs", but I cannot see the "rule of thumb" or any other specific intentions with this rule.

Detailed version:

Let me quote:

Classes that have custom destructors, copy/move constructors or copy/move assignment operators should deal exclusively with ownership. Other classes should not have custom destructors, copy/move constructors or copy/move assignment operators.

What does this mean? What do they mean by ownership, ownership of what? They have also showed an example code(I guess it is connected to the introduction):

class rule_of_zero
{
    std::string cppstring;
 public:
    rule_of_zero(const std::string& arg) : cppstring(arg) {}
};

What do they want to show with this, I am really lost on this one.

Also, they are also talking about the scenario when you are dealing with a polymorphic class and the destructor is being declared public and virtual and the fact that this block implicit moves. Therefore, you have to declare them all as defaulted:

class base_of_five_defaults
{
 public:
    base_of_five_defaults(const base_of_five_defaults&) = default;
    base_of_five_defaults(base_of_five_defaults&&) = default;
    base_of_five_defaults& operator=(const base_of_five_defaults&) = default;
    base_of_five_defaults& operator=(base_of_five_defaults&&) = default;
    virtual ~base_of_five_defaults() = default;
};

Does this mean that whenever you have a base class with a destructor that is declared both public and virtual, you really have to declare all the other special member function as defaulted? If so, I do not see why.

I know that this is a lot of confusion in one place.

Rule of Zero confusion?

1 Answers