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✨ Add optional tombstones for product types #265

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Merged
merged 1 commit into from
Aug 14, 2025
Merged

✨ Add optional tombstones for product types #265

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16 changes: 16 additions & 0 deletions include/stdx/optional.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,11 @@ template <typename T, typename = void> struct tombstone_traits {
}
};

template <typename T, typename = void> constexpr auto has_tombstone_v = true;
template <typename T>
constexpr auto has_tombstone_v<
T, std::void_t<typename tombstone_traits<T>::unspecialized>> = false;

template <typename T>
struct tombstone_traits<T, std::enable_if_t<std::is_floating_point_v<T>>> {
constexpr auto operator()() const {
Expand All @@ -35,6 +40,17 @@ struct tombstone_traits<T, std::enable_if_t<std::is_pointer_v<T>>> {
constexpr auto operator()() const { return nullptr; }
};

template <template <typename...> typename L, typename T, typename... Ts>
struct tombstone_traits<
L<T, Ts...>,
std::enable_if_t<std::is_constructible_v<L<T, Ts...>, T, Ts...> and
(has_tombstone_v<T> and ... and has_tombstone_v<Ts>)>> {
constexpr auto operator()() const {
return L<T, Ts...>{tombstone_traits<T>{}(),
tombstone_traits<Ts>{}()...};
}
};

template <auto V> struct tombstone_value {
constexpr auto operator()() const {
if constexpr (stdx::is_cx_value_v<decltype(V)>) {
Expand Down
35 changes: 28 additions & 7 deletions test/optional.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,14 @@ template <> struct stdx::tombstone_traits<S> {
constexpr auto operator()() const { return S{-1}; }
};

TEST_CASE("tombstone detection", "[optional]") {
STATIC_REQUIRE(stdx::has_tombstone_v<E>);
STATIC_REQUIRE(stdx::has_tombstone_v<S>);
STATIC_REQUIRE(stdx::has_tombstone_v<float>);
STATIC_REQUIRE(stdx::has_tombstone_v<int *>);
STATIC_REQUIRE(not stdx::has_tombstone_v<int>);
}

TEST_CASE("sizeof(optional<T>) == sizeof(T)", "[optional]") {
using O1 = stdx::optional<E>;
STATIC_REQUIRE(sizeof(O1) == sizeof(E));
Expand Down Expand Up @@ -428,18 +436,31 @@ TEST_CASE("tombstone with non-structural value", "[optional]") {
}
#endif

#if __cplusplus >= 202002L
namespace {
template <typename T>
using my_optional = stdx::conditional_t<requires {
typename stdx::tombstone_traits<T>::unspecialized;
}, std::optional<T>, stdx::optional<T>>;
using my_optional = stdx::conditional_t<stdx::has_tombstone_v<T>,
stdx::optional<T>, std::optional<T>>;
} // namespace

TEST_CASE("select optional implementation based on whether tombstone traits "
"are present",
"[optional]") {
static_assert(std::is_same_v<my_optional<S>, stdx::optional<S>>);
static_assert(std::is_same_v<my_optional<int>, std::optional<int>>);
STATIC_REQUIRE(std::is_same_v<my_optional<S>, stdx::optional<S>>);
STATIC_REQUIRE(std::is_same_v<my_optional<int>, std::optional<int>>);
}

TEST_CASE("product types have tombstones iff their components do",
"[optional]") {
STATIC_REQUIRE(not stdx::has_tombstone_v<std::tuple<>>);
STATIC_REQUIRE(stdx::has_tombstone_v<std::tuple<E, S>>);
STATIC_REQUIRE(not stdx::has_tombstone_v<std::tuple<int>>);
STATIC_REQUIRE(stdx::has_tombstone_v<std::pair<E, S>>);
}

TEST_CASE("tombstone traits for product types come from components",
"[optional]") {
constexpr auto o = stdx::optional<std::tuple<E, S, float>>{};
STATIC_REQUIRE(not o);
STATIC_REQUIRE(*o == std::tuple{E{0xffu}, S{-1},
std::numeric_limits<float>::infinity()});
}
#endif