🎨 Make apply work with types that implement the tuple protocol

.github/workflows/asciidoctor-ghpages.yml

@@ -50,7 +50,7 @@ jobs:
         env:
           cache-name: cpm-cache-0
         id: cpm-cache-restore
-        uses: actions/cache/restore@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5.0.3
+        uses: actions/cache/restore@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5.0.4
         with:
           path: ~/cpm-cache
           key: ${{runner.os}}-${{env.cache-name}}-${{ hashFiles('**/CMakeLists.txt', 'cmake/**') }}
@@ -64,7 +64,7 @@ jobs:
         env:
           cache-name: cpm-cache-0
         if: steps.cpm-cache-restore.outputs.cache-hit != 'true'
-        uses: actions/cache/save@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5.0.3
+        uses: actions/cache/save@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5.0.4
         with:
           path: ~/cpm-cache
           key: ${{runner.os}}-${{env.cache-name}}-${{ hashFiles('**/CMakeLists.txt', 'cmake/**') }}

include/stdx/tuple.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <stdx/type_traits.hpp>
 #include <stdx/udls.hpp>
 #include <stdx/utility.hpp>
 
@@ -335,18 +336,6 @@ struct tuple_impl<std::index_sequence<Is...>, index_function_list<Fs...>, Ts...>
     constexpr static auto size =
         std::integral_constant<std::size_t, sizeof...(Ts)>{};
 
-    [[nodiscard]] constexpr static auto fill_inner_indices(index_pair *p)
-        -> index_pair * {
-        ((p++->inner = Is), ...);
-        return p;
-    }
-    [[nodiscard]] constexpr static auto
-    fill_outer_indices(index_pair *p, [[maybe_unused]] std::size_t n)
-        -> index_pair * {
-        ((p++->outer = (static_cast<void>(Is), n)), ...);
-        return p;
-    }
-
   private:
     template <typename Funcs, typename... Us>
         requires(... and std::equality_comparable_with<Ts, Us>)
@@ -407,18 +396,56 @@ tuple_impl(Ts...)
 template <typename T> constexpr auto tuple_size_v = T::size();
 template <typename T, std::size_t N>
 constexpr auto tuple_size_v<std::array<T, N>> = N;
+template <typename T, typename U>
+constexpr auto tuple_size_v<std::pair<T, U>> = std::size_t{2};
 template <typename T, T N>
 constexpr auto tuple_size_v<make_integer_sequence<T, N>> = std::size_t{N};
 
-template <std::size_t I, typename T>
-using tuple_element_t = typename T::template element_t<I>;
-
 template <typename T>
 concept tuple_comparable = requires { typename T::common_tuple_comparable; };
 
 template <typename T>
 concept tuplelike = requires { typename remove_cvref_t<T>::is_tuple; };
 
+template <std::size_t I, typename T> struct tuple_element;
+
+template <std::size_t I, tuplelike T> struct tuple_element<I, T> {
+    using type = typename T::template element_t<I>;
+};
+
+template <std::size_t I, typename T, std::size_t N>
+struct tuple_element<I, std::array<T, N>> {
+    using type = T;
+};
+
+template <std::size_t I, typename T, typename U>
+struct tuple_element<I, std::pair<T, U>> {
+    using type = nth_t<I, T, U>;
+};
+
+template <std::size_t I, typename T>
+using tuple_element_t = typename tuple_element<I, T>::type;
+
+namespace detail {
+template <typename T>
+concept has_vacuous_tuple_protocol = requires {
+    {
+        tuple_size_v<std::remove_cvref_t<T>>
+    } -> std::same_as<std::size_t const &>;
+};
+template <typename T>
+concept is_vacuous_tuple = tuple_size_v<std::remove_cvref_t<T>> == 0;
+} // namespace detail
+
+template <typename T>
+concept has_tuple_protocol =
+    detail::has_vacuous_tuple_protocol<T> and
+    (detail::is_vacuous_tuple<T> or requires(T &t) {
+        {
+            get<0>(t)
+        } -> std::same_as<tuple_element_t<0, std::remove_cvref_t<T>> &>;
+    });
+
 template <typename... Ts>
 class tuple : public detail::tuple_impl<std::index_sequence_for<Ts...>,
                                         detail::index_function_list<>, Ts...> {
@@ -524,6 +551,5 @@ class one_of : public detail::tuple_impl<std::index_sequence_for<Ts...>,
     }
 };
 template <typename... Ts> one_of(Ts...) -> one_of<Ts...>;
-
 } // namespace v1
 } // namespace stdx

include/stdx/tuple_algorithms.hpp

@@ -14,27 +14,49 @@
 
 namespace stdx {
 inline namespace v1 {
-template <typename F, tuplelike... Ts> constexpr auto apply(F &&f, Ts &&...ts) {
+namespace detail {
+template <has_tuple_protocol T>
+[[nodiscard]] constexpr auto fill_inner_indices([[maybe_unused]] index_pair *p)
+    -> index_pair * {
+    return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
+        ((p++->inner = Is), ...);
+        return p;
+    }(std::make_index_sequence<tuple_size_v<T>>{});
+}
+
+template <has_tuple_protocol T>
+[[nodiscard]] constexpr auto fill_outer_indices([[maybe_unused]] index_pair *p,
+                                                [[maybe_unused]] std::size_t n)
+    -> index_pair * {
+    return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
+        ((p++->outer = (static_cast<void>(Is), n)), ...);
+        return p;
+    }(std::make_index_sequence<tuple_size_v<T>>{});
+}
+} // namespace detail
+
+template <typename F, has_tuple_protocol... Ts>
+constexpr auto apply(F &&f, Ts &&...ts) {
     constexpr auto total_num_elements =
         (std::size_t{} + ... + stdx::tuple_size_v<std::remove_cvref_t<Ts>>);
 
     [[maybe_unused]] constexpr auto element_indices =
         [&]() -> std::array<detail::index_pair, total_num_elements> {
         std::array<detail::index_pair, total_num_elements> indices{};
         [[maybe_unused]] auto p = indices.data();
-        ((p = std::remove_cvref_t<Ts>::fill_inner_indices(p)), ...);
+        ((p = detail::fill_inner_indices<std::remove_cvref_t<Ts>>(p)), ...);
         [[maybe_unused]] auto q = indices.data();
         [[maybe_unused]] std::size_t n{};
-        ((q = std::remove_cvref_t<Ts>::fill_outer_indices(q, n++)), ...);
+        ((q = detail::fill_outer_indices<std::remove_cvref_t<Ts>>(q, n++)),
+         ...);
         return indices;
     }();
 
     [[maybe_unused]] auto outer_tuple =
         stdx::tuple<Ts &&...>{std::forward<Ts>(ts)...};
     return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
-        return std::forward<F>(f)(
-            std::move(outer_tuple)[index<element_indices[Is].outer>]
-                                  [index<element_indices[Is].inner>]...);
+        return std::forward<F>(f)(get<element_indices[Is].inner>(
+            get<element_indices[Is].outer>(std::move(outer_tuple)))...);
     }(std::make_index_sequence<total_num_elements>{});
 }
 
@@ -51,10 +73,11 @@ template <tuplelike... Ts> [[nodiscard]] constexpr auto tuple_cat(Ts &&...ts) {
             [&]() -> std::array<detail::index_pair, total_num_elements> {
             std::array<detail::index_pair, total_num_elements> indices{};
             auto p = indices.data();
-            ((p = std::remove_cvref_t<Ts>::fill_inner_indices(p)), ...);
+            ((p = detail::fill_inner_indices<std::remove_cvref_t<Ts>>(p)), ...);
             auto q = indices.data();
             std::size_t n{};
-            ((q = std::remove_cvref_t<Ts>::fill_outer_indices(q, n++)), ...);
+            ((q = detail::fill_outer_indices<std::remove_cvref_t<Ts>>(q, n++)),
+             ...);
             return indices;
         }();
 

test/indexed_tuple.cpp

@@ -17,6 +17,16 @@ template <typename Key, typename Value> struct map_entry {
 template <typename T> using key_for = typename T::key_t;
 } // namespace
 
+TEST_CASE("indexed_tuple is tuplelike", "[tuple]") {
+    auto t = stdx::indexed_tuple{1, 2, 3};
+    STATIC_CHECK(stdx::tuplelike<decltype(t)>);
+}
+
+TEST_CASE("indexed_tuple has tuple protocol", "[tuple]") {
+    auto t = stdx::indexed_tuple{1, 2, 3};
+    STATIC_CHECK(stdx::has_tuple_protocol<decltype(t)>);
+}
+
 TEST_CASE("make_indexed_tuple", "[indexed_tuple]") {
     STATIC_REQUIRE(stdx::make_indexed_tuple<>() == stdx::indexed_tuple{});
     STATIC_REQUIRE(stdx::make_indexed_tuple<>(1, 2, 3) ==

test/tuple.cpp

@@ -7,6 +7,7 @@
 #include <catch2/catch_template_test_macros.hpp>
 #include <catch2/catch_test_macros.hpp>
 
+#include <array>
 #include <compare>
 #include <type_traits>
 #include <utility>
@@ -44,6 +45,29 @@ TEST_CASE("multi element tuple", "[tuple]") {
     STATIC_CHECK(T::size() == 2);
 }
 
+TEST_CASE("tuple is tuplelike", "[tuple]") {
+    auto t = stdx::tuple{1, 2, 3};
+    STATIC_CHECK(stdx::tuplelike<decltype(t)>);
+    STATIC_CHECK(stdx::tuplelike<stdx::tuple<>>);
+}
+
+TEST_CASE("tuple has tuple protocol", "[tuple]") {
+    auto t = stdx::tuple{1, 2, 3};
+    STATIC_CHECK(stdx::has_tuple_protocol<decltype(t)>);
+    STATIC_CHECK(stdx::has_tuple_protocol<stdx::tuple<>>);
+}
+
+TEST_CASE("std::array has tuple protocol", "[tuple_algorithms]") {
+    auto t = std::array{1, 2, 3};
+    STATIC_CHECK(stdx::has_tuple_protocol<decltype(t)>);
+    STATIC_CHECK(stdx::has_tuple_protocol<std::array<int, 0>>);
+}
+
+TEST_CASE("std::pair has tuple protocol", "[tuple_algorithms]") {
+    auto t = std::pair{1, 2};
+    STATIC_CHECK(stdx::has_tuple_protocol<decltype(t)>);
+}
+
 namespace {
 template <auto N> struct empty {};
 } // namespace

test/tuple_algorithms.cpp

@@ -116,6 +116,16 @@ TEST_CASE("apply preserves argument order", "[tuple_algorithms]") {
     CHECK(called == 1);
 }
 
+TEST_CASE("apply works on std::array", "[tuple_algorithms]") {
+    STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); },
+                               std::array{1, 2, 3, 4}) == 10);
+}
+
+TEST_CASE("apply works on std::pair", "[tuple_algorithms]") {
+    STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); },
+                               std::pair{1, 2}) == 3);
+}
+
 TEST_CASE("variadic apply", "[tuple_algorithms]") {
     STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); }) == 0);
     STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); },
@@ -161,6 +171,24 @@ TEST_CASE("variadic apply preserves argument order", "[tuple_algorithms]") {
     CHECK(called == 1);
 }
 
+TEST_CASE("variadic apply works on std::array", "[tuple_algorithms]") {
+    STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); },
+                               std::array{1, 2, 3, 4},
+                               std::array{1, 2, 3, 4, 5}) == 25);
+}
+
+TEST_CASE("variadic apply works on std::pair", "[tuple_algorithms]") {
+    STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); },
+                               std::pair{1, 2}, std::pair{3, 4}) == 10);
+}
+
+TEST_CASE("variadic apply works on heterogeneous arguments",
+          "[tuple_algorithms]") {
+    STATIC_REQUIRE(stdx::apply([](auto... xs) { return (0 + ... + xs); },
+                               std::array{1, 2}, std::pair{3, 4},
+                               stdx::tuple{5, 6}) == 21);
+}
+
 TEST_CASE("join", "[tuple_algorithms]") {
     constexpr auto t = stdx::tuple{1, 2, 3};
     STATIC_REQUIRE(t.join(std::plus{}) == 6);