add intial variant_alternative implementation (#161 http://en.cppreference.com/w/cpp/utility/variant/variant_alternative)

Author: artemp

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 +1023,32 @@ ResultType const& get_unchecked(T const& var)
 {
     return var.template get_unchecked<ResultType>();
 }
+
+// variant_alternative
+template <std::size_t Index, typename...Types>
+struct variant_alternative {};
+
+template <std::size_t Index, typename First, typename...Types>
+struct variant_alternative<Index, First, Types...>
+{
+    using type = typename std::conditional<(sizeof...(Types) == Index),
+                                           First,
+                                           typename variant_alternative<Index, Types...>::type>::type;
+};
+
+template <std::size_t Index>
+struct variant_alternative<Index>
+{
+    using type = void;
+};
+
+template <std::size_t Index, typename...Types>
+struct variant_alternative<Index, variant<Types...>>
+{
+    using type = typename variant_alternative< sizeof...(Types) - Index - 1, Types...>::type;
+};
+
+
 } // namespace util
 } // namespace mapbox
 
@@ -1035,6 +1061,7 @@ 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 +1,23 @@
+#include "catch.hpp"
+
+#include <mapbox/variant.hpp>
+#include <mapbox/variant_io.hpp>
+
+#include <string>
+#include <iostream>
+
+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;
+
+    std::cerr << typeid(type_0).name() << std::endl;
+    std::cerr << typeid(type_1).name() << std::endl;
+    std::cerr << typeid(type_2).name() << std::endl;
+    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);
+}