add variant_size helper

mapbox · Oct 23, 2017 · 835ebc1 · 835ebc1
1 parent 30560e1
commit 835ebc1
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Showing 2 changed files with 45 additions and 1 deletion.
diff --git a/include/mapbox/variant.hpp b/include/mapbox/variant.hpp
@@ -1023,6 +1023,29 @@ 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>
@@ -1060,6 +1083,17 @@ struct variant_alternative<0, variant<First, Types...>>
 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

diff --git a/test/t/variant_alternative.cpp b/test/t/variant_alternative.cpp
@@ -5,7 +5,7 @@
 
 #include <string>
 
-TEST_CASE("variant alternative", "[types]")
+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;
@@ -19,3 +19,13 @@ TEST_CASE("variant alternative", "[types]")
     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);
+}