Merge pull request #119 from elbeno/improve-template-for-each

🆕 Allow `template_for_each` to iterate `std::integer_sequence`

docs/type_traits.adoc

@@ -152,6 +152,17 @@ stdx::template_for_each<L2>([&] <auto V> () { y += V; });
 // y is now 3
 ----
 
+`template_for_each` can also be used with a
+https://en.cppreference.com/w/cpp/utility/integer_sequence[`std::integer_sequence`]:
+
+[source,cpp]
+----
+using L3 = stdx::make_index_sequence<3>;
+int y{};
+stdx::template_for_each<L3>([&] <auto V> () { y += V; });
+// y is now 3
+----
+
 NOTE: A primary use case of `template_for_each` is to be able to use a list of
 tag types without those types having to be complete.
 

include/stdx/type_traits.hpp

@@ -119,9 +119,9 @@ template <typename...> struct type_list {};
 template <auto...> struct value_list {};
 
 template <typename L> struct for_each_t {
-    static_assert(
-        always_false_v<L>,
-        "template_for_each must be called with a type list (or value list)");
+    static_assert(always_false_v<L>,
+                  "template_for_each must be called with a type list, "
+                  "value_list, or std::integer_sequence");
 };
 
 template <template <typename...> typename L, typename... Ts>
@@ -136,6 +136,12 @@ struct for_each_t<L<Vs...>> {
         (f.template operator()<Vs>(), ...);
     }
 };
+template <template <typename, auto...> typename L, typename T, T... Vs>
+struct for_each_t<L<T, Vs...>> {
+    template <typename F> constexpr auto operator()(F &&f) const {
+        (f.template operator()<Vs>(), ...);
+    }
+};
 
 template <typename L> constexpr static auto template_for_each = for_each_t<L>{};
 

test/type_traits.cpp

@@ -4,6 +4,7 @@
 
 #include <cstddef>
 #include <type_traits>
+#include <utility>
 
 namespace {
 template <typename> struct unary_t {};
@@ -130,6 +131,13 @@ TEST_CASE("template_for_each with empty value list", "[type_traits]") {
     CHECK(value == 17);
 }
 
+TEST_CASE("template_for_each with index sequence", "[type_traits]") {
+    value = 0;
+    using L = std::make_index_sequence<3>;
+    stdx::template_for_each<L>(add_value{});
+    CHECK(value == 3);
+}
+
 TEST_CASE("is_same_unqualified", "[type_traits]") {
     static_assert(stdx::is_same_unqualified_v<int, int>);
     static_assert(not stdx::is_same_unqualified_v<int, void>);