Make using WebContentsUserData simpler.

chrome/browser/common/web_contents_user_data.h

Index: chrome/browser/common/web_contents_user_data.h
diff --git a/chrome/browser/common/web_contents_user_data.h b/chrome/browser/common/web_contents_user_data.h
index 2cffb588e864a5829bf989fe13b4586ca352d3a8..364997ebd93c428d62979db2e0ca0d1d3344c2e5 100644
--- a/chrome/browser/common/web_contents_user_data.h
+++ b/chrome/browser/common/web_contents_user_data.h
@@ -18,12 +18,11 @@
 //   // ... more public stuff here ...
 //  private:
 //   explicit FooTabHelper(content::WebContents* contents);
-//   static int kUserDataKey;
 //   friend class WebContentsUserData<FooTabHelper>;
 //   // ... more private stuff here ...
 // }
 // --- in foo_tab_helper.cc ---
-// int FooTabHelper::kUserDataKey;
+// DEFINE_WEB_CONTENTS_USER_DATA_KEY(FooTabHelper)
 //
 template <typename T>
 class WebContentsUserData : public base::SupportsUserData::Data {
@@ -32,18 +31,33 @@ class WebContentsUserData : public base::SupportsUserData::Data {
   // If an instance is already attached, does nothing.
   static void CreateForWebContents(content::WebContents* contents) {
     if (!FromWebContents(contents))
-      contents->SetUserData(&T::kUserDataKey, new T(contents));
+      contents->SetUserData(&kLocatorKey, 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));
+    return static_cast<T*>(contents->GetUserData(&kLocatorKey));
   }
   static const T* FromWebContents(const content::WebContents* contents) {
-    return static_cast<const T*>(contents->GetUserData(&T::kUserDataKey));
+    return static_cast<const T*>(contents->GetUserData(&kLocatorKey));
   }
+

[awong, 2012/10/10 17:03:46]

Can this be private?

[Avi (use Gerrit), 2012/10/10 18:18:54]

I don't know. Let me try. I'd be happy if it were.

+  // The user data key.
+  static int kLocatorKey;
 };
 
+// The macro to define the locator key. This key must be defined in the .cc file
+// of the tab helper otherwise different instances for different template types
+// will be collapsed by the Visual Studio linker.

[awong, 2012/10/10 17:03:46]

This comment still makes me a bit uneasy.  Since we effectively have an unused
variable, with WPO, what special about having a definition in the .cc instead of
the .h that's keeping the linker from merging everything?

[Avi (use Gerrit), 2012/10/10 18:18:54]

I wish I knew. I'm trying to remember who I had the conversation with who said
that this made it work. This also helps with the component builds to ensure that
it's single.

+//
+// The "= 0" is surprising, but is required to effect a definition rather than
+// a declaration. Without it, this would be merely a declaration of a template
+// specialization. (C++98: 14.7.3.15; C++11: 14.7.3.13)
+//
+#define DEFINE_WEB_CONTENTS_USER_DATA_KEY(TYPE) \
+template<>                                      \
+int WebContentsUserData<TYPE>::kLocatorKey = 0;
+
 #endif  // CHROME_BROWSER_COMMON_WEB_CONTENTS_USER_DATA_H_