1 files changed, 73 insertions, 0 deletions
diff --git a/xpcom/ds/SimpleEnumerator.h b/xpcom/ds/SimpleEnumerator.h
new file mode 100644
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+++ b/xpcom/ds/SimpleEnumerator.h
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+/* -*- 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/. */
+
+#ifndef mozilla_SimpleEnumerator_h
+#define mozilla_SimpleEnumerator_h
+
+#include "nsCOMPtr.h"
+#include "nsISimpleEnumerator.h"
+
+namespace mozilla {
+
+/**
+ * A wrapper class around nsISimpleEnumerator to support ranged iteration. This
+ * requires every element in the enumeration to implement the same interface, T.
+ * If any element does not implement this interface, the enumeration ends at
+ * that element, and triggers an assertion in debug builds.
+ *
+ * Typical usage looks something like:
+ *
+ *   for (auto& docShell : SimpleEnumerator<nsIDocShell>(docShellEnum)) {
+ *     docShell.LoadURI(...);
+ *   }
+ */
+
+template <typename T>
+class SimpleEnumerator final {
+ public:
+  explicit SimpleEnumerator(nsISimpleEnumerator* aEnum) : mEnum(aEnum) {}
+
+  class Entry {
+   public:
+    explicit Entry(T* aPtr) : mPtr(aPtr) {}
+
+    explicit Entry(nsISimpleEnumerator& aEnum) : mEnum(&aEnum) { ++*this; }
+
+    const nsCOMPtr<T>& operator*() {
+      MOZ_ASSERT(mPtr);
+      return mPtr;
+    }
+
+    Entry& operator++() {
+      MOZ_ASSERT(mEnum);
+      nsCOMPtr<nsISupports> next;
+      if (NS_SUCCEEDED(mEnum->GetNext(getter_AddRefs(next)))) {
+        mPtr = do_QueryInterface(next);
+        MOZ_ASSERT(mPtr);
+      } else {
+        mPtr = nullptr;
+      }
+      return *this;
+    }
+
+    bool operator!=(const Entry& aOther) const { return mPtr != aOther.mPtr; }
+
+   private:
+    nsCOMPtr<T> mPtr;
+    nsCOMPtr<nsISimpleEnumerator> mEnum;
+  };
+
+  Entry begin() { return Entry(*mEnum); }
+
+  Entry end() { return Entry(nullptr); }
+
+ private:
+  nsCOMPtr<nsISimpleEnumerator> mEnum;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_SimpleEnumerator_h