base: Add a new type of CallbackQueue.

This type is templated on what arguments the callbacks in it accept, and
it inherits directly from std::list instead of containing one and
forwarding selected members.

This version is called CallbackQueue2, but once all CallbackQueue
instances have been replaced it will be renamed to CallbackQueue.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: I32ab7454ea8c6a2af31cbcf5d4932a069ace1cb5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32642
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/callback.hh

@@ -29,6 +29,7 @@
 #ifndef __BASE_CALLBACK_HH__
 #define __BASE_CALLBACK_HH__
 
+#include <functional>
 #include <list>
 #include <string>
 
@@ -79,6 +80,21 @@ class MakeCallback : public Callback
     void process() { (object->*F)(); }
 };
 
+class CallbackQueue2 : public std::list<std::function<void()>>
+{
+  public:
+    using Base = std::list<std::function<void()>>;
+
+    using Base::Base;
+
+    void
+    process()
+    {
+        for (auto &f: *this)
+            f();
+    }
+};
+
 class CallbackQueue
 {
   protected: