Std :: experimental :: optional <T> implementation: Constexpr constructor confusion

Question

While the implementation of std::experimental::optional ( cppreference.com ) was confused by the specification of a specific constructor, namely:

 constexpr optional( const T& value ); // (4) 

( Source )

This constructor allows optional<T> for the trivially destructible type T in the context of constexpr . While the first requirement, namely, to disable the user-supplied destructor in this case, in order to make optional<T> alphabetic type, needs to be decided, I don’t know how to get around the restriction of placement-new, which is not allowed in constexpr .

It seemed to me that I should use optional<T> with std::aligned_storage<T> so that the types T not constructive by default and satisfy any alignment requirements, if applicable. But, as I said, constexpr forbids me to use a new placement in this particular constructor.

I had a lot of coffee and I don’t see the obvious solution here?

thanks

Answer 1

I do not know how to get around the placement constraint - new is not allowed in constexpr.

This is the correct diagnostic, literal type, constexpr and new expressions do not mix. The easiest way to fulfill the various requirements of std::experimental::optional<T> is to implement it with options. Positively, a union should be involved at some point. Quick sketch:

 template<typename Val> struct optional { union { Val optional_value; unsigned char dummy_byte; }; bool filled; // post-condition: no-value state constexpr optional() : dummy_byte {} , filled(false) {} // post-condition: has-value state constexpr optional(Val const& val) : optional_value(val) , filled(true) {} // other special members omitted for brevity }; 

Live on coliru

In fact, in the old series of optional sentences used for there is a paragraph on the method to demonstrate that the requirements put forward by it were reasonable in general. (Currently, std::experimental::optional lives in various TS candidates for the library’s fundamental funds.)