Make it possible to use ThreadLocalStorage::Slot as a static without

base/threading/thread_local_storage.h

 // Copyright (c) 2011 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 BASE_THREADING_THREAD_LOCAL_STORAGE_H_
 #define BASE_THREADING_THREAD_LOCAL_STORAGE_H_
 #pragma once
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 
 #if defined(OS_POSIX)
 #include <pthread.h>
 #endif
 
 namespace base {
 
 // Wrapper for thread local storage.  This class doesn't do much except provide
 // an API for portability.
 class BASE_EXPORT ThreadLocalStorage {
  public:
 
   // Prototype for the TLS destructor function, which can be optionally used to
   // cleanup thread local storage on thread exit.  'value' is the data that is
   // stored in thread local storage.
   typedef void (*TLSDestructorFunc)(void* value);
 
+  // StaticSlot uses its own struct initializer-list style static
+  // initialization, as base's LINKER_INITIALIZED requires a constructor and on
+  // some compilers (notably gcc 4.4) this still ends up needing runtime
+  // initialization.
+  #define TLS_INITIALIZER {0}
+
   // A key representing one value stored in TLS.
-  class BASE_EXPORT Slot {
-   public:
-    explicit Slot(TLSDestructorFunc destructor = NULL);
-
-    // This constructor should be used for statics.
-    // It returns an uninitialized Slot.
-    explicit Slot(base::LinkerInitialized x) {}
-
+  // Initialize like
+  //   ThreadLocalStorage::StaticSlot my_slot = TLS_INITIALIZER;
+  // If you're not using a static variable, use the convenience class
+  // ThreadLocalStorage::Slot (below) instead.
+  struct BASE_EXPORT StaticSlot {
     // Set up the TLS slot.  Called by the constructor.
     // 'destructor' is a pointer to a function to perform per-thread cleanup of
     // this object.  If set to NULL, no cleanup is done for this TLS slot.
     // Returns false on error.
     bool Initialize(TLSDestructorFunc destructor);
 
     // Free a previously allocated TLS 'slot'.
     // If a destructor was set for this slot, removes
     // the destructor so that remaining threads exiting
     // will not free data.
     void Free();
 
     // Get the thread-local value stored in slot 'slot'.
     // Values are guaranteed to initially be zero.
     void* Get() const;
 
     // Set the thread-local value stored in slot 'slot' to
     // value 'value'.
     void Set(void* value);
 
     bool initialized() const { return initialized_; }
 
-   private:
     // The internals of this struct should be considered private.
     bool initialized_;
 #if defined(OS_WIN)
     int slot_;
 #elif defined(OS_POSIX)
     pthread_key_t key_;
 #endif
 
+  };
+
+  // A convenience wrapper around StaticSlot with a constructor. Can be used
+  // as a member variable.
+  class BASE_EXPORT Slot : public StaticSlot {
+   public:
+    // Calls StaticSlot::Initialize().
+    explicit Slot(TLSDestructorFunc destructor = NULL);
+
+   private:
+    using StaticSlot::initialized_;
+#if defined(OS_WIN)
+    using StaticSlot::slot_;
+#elif defined(OS_POSIX)
+    using StaticSlot::key_;
+#endif
     DISALLOW_COPY_AND_ASSIGN(Slot);
   };
 
 #if defined(OS_WIN)
   // Function called when on thread exit to call TLS
   // destructor functions.  This function is used internally.
   static void ThreadExit();
 
  private:
   // Function to lazily initialize our thread local storage.
   static void **Initialize();
 
   static long tls_key_;
   static long tls_max_;
 #endif  // OS_WIN
 
   DISALLOW_COPY_AND_ASSIGN(ThreadLocalStorage);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_LOCAL_STORAGE_H_