Merge pull request #162 from mapbox/variant_alternative

add `variant_alternative` and `variant_size` implementations

Makefile

@@ -99,7 +99,7 @@ out/%.o: test/t/%.cpp Makefile $(ALL_HEADERS)
 	mkdir -p ./out
 	$(CXX) -c -o $@ $< -Iinclude -isystem test/include $(FINAL_CXXFLAGS)
 
-out/unit: out/unit.o out/binary_visitor_1.o out/binary_visitor_2.o out/binary_visitor_3.o out/binary_visitor_4.o out/binary_visitor_5.o out/binary_visitor_6.o out/issue21.o out/issue122.o out/mutating_visitor.o out/optional.o out/recursive_wrapper.o out/sizeof.o out/unary_visitor.o out/variant.o
+out/unit: out/unit.o out/binary_visitor_1.o out/binary_visitor_2.o out/binary_visitor_3.o out/binary_visitor_4.o out/binary_visitor_5.o out/binary_visitor_6.o out/issue21.o out/issue122.o out/mutating_visitor.o out/optional.o out/recursive_wrapper.o out/sizeof.o out/unary_visitor.o out/variant.o out/variant_alternative.o
 	mkdir -p ./out
 	$(CXX) -o $@ $^ $(LDFLAGS)
 

include/mapbox/variant.hpp

@@ -858,7 +858,7 @@ class variant
 
     VARIANT_INLINE int which() const noexcept
     {
-        return static_cast<int>(sizeof...(Types)-type_index - 1);
+        return static_cast<int>(sizeof...(Types) - type_index - 1);
     }
 
     template <typename T, typename std::enable_if<
@@ -1023,6 +1023,78 @@ ResultType const& get_unchecked(T const& var)
 {
     return var.template get_unchecked<ResultType>();
 }
+// variant_size
+template <typename T>
+struct variant_size;
+
+//variable templates is c++14
+//template <typename T>
+//constexpr std::size_t variant_size_v = variant_size<T>::value;
+
+template <typename T>
+struct variant_size<const T>
+    : variant_size<T> {};
+
+template <typename T>
+struct variant_size<volatile T>
+    : variant_size<T> {};
+
+template <typename T>
+struct variant_size<const volatile T>
+    : variant_size<T> {};
+
+template <typename... Types>
+struct variant_size<variant<Types...>>
+    : std::integral_constant<std::size_t, sizeof...(Types)> {};
+
+// variant_alternative
+template <std::size_t Index, typename T>
+struct variant_alternative;
+
+#if defined(__clang__)
+#if __has_builtin(__type_pack_element)
+#define has_type_pack_element
+#endif
+#endif
+
+#if defined(has_type_pack_element)
+template <std::size_t Index, typename ...Types>
+struct variant_alternative<Index, variant<Types...>>
+{
+    static_assert(sizeof...(Types) > Index , "Index out of range");
+    using type = __type_pack_element<Index, Types...>;
+};
+#else
+template <std::size_t Index, typename First, typename...Types>
+struct variant_alternative<Index, variant<First, Types...>>
+    : variant_alternative<Index - 1, variant<Types...>>
+{
+    static_assert(sizeof...(Types) > Index -1 , "Index out of range");
+};
+
+template <typename First, typename...Types>
+struct variant_alternative<0, variant<First, Types...>>
+{
+    using type = First;
+};
+
+#endif
+
+template <size_t Index, typename T>
+using variant_alternative_t = typename variant_alternative<Index, T>::type;
+
+template <size_t Index, typename T>
+struct variant_alternative<Index, const T>
+    : std::add_const<variant_alternative<Index, T>> {};
+
+template <size_t Index, typename T>
+struct variant_alternative<Index, volatile T>
+    : std::add_volatile<variant_alternative<Index, T>> {};
+
+template <size_t Index, typename T>
+struct variant_alternative<Index, const volatile T>
+    : std::add_cv<variant_alternative<Index, T>> {};
+
 } // namespace util
 } // namespace mapbox
 
@@ -1035,6 +1107,7 @@ struct hash< ::mapbox::util::variant<Types...>> {
         return ::mapbox::util::apply_visitor(::mapbox::util::detail::hasher{}, v);
     }
 };
+
 }
 
 #endif // MAPBOX_UTIL_VARIANT_HPP

test/t/variant_alternative.cpp

@@ -0,0 +1,31 @@
+#include "catch.hpp"
+
+#include <mapbox/variant.hpp>
+#include <mapbox/variant_io.hpp>
+
+#include <string>
+
+TEST_CASE("variant_alternative", "[types]")
+{
+    using variant_type =  mapbox::util::variant<int, double, std::string>;
+    using type_0 = mapbox::util::variant_alternative<0, variant_type>::type;
+    using type_1 = mapbox::util::variant_alternative<1, variant_type>::type;
+    using type_2 = mapbox::util::variant_alternative<2, variant_type>::type;
+    //using type_3 = mapbox::util::variant_alternative<3, variant_type>::type; // compile error
+    constexpr bool check_0 = std::is_same<int, type_0>::value;
+    constexpr bool check_1 = std::is_same<double, type_1>::value;
+    constexpr bool check_2 = std::is_same<std::string, type_2>::value;
+    CHECK(check_0);
+    CHECK(check_1);
+    CHECK(check_2);
+}
+
+TEST_CASE("variant_size", "[types]")
+{
+    constexpr auto value_0 = mapbox::util::variant_size<mapbox::util::variant<>>::value;
+    constexpr auto value_1 = mapbox::util::variant_size<mapbox::util::variant<int>>::value;
+    constexpr auto value_2 = mapbox::util::variant_size<mapbox::util::variant<int, std::string>>::value;
+    CHECK(value_0 == 0);
+    CHECK(value_1 == 1);
+    CHECK(value_2 == 2);
+}