optional<T> in a possible C++20 future

template<typename _Tp>
class optional
: private _Optional_base<_Tp>,
private _Enable_copy_move<
// Copy constructor.
// Copy assignment.
__and_<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>::value,
// Move constructor.
// Move assignment.
__and_<is_move_constructible<_Tp>, is_move_assignable<_Tp>>::value,
// Unique tag type.
/* ... */
template <typename T>
class optional {
struct empty { };
union {
empty _;
T val_;
bool engaged_;
optional(optional const& ) requires is_trivially_copy_constructible_v<T> && is_copy_constructible_v<T> = default;
optional(optional const& rhs) requires is_copy_constructible_v<T>
: engaged_(rhs.engaged_)
if (engaged_) {
new (val_) T(rhs.val_);




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Barry Revzin

Barry Revzin

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