Function optional

ext/include/boost/optional/optional.hpp:935–935

Creates an optional uninitialized. No-throw

    // Creates an optional<T> uninitialized.
    // No-throw
    optional( none_t none_ ) BOOST_NOEXCEPT : base(none_) {}

    // Creates an optional<T> initialized with 'val'.
    // Can throw if T::T(T const&) does