Bug 1163329 - Add a utility for expanding a tuple into a variadic argument list to MFBT. r=froydnj

--HG--
extra : rebase_source : e6aaf87a75bfcbebdbd93824a0927ac89a9d2f78
extra : source : d2ccca76a74b83715edefb4414661f4a265021a3

mfbt/IndexSequence.h

@@ -0,0 +1,143 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A utility for expanding a tuple into a variadic argument list.
+ * Based on std::index_sequence. */
+
+/**
+ * Example usage:
+ *
+ * Problem:
+ *
+ *   You have a variadic function Foo:
+ *
+ *     template <typename... Args> void Foo(Args...);
+ *
+ *   And a variadic function Bar, which contains a tuple:
+ *
+ *     template <typename... Args>
+ *     void Bar() {
+ *       // ...
+ *       Tuple<Args...> t;
+ *     }
+ *
+ *   And inside Bar, you want to call Foo with the elements of the tuple as
+ *   arguments to Foo.
+ *
+ *   You want to write:
+ *
+ *     Foo(Get<0>(t), Get<1>(t), ..., Get<N>(t))
+ *
+ *   but you can't literally write that, because N is different for different
+ *   instantiations of Bar.
+ *
+ * Solution:
+ *
+ *   Write a helper function which takes the tuple, and an index sequence
+ *   containing indices corresponding to the tuple indices.
+ *
+ *     template <typename... Args, size_t... Indices>
+ *     void Helper(const Tuple<Args...>& t, IndexSequence<Indices>)
+ *     {
+ *       Foo(Get<Indices>(t)...);
+ *     }
+ *
+ *   Assuming 'Indices...' are 0, 1, ..., N - 1, where N is the size of the
+ *   tuple, pack expansion will expand the pack 'Get<Indices>(t)...' to
+ *   'Get<0>(t), Get<1>(t), ..., Get<N>(t)'.
+ *
+ *   Finally, call the helper, creating the index sequence to pass in like so:
+ *
+ *     template <typename... Args>
+ *     void Bar() {
+ *       // ...
+ *       Tuple<Args...> t;
+ *       Helper(t, typename IndexSequenceFor<Args...>::Type());
+ *     }
+ */
+
+#ifndef mozilla_IndexSequence_h
+#define mozilla_IndexSequence_h
+
+#include "mozilla/Attributes.h"
+
+#include <stddef.h>
+
+namespace mozilla {
+
+/**
+ * Represents a compile-time sequence of integer indices.
+ */
+template<size_t... Indices>
+struct IndexSequence
+{
+  static MOZ_CONSTEXPR size_t Size() { return sizeof...(Indices); }
+};
+
+namespace detail {
+
+// Helpers used by MakeIndexSequence.
+
+template<size_t... Indices>
+struct IndexTuple
+{
+  typedef IndexTuple<Indices..., sizeof...(Indices)> Next;
+};
+
+// Builds IndexTuple<0, 1, ..., N - 1>.
+template<size_t N>
+struct BuildIndexTuple
+{
+  typedef typename BuildIndexTuple<N - 1>::Type::Next Type;
+};
+
+template<>
+struct BuildIndexTuple<0>
+{
+  typedef IndexTuple<> Type;
+};
+
+template<size_t N, typename IndexTuple>
+struct MakeIndexSequenceImpl;
+
+template<size_t N, size_t... Indices>
+struct MakeIndexSequenceImpl<N, IndexTuple<Indices...>>
+{
+  typedef IndexSequence<Indices...> Type;
+};
+
+}  // namespace detail
+
+/**
+ * A utility for building an IndexSequence of consecutive indices.
+ * MakeIndexSequence<N>::Type evaluates to IndexSequence<0, 1, .., N - 1>.
+ * Note: unlike std::make_index_sequence, this is not an alias template
+ * to work around bugs in MSVC 2013.
+ */
+template<size_t N>
+struct MakeIndexSequence
+{
+  typedef typename detail::MakeIndexSequenceImpl<N,
+    typename detail::BuildIndexTuple<N>::Type>::Type Type;
+};
+
+/**
+ * A utility for building an IndexSequence of consecutive indices
+ * corresponding to a variadic argument list.
+ * IndexSequenceFor<Types...> evaluates to IndexSequence<0, 1, ..., N - 1>
+ * where N is the number of types in Types.
+ * Note: unlike std::index_sequence_for, this is not an alias template
+ * to work around bugs in MSVC 2013.
+ */
+template<typename... Types>
+struct IndexSequenceFor
+{
+  typedef typename MakeIndexSequence<sizeof...(Types)>::Type Type;
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_IndexSequence_h */

mfbt/moz.build

@@ -37,6 +37,7 @@ EXPORTS.mozilla = [
     'FloatingPoint.h',
     'GuardObjects.h',
     'HashFunctions.h',
+    'IndexSequence.h',
     'IntegerPrintfMacros.h',
     'IntegerRange.h',
     'IntegerTypeTraits.h',