Merge pull request #120 from elbeno/apply-sequence

🆕 Add `apply_sequence`

Author: elbeno

docs/type_traits.adoc

@@ -36,6 +36,38 @@ auto f(T) {
 };
 ----
 
+=== `apply_sequence`
+
+A xref:type_traits.adoc#_type_list_and_value_list[`type_list` or a `value_list`]
+can be unpacked and passed as individual template arguments with
+`apply_sequence`. A function object whose call operator is a variadic function
+template with no runtime arguments is called with the pack of arguments.
+
+[source,cpp]
+----
+using L1 = stdx::type_list<std::integral_constant<int, 1>,
+                           std::integral_constant<int, 2>>;
+int x = stdx::apply_sequence<L1>([&] <typename... Ts> () { return (0 + ... + Ts::value); });
+// x is 3
+
+using L2 = stdx::value_list<1, 2>;
+int y = stdx::apply_sequence<L1>([&] <auto... Vs> () { return (0 + ... + Vs); });
+// y is 3
+----
+
+`apply_sequence` 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>;
+auto y = stdx::apply_sequence<L3>([&] <auto... Vs> () { y += V; });
+// y is 3
+----
+
+NOTE: If the function iterates the pack by folding over `operator,` then
+xref:type_traits.adoc#_template_for_each[`template_for_each`] is probably what you want.
+
 === `is_function_object_v`
 
 `is_function_object_v` is a variable template that detects whether a type is a
@@ -52,6 +84,17 @@ stdx::is_function_object_v<decltype(lam)>;       // true
 stdx::is_function_object_v<decltype(gen_lam)>;   // true
 ----
 
+=== `is_same_unqualified_v`
+
+`is_same_unqualified_v` is a variable template that detects whether a two types
+are the same are removing top-level cv-qualifications and references, if any.
+
+[source,cpp]
+----
+stdx::is_same_unqualified_v<int, int const&>; // true
+stdx::is_same_unqualified_v<int, void>;       // false
+----
+
 === `is_specialization_of_v`
 
 `is_specialization_of_v` is a variable template that detects whether a type is a
@@ -112,31 +155,12 @@ NOTE: Detecting structurality of a type is not yet possible in the general case,
 so there are certain structural types for which this trait will be `false`. In
 practice those types should be rare, and there should be no false positives.
 
-=== `type_or_t`
-
-`type_or_t` is an alias template that selects a type based on whether or not it
-passes a predicate. If not, a default is returned.
-
-[source,cpp]
-----
-using A = int *;
-using T = stdx::type_or_t<std::is_pointer, A>;        // A
-
-using B = int;
-using X = stdx::type_or_t<std::is_pointer, B>;        // void (implicit default)
-using Y = stdx::type_or_t<std::is_pointer, B, float>; // float (explicit default)
-----
-
-=== `type_list` and `value_list`
-
-`type_list` is an empty `struct` templated over any number of types.
-`value_list` is an empty `struct` templated over any number of NTTPs.
-
 === `template_for_each`
 
-A `type_list` or a `value_list` can be iterated with `template_for_each`. A
-function object whose call operator is a unary function template with no runtime
-arguments is passed to each of these functions.
+A xref:type_traits.adoc#_type_list_and_value_list[`type_list` or a `value_list`]
+can be iterated with `template_for_each`. A function object whose call operator
+is a unary function template with no runtime arguments is called with each
+element of the list.
 
 [source,cpp]
 ----
@@ -158,21 +182,31 @@ https://en.cppreference.com/w/cpp/utility/integer_sequence[`std::integer_sequenc
 [source,cpp]
 ----
 using L3 = stdx::make_index_sequence<3>;
-int y{};
+std::size_t 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.
 
-=== `is_same_unqualified_v`
+=== `type_or_t`
 
-`is_same_unqualified_v` is a variable template that detects whether a two types
-are the same are removing top-level cv-qualifications and references, if any.
+`type_or_t` is an alias template that selects a type based on whether or not it
+passes a predicate. If not, a default is returned.
 
 [source,cpp]
 ----
-stdx::is_same_unqualified_v<int, int const&>; // true
-stdx::is_same_unqualified_v<int, void>;       // false
+using A = int *;
+using T = stdx::type_or_t<std::is_pointer, A>;        // A
+
+using B = int;
+using X = stdx::type_or_t<std::is_pointer, B>;        // void (implicit default)
+using Y = stdx::type_or_t<std::is_pointer, B, float>; // float (explicit default)
 ----
+
+=== `type_list` and `value_list`
+
+`type_list` is an empty `struct` templated over any number of types.
+`value_list` is an empty `struct` templated over any number of NTTPs.
+

include/stdx/type_traits.hpp

@@ -118,6 +118,7 @@ constexpr bool is_scoped_enum_v =
 template <typename...> struct type_list {};
 template <auto...> struct value_list {};
 
+namespace detail {
 template <typename L> struct for_each_t {
     static_assert(always_false_v<L>,
                   "template_for_each must be called with a type list, "
@@ -142,8 +143,40 @@ struct for_each_t<L<T, Vs...>> {
         (f.template operator()<Vs>(), ...);
     }
 };
+} // namespace detail
+
+template <typename L>
+constexpr static auto template_for_each = detail::for_each_t<L>{};
+
+namespace detail {
+template <typename L> struct apply_sequence_t {
+    static_assert(always_false_v<L>,
+                  "apply_sequence must be called with a type list, "
+                  "value_list, or std::integer_sequence");
+};
+
+template <template <typename...> typename L, typename... Ts>
+struct apply_sequence_t<L<Ts...>> {
+    template <typename F> constexpr auto operator()(F &&f) const {
+        return f.template operator()<Ts...>();
+    }
+};
+template <template <auto...> typename L, auto... Vs>
+struct apply_sequence_t<L<Vs...>> {
+    template <typename F> constexpr auto operator()(F &&f) const {
+        return f.template operator()<Vs...>();
+    }
+};
+template <template <typename, auto...> typename L, typename T, T... Vs>
+struct apply_sequence_t<L<T, Vs...>> {
+    template <typename F> constexpr auto operator()(F &&f) const {
+        return f.template operator()<Vs...>();
+    }
+};
+} // namespace detail
 
-template <typename L> constexpr static auto template_for_each = for_each_t<L>{};
+template <typename L>
+constexpr static auto apply_sequence = detail::apply_sequence_t<L>{};
 
 template <typename T, typename U>
 constexpr bool is_same_unqualified_v =

test/type_traits.cpp

@@ -100,6 +100,15 @@ struct add_value {
     }
     template <auto T> constexpr auto operator()() const -> void { value += T; }
 };
+
+struct add_values {
+    template <typename... Ts> constexpr auto operator()() const {
+        return (0 + ... + Ts::value);
+    }
+    template <auto... Ts> constexpr auto operator()() const {
+        return (0 + ... + Ts);
+    }
+};
 } // namespace
 
 TEST_CASE("template_for_each with type list", "[type_traits]") {
@@ -138,6 +147,22 @@ TEST_CASE("template_for_each with index sequence", "[type_traits]") {
     CHECK(value == 3);
 }
 
+TEST_CASE("apply_sequence with type list", "[type_traits]") {
+    using L = stdx::type_list<std::integral_constant<int, 1>,
+                              std::integral_constant<int, 2>>;
+    CHECK(stdx::apply_sequence<L>(add_values{}) == 3);
+}
+
+TEST_CASE("apply_sequence with value list", "[type_traits]") {
+    using L = stdx::value_list<1, 2>;
+    CHECK(stdx::apply_sequence<L>(add_values{}) == 3);
+}
+
+TEST_CASE("apply_sequence with index sequence", "[type_traits]") {
+    using L = std::make_index_sequence<3>;
+    CHECK(stdx::apply_sequence<L>(add_values{}) == 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>);