Add support for value wrappers

Wrapper is a class that wraps another type. Examples of wrappers are
various smart pointers, holders, etc. A wrapper can be transparent or
it can handle the NULL semantics.

The new odb::nullable class template is a NULL wrapper that helps
to add the NULL semantics to a value type.

New test: common/wrapper.

1 files changed, 40 insertions, 0 deletions

diff --git a/odb/details/wrapper-p.hxx b/odb/details/wrapper-p.hxx
new file mode 100644
index 0000000..7736fc9
--- /dev/null
+++ b/odb/details/wrapper-p.hxx
@@ -0,0 +1,40 @@
+// file      : odb/details/wrapper-p.hxx
+// author    : Boris Kolpackov <boris@codesynthesis.com>
+// copyright : Copyright (c) 2009-2011 Code Synthesis Tools CC
+// license   : GNU GPL v2; see accompanying LICENSE file
+
+#ifndef ODB_DETAILS_WRAPPER_P_HXX
+#define ODB_DETAILS_WRAPPER_P_HXX
+
+#include <odb/pre.hxx>
+
+#include <odb/wrapper-traits.hxx>
+
+#include <odb/details/meta/answer.hxx>
+
+namespace odb
+{
+  namespace details
+  {
+    // GCC doesn't like these to be inside wrapper_p.
+    //
+    template <typename T>
+    meta::no
+    wrapper_p_test (...);
+
+    template <typename T>
+    meta::yes
+    wrapper_p_test (typename wrapper_traits<T>::wrapped_type*);
+
+    template <typename T>
+    struct wrapper_p
+    {
+      static const bool r =
+        sizeof (wrapper_p_test<T> (0)) == sizeof (meta::yes);
+    };
+  }
+}
+
+#include <odb/post.hxx>
+
+#endif // ODB_DETAILS_WRAPPER_P_HXX