Using std::optional as a class member of class A

Question

Why isn't it possible to define a class member of type std::optional<A> in class A?

Example:

#include <optional>

class A {
public:
    A(std::optional<A> optional = {}) : optional_(optional) { }
    ~A() = default;
private:
    std::optional<A> optional_;
};

Compiler error:

error: incomplete type 'A' used in type trait expression
   : public integral_constant<bool, __is_trivially_destructible(_Tp)> {};
                                    ^
error: incomplete type 'A' used in type trait expression
   : public integral_constant<bool, __is_constructible(_Tp, _Args...)>
                                    ^

When constructing an `A`, you must allocate enough memory to store all of `A`'s members. But an `A` may contain an `A`, which may contain an `A`, which may contain an `A`... ad infinitum. If `A` could hold a `std::optional`, it would require infinite memory on the stack. — 0x5453, Aug 10 '20 at 20:54

score 4 · Accepted Answer · answered Aug 10 '20 at 20:52

4

optional<T> requires that T is a complete type (and therefore has a known size). Until the class A's definition is finished, A is an incomplete type. Hence the error.

There is no way to have any type T hold a member variable of type optional<T>, since optional<T> itself has a member variable of type T. You can heap-allocate an optional<T> (or just a T, since you'd be storing a pointer that could be nullptr).

answered Aug 10 '20 at 20:52

Nicol Bolas

449,505
63
781
982

Thanks. What's the common way to define recursive optionals in modern c++, simplest example to implement a tree? I've tried `std::unique_ptr`, too, which also doesn't work. But it works with a `std::shared_ptr `. – Yannic Aug 10 '20 at 21:06
4

@Yannic `std::unique_ptr` for exclusive ownership. If it "doesn't work" (which is a very vague description) then post a question with those details. – cdhowie Aug 10 '20 at 21:11
1

@Yannic [`unique_ptr`](https://godbolt.org/z/M9Todc) example (with moving only, no copying) – Ted Lyngmo Aug 10 '20 at 21:18
Nevermind! Mixed things up and got confused, but got it now – Yannic Aug 10 '20 at 21:19

Using std::optional as a class member of class A

1 Answers1