Revert "Revert 159158 - Make using WebContentsUserData simpler."

This reverts commit r159277.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@159280 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 21 insertions, 13 deletions

diff --git a/chrome/browser/common/web_contents_user_data.h b/chrome/browser/common/web_contents_user_data.h
index 2cffb58..364997e 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));
   }
+
+  // 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.
+//
+// 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_
diff --git a/chrome/browser/common/web_contents_user_data_unittest.cc b/chrome/browser/common/web_contents_user_data_unittest.cc
index cb35bac..1ee3a6a 100644
--- a/chrome/browser/common/web_contents_user_data_unittest.cc
+++ b/chrome/browser/common/web_contents_user_data_unittest.cc
@@ -11,15 +11,12 @@
 #include "content/public/test/web_contents_tester.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
-namespace {
-
 class WebContentsAttachedClass1
     : public WebContentsUserData<WebContentsAttachedClass1> {
  public:
   virtual ~WebContentsAttachedClass1() {}
  private:
   explicit WebContentsAttachedClass1(content::WebContents* contents) {}
-  static int kUserDataKey;
   friend class WebContentsUserData<WebContentsAttachedClass1>;
 };
 
@@ -29,14 +26,11 @@ class WebContentsAttachedClass2
   virtual ~WebContentsAttachedClass2() {}
  private:
   explicit WebContentsAttachedClass2(content::WebContents* contents) {}
-  static int kUserDataKey;
   friend class WebContentsUserData<WebContentsAttachedClass2>;
 };
 
-int WebContentsAttachedClass1::kUserDataKey;
-int WebContentsAttachedClass2::kUserDataKey;
-
-}  // namespace
+DEFINE_WEB_CONTENTS_USER_DATA_KEY(WebContentsAttachedClass1)
+DEFINE_WEB_CONTENTS_USER_DATA_KEY(WebContentsAttachedClass2)
 
 typedef ChromeRenderViewHostTestHarness WebContentsUserDataTest;