Create base class for TabHelpers.

chrome/browser/tab_contents/web_contents_user_data.h

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_TAB_CONTENTS_WEB_CONTENTS_USER_DATA_H_
+#define CHROME_BROWSER_TAB_CONTENTS_WEB_CONTENTS_USER_DATA_H_
+
+#include "base/supports_user_data.h"
+#include "content/public/browser/web_contents.h"
+
+// A base class for classes attached to, and scoped to, the lifetime of a
+// WebContents. For example:
+//
+// --- in foo_tab_helper.h ---
+// class FooTabHelper : public WebContentsUserData<FooTabHelper> {
+//  public:
+//   explicit FooTabHelper(content::WebContents* contents);
+//   virtual ~FooTabHelper();
+//   static int kUserDataKey;

[sky, 2012/09/12 00:03:13]

If this isn't constant, shouldn't it be named user_data_key and with the rest of
the fields? Where is it expected this will be set? If it's public, might others
mutate it?

[Avi (use Gerrit), 2012/09/12 00:08:44]

The key to the puzzle is that its value is irrelevant; its address is constant,
and that's all that matters.
It's not supposed to be set.
It doesn't matter if they do.

+//  // ... more stuff here ...
+// }
+// --- in foo_tab_helper.cc ---
+// int FooTabHelper::kUserDataKey;
+//
+template <typename T>
+class WebContentsUserData : public base::SupportsUserData::Data {
+ public:
+  // Creates an object of type T, and attaches it to the specified WebContents.
+  static void CreateForWebContents(content::WebContents* contents) {
+    contents->SetUserData(&T::kUserDataKey, new T(contents));
+  }
+
+  // Retrieves the instance of type T that was attached to the specified
+  // WebContents (via CreateForWebContents above) and returns it. If no instance
+  // of the type was attached, returns NULL.
+  static T* FromWebContents(content::WebContents* contents) {
+    return static_cast<T*>(contents->GetUserData(&T::kUserDataKey));
+  }
+};
+
+#endif  // CHROME_BROWSER_TAB_CONTENTS_WEB_CONTENTS_USER_DATA_H_