Bug 1338389 - Index-based Variant::is<N>, as<N>, and extract<N> - r=froydnj

MozReview-Commit-ID: C5iga0Eb1tH

--HG--
extra : rebase_source : d88cf614318cc8544d7ab52315e015a7ca4e5efd

mfbt/Variant.h

@@ -25,6 +25,22 @@ class Variant;
 
 namespace detail {
 
+// Nth<N, types...>::Type is the Nth type (0-based) in the list of types Ts.
+template<size_t N, typename... Ts>
+struct Nth;
+
+template<typename T, typename... Ts>
+struct Nth<0, T, Ts...>
+{
+  using Type = T;
+};
+
+template<size_t N, typename T, typename... Ts>
+struct Nth<N, T, Ts...>
+{
+  using Type = typename Nth<N - 1, Ts...>::Type;
+};
+
 template <typename...>
 struct FirstTypeIsInRest;
 
@@ -180,31 +196,31 @@ struct VariantImplementation<Tag, N, T>
 
   template<typename Variant>
   static void copyConstruct(void* aLhs, const Variant& aRhs) {
-    ::new (KnownNotNull, aLhs) T(aRhs.template as<T>());
+    ::new (KnownNotNull, aLhs) T(aRhs.template as<N>());
   }
 
   template<typename Variant>
   static void moveConstruct(void* aLhs, Variant&& aRhs) {
-    ::new (KnownNotNull, aLhs) T(aRhs.template extract<T>());
+    ::new (KnownNotNull, aLhs) T(aRhs.template extract<N>());
   }
 
   template<typename Variant>
   static void destroy(Variant& aV) {
-    aV.template as<T>().~T();
+    aV.template as<N>().~T();
   }
 
   template<typename Variant>
   static bool
   equal(const Variant& aLhs, const Variant& aRhs) {
-      return aLhs.template as<T>() == aRhs.template as<T>();
+      return aLhs.template as<N>() == aRhs.template as<N>();
   }
 
   template<typename Matcher, typename ConcreteVariant>
   static auto
   match(Matcher&& aMatcher, ConcreteVariant& aV)
-    -> decltype(aMatcher.match(aV.template as<T>()))
+    -> decltype(aMatcher.match(aV.template as<N>()))
   {
-    return aMatcher.match(aV.template as<T>());
+    return aMatcher.match(aV.template as<N>());
   }
 };
 
@@ -222,8 +238,8 @@ struct VariantImplementation<Tag, N, T, Ts...>
 
   template<typename Variant>
   static void copyConstruct(void* aLhs, const Variant& aRhs) {
-    if (aRhs.template is<T>()) {
-      ::new (KnownNotNull, aLhs) T(aRhs.template as<T>());
+    if (aRhs.template is<N>()) {
+      ::new (KnownNotNull, aLhs) T(aRhs.template as<N>());
     } else {
       Next::copyConstruct(aLhs, aRhs);
     }
@@ -231,8 +247,8 @@ struct VariantImplementation<Tag, N, T, Ts...>
 
   template<typename Variant>
   static void moveConstruct(void* aLhs, Variant&& aRhs) {
-    if (aRhs.template is<T>()) {
-      ::new (KnownNotNull, aLhs) T(aRhs.template extract<T>());
+    if (aRhs.template is<N>()) {
+      ::new (KnownNotNull, aLhs) T(aRhs.template extract<N>());
     } else {
       Next::moveConstruct(aLhs, Move(aRhs));
     }
@@ -240,8 +256,8 @@ struct VariantImplementation<Tag, N, T, Ts...>
 
   template<typename Variant>
   static void destroy(Variant& aV) {
-    if (aV.template is<T>()) {
-      aV.template as<T>().~T();
+    if (aV.template is<N>()) {
+      aV.template as<N>().~T();
     } else {
       Next::destroy(aV);
     }
@@ -249,9 +265,9 @@ struct VariantImplementation<Tag, N, T, Ts...>
 
   template<typename Variant>
   static bool equal(const Variant& aLhs, const Variant& aRhs) {
-    if (aLhs.template is<T>()) {
-      MOZ_ASSERT(aRhs.template is<T>());
-      return aLhs.template as<T>() == aRhs.template as<T>();
+    if (aLhs.template is<N>()) {
+      MOZ_ASSERT(aRhs.template is<N>());
+      return aLhs.template as<N>() == aRhs.template as<N>();
     } else {
       return Next::equal(aLhs, aRhs);
     }
@@ -260,10 +276,10 @@ struct VariantImplementation<Tag, N, T, Ts...>
   template<typename Matcher, typename ConcreteVariant>
   static auto
   match(Matcher&& aMatcher, ConcreteVariant& aV)
-    -> decltype(aMatcher.match(aV.template as<T>()))
+    -> decltype(aMatcher.match(aV.template as<N>()))
   {
-    if (aV.template is<T>()) {
-      return aMatcher.match(aV.template as<T>());
+    if (aV.template is<N>()) {
+      return aMatcher.match(aV.template as<N>());
     } else {
       // If you're seeing compilation errors here like "no matching
       // function for call to 'match'" then that means that the
@@ -349,6 +365,7 @@ struct AsVariantTemporary
  *     Variant<char, uint32_t> v1 = Foo();  // v1 holds char('x').
  *
  * All access to the contained value goes through type-safe accessors.
+ * Either the stored type, or the type index may be provided.
  *
  *     void
  *     Foo(Variant<A, B, C> v)
@@ -356,6 +373,8 @@ struct AsVariantTemporary
  *       if (v.is<A>()) {
  *         A& ref = v.as<A>();
  *         ...
+ *       } else (v.is<1>()) { // Same as v.is<B> in this case.
+ *         ...
  *       } else {
  *         ...
  *       }
@@ -382,8 +401,8 @@ struct AsVariantTemporary
  *     auto ptr = v.extract<UniquePtr<A>>();
  *
  * Finally, you can exhaustively match on the contained variant and branch into
- * different code paths depending which type is contained. This is preferred to
- * manually checking every variant type T with is<T>() because it provides
+ * different code paths depending on which type is contained. This is preferred
+ * to manually checking every variant type T with is<T>() because it provides
  * compile-time checking that you handled every type, rather than runtime
  * assertion failures.
  *
@@ -533,7 +552,7 @@ public:
   }
 
   /** Move assignment from AsVariant(). */
-  template <typename T>
+  template<typename T>
   Variant& operator=(detail::AsVariantTemporary<T>&& aValue)
   {
     this->~Variant();
@@ -554,6 +573,14 @@ public:
     return Impl::template tag<T>() == tag;
   }
 
+  template<size_t N>
+  bool is() const
+  {
+    static_assert(N < sizeof...(Ts),
+                  "provided an index outside of this Variant's type list");
+    return N == size_t(tag);
+  }
+
   /**
    * Operator == overload that defers to the variant type's operator==
    * implementation if the rhs is tagged as the same type as this one.
@@ -582,6 +609,15 @@ public:
     return *static_cast<T*>(ptr());
   }
 
+  template<size_t N>
+  typename detail::Nth<N, Ts...>::Type& as()
+  {
+    static_assert(N < sizeof...(Ts),
+                  "provided an index outside of this Variant's type list");
+    MOZ_RELEASE_ASSERT(is<N>());
+    return *static_cast<typename detail::Nth<N, Ts...>::Type*>(ptr());
+  }
+
   /** Immutable const reference. */
   template<typename T>
   const T& as() const {
@@ -591,6 +627,15 @@ public:
     return *static_cast<const T*>(ptr());
   }
 
+  template<size_t N>
+  const typename detail::Nth<N, Ts...>::Type& as() const
+  {
+    static_assert(N < sizeof...(Ts),
+                  "provided an index outside of this Variant's type list");
+    MOZ_RELEASE_ASSERT(is<N>());
+    return *static_cast<const typename detail::Nth<N, Ts...>::Type*>(ptr());
+  }
+
   /**
    * Extract the contained variant value from this container into a temporary
    * value.  On completion, the value in the variant will be in a
@@ -605,6 +650,15 @@ public:
     return T(Move(as<T>()));
   }
 
+  template<size_t N>
+  typename detail::Nth<N, Ts...>::Type extract()
+  {
+    static_assert(N < sizeof...(Ts),
+                  "provided an index outside of this Variant's type list");
+    MOZ_RELEASE_ASSERT(is<N>());
+    return typename detail::Nth<N, Ts...>::Type(Move(as<N>()));
+  }
+
   // Exhaustive matching of all variant types on the contained value.
 
   /** Match on an immutable const reference. */

mfbt/tests/TestVariant.cpp

@@ -28,6 +28,12 @@ testSimple()
   MOZ_RELEASE_ASSERT(v.is<uint64_t>());
   MOZ_RELEASE_ASSERT(!v.is<uint32_t>());
   MOZ_RELEASE_ASSERT(v.as<uint64_t>() == 1);
+
+  MOZ_RELEASE_ASSERT(v.is<1>());
+  MOZ_RELEASE_ASSERT(!v.is<0>());
+  static_assert(mozilla::IsSame<decltype(v.as<1>()), uint64_t&>::value,
+                "as<1>() should return a uint64_t");
+  MOZ_RELEASE_ASSERT(v.as<1>() == 1);
 }
 
 static void