Add helper for converting designated initializers to C++

Unfortunately, C++ does not support designated initializers.  Add a function
that helps fill their place.

Use example:

-static struct netisr_handler ether_nh = {
-       .nh_name = "ether",
-       .nh_handler = ether_nh_input,
-       .nh_proto = NETISR_ETHER,
-       .nh_policy = NETISR_POLICY_SOURCE,
-       .nh_dispatch = NETISR_DISPATCH_DIRECT,
-};
+static netisr_handler ether_nh = initialize_with([] (netisr_handler& x) {
+       x.nh_name = "ether";
+       x.nh_handler = ether_nh_input;
+       x.nh_proto = NETISR_ETHER;
+       x.nh_policy = NETISR_POLICY_SOURCE;
+       x.nh_dispatch = NETISR_DISPATCH_DIRECT;
+});

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

include/osv/initialize.hh

+/*
+ * Copyright (C) 2013 Cloudius Systems, Ltd.
+ *
+ * This work is open source software, licensed under the terms of the
+ * BSD license as described in the LICENSE file in the top-level directory.
+ */
+
+#ifndef INITIALIZE_HH_
+#define INITIALIZE_HH_
+
+#include <type_traits>
+
+// helper to find out the argument type of a function object
+template <typename T>
+struct first_argument_type_of;
+
+// special case: free function
+template <typename T, typename Ret>
+struct first_argument_type_of<Ret (T)> {
+    typedef T type;
+};
+
+// special case: member function pointer
+template <typename T1, typename T2, typename Ret>
+struct first_argument_type_of<Ret (T1::*)(T2)> {
+    typedef T2 type;
+};
+
+// special case: member function pointer (const)
+template <typename T1, typename T2, typename Ret>
+struct first_argument_type_of<Ret (T1::*)(T2) const> {
+    typedef T2 type;
+};
+
+// general case: class, look at operator()(T)
+template <typename T>
+struct first_argument_type_of {
+    typedef typename first_argument_type_of<decltype(&T::operator())>::type type;
+};
+
+template <typename T>
+using first_argument_no_ref = typename std::remove_reference<typename first_argument_type_of<T>::type>::type;
+
+/// \file Initialization helper for C designated initializers
+
+/// \fn T initialize(func f)
+/// \brief C compatibility for designated initializers
+///
+/// Unfortunately C++ does not support designated initializers; so
+/// this function helps fill its place by default-initializing an object
+/// and then calling a user supplied function (usually a lambda) to complete
+/// initialization.
+template <typename func>
+auto initialize_with(func f) -> first_argument_no_ref<func>
+{
+    first_argument_no_ref<func> val = {};
+    f(val);
+    return val;
+}
+
+
+#endif /* INITIALIZE_HH_ */