I’m a huge proponent of clearly distinguishing value-semantic types (like string and unique_ptr<int>) from classically polymorphic, inheritance-based types (like memory_resource and exception).

• Never inherit from a value-semantic type (and especially not from std types).

• Never copy, move, assign, or swap a polymorphic type.

This means that polymorphic types should usually =delete all their SMFs, like this:

struct Base1 {
    explicit Base1(some, args);

    Base1(Base1&&) = delete;
    Base1(const Base1&) = delete;
    Base1& operator=(Base1&&) = delete;
    Base1& operator=(const Base1&) = delete;

    virtual ~Base1() = default;
};

But I admit that in my own codebases I often just do this:

struct Base2 {
    explicit Base2(some, args);
    virtual ~Base2() = default;
};

The compiler will still quietly generate a defaulted copy constructor and a defaulted copy-assignment operator for Base2; but that happens only for historical reasons (namely, that we didn’t really understand the importance of the Rule of Three until after C++98 was standardized), and they won’t get codegenned unless someone actually uses them, which I won’t. In fact, C++11 deprecated the implicit generating of copy operations for Base2; one day Base2 might become the really and truly recommended way to write a polymorphic class type.

But as of this writing, Base2 implicitly provides copy operations, which technically permits anyone — say, a new-hire coworker of yours — to accidentally (or intentionally) write code that copies or swaps something derived from Base2. In codebases I control, I don’t mind that that’s technically possible; but to anyone who asks, I’ll recommend the more explicit Base1 style.

A couple weeks ago, I learned another advantage of Base1 over Base2.

Base2 can trigger deprecation warnings

Clang diagnoses deprecated features under the (off-by-default) -Wdeprecated flag. If you turn it on, you’ll see a warning every time you use the deprecated implicitly-defaulted copy operations of Base2. (Godbolt.)

void test() {
    Base2 b;
    auto copy = b;
}

warning: definition of implicit copy constructor for 'Base2' is deprecated
because it has a user-declared destructor [-Wdeprecated-copy-with-dtor]
    virtual ~Base2() = default;
            ^
note: in implicit copy constructor for 'Base2' first required here
    auto copy = b;
                ^

This is good in general, but there are a couple ways to trigger this warning that can be a little annoying.

Intentional cloning

First, here’s the one situation I know where one might copy a polymorphic type intentionally: the “clone” pattern.

struct Animal {
    explicit Animal() = default;
    virtual std::unique_ptr<Animal> clone() const = 0;
    virtual std::string noise() const = 0;
    virtual ~Animal() = default;
};

struct Cat : public Animal {
    std::unique_ptr<Animal> clone() const override {
        return std::make_unique<Cat>(*this);
    }
    std::string noise() const override {
        return "meow";
    }
};

int main() {
    std::unique_ptr<Animal> a = std::make_unique<Cat>();
    auto b = a->clone();
    assert(b->noise() == "meow");
}

Cat::clone calls the implicitly defaulted Cat(const Cat&): no problem. Then Cat(const Cat&) tries to copy its Animal base-class subobject via the Animal(const Animal&) that was implicitly generated despite Animal’s user-declared destructor: that’s deprecated.

The solution here is either to completely refactor your codebase to eliminate the “clone” pattern (my preference, if you can swing it); or else to mildly edit it so that the code clearly reflects your intent. You intend Animal to be copyable, and it has a user-declared destructor; so, follow the Rule of Three!

struct Animal {
    explicit Animal() = default;
    virtual std::unique_ptr<Animal> clone() const = 0;
    virtual std::string noise() const = 0;
    Animal(const Animal&) = default;
    Animal& operator=(const Animal&) = delete;
    virtual ~Animal() = default;
};

It might even be worthwhile to mark that copy constructor as protected, since you intend it to be used only by the child class in this very specific scenario.

Notice that -Wdeprecated is triggered by the author of Cat::clone, but the fix needs to be made by the author of Animal.

__declspec(dllexport)

Someone on the cpplang Slack reported a novel way to accidentally trigger -⁠Wdeprecated-copy-dtor (Godbolt): use the Windows compiler clang-cl and mark a polymorphic class with __declspec(dllexport).

When you mark a class with dllexport, the compiler codegens all of its member functions, even the implicitly defaulted special members that wouldn’t ordinarily get instantiated until they were actually used. This bears some resemblance to explicit template instantiation; see “Don’t explicitly instantiate std templates” (2021-08-06). The result: you’re effectively “using” the deprecated copy operations, even though you didn’t really intend to.

struct Animal {
    explicit Animal() = default;
    virtual std::string noise() const = 0;
    virtual ~Animal() = default;
};

struct __declspec(dllexport) Cat : public Animal {
    explicit Cat() = default;
    std::string noise() const override {
        return "meow";
    }
};

warning: definition of implicit copy constructor for 'Animal' is deprecated
because it has a user-declared destructor [-Wdeprecated-copy-with-dtor]
  virtual ~Animal() = default;
          ^
note: in implicit copy constructor for 'Animal' first required here
  struct __declspec(dllexport) Cat : public Animal {
                               ^
note: in implicit copy constructor for 'Cat' first required here
note: due to 'Cat' being dllexported
  struct __declspec(dllexport) Cat : public Animal {
                    ^

There are at least three ways you might address this warning:

• =delete the copy operations of Animal; i.e., convert it from Base2 style to Base1 style. (Godbolt.) This is the best solution, but only the author of Animal can do it.

• =delete the copy operations of Cat. (Godbolt.) This is the most surgical solution, but tedious and error-prone if you have dozens of derived classes.

• Use #pragma GCC diagnostic to suppress the warning around the definition of Animal. (Godbolt.) If you can’t modify Animal, then this is the simplest solution, at the cost of a tidbit of compiler-specific cruft. In industry codebases, I recommend always wrapping each third-party dependency (asio, openssl, protobuf, re2…) in a header file that you control, specifically so that you can maintain this kind of cruft inside that single header instead of touching dozens of files.

UPDATE, 2023-05-06: DV Henkel-Wallace writes in with a fourth option!

• Introduce a class ShimAnimal between Animal and Cat. =delete the copy operations of ShimAnimal, and make Cat inherit from ShimAnimal instead of Animal. (Godbolt.) This is just as surgical as the second solution above, and much less error-prone; its only downside is that it requires breaking up our nice flat hierarchy and making Cat a grandchild, which is a solution that won’t naturally occur to me, personally. See “Inheritance is for sharing an interface” (2020-10-09).

Note that GCC trunk has a similar warning -Wdeprecated-copy-dtor; but it’s not included in GCC’s -Wdeprecated, and even when you pass it explicitly, it seems to trigger only on user-provided destructors like ~B() {}, not merely user-declared ones like ~B() = default. I prefer Clang’s behavior, since it matches the letter of the Standard re what’s actually deprecated and therefore re what might actually change behavior someday in the future.