From 9ac9bb98f307156c02fa19ea747cf5eb1d2dcadf Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 4 Aug 2011 13:29:42 +0200
Subject: 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.
---
 odb/wrapper-traits.hxx | 153 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 153 insertions(+)
 create mode 100644 odb/wrapper-traits.hxx

(limited to 'odb/wrapper-traits.hxx')

diff --git a/odb/wrapper-traits.hxx b/odb/wrapper-traits.hxx
new file mode 100644
index 0000000..fdfb872
--- /dev/null
+++ b/odb/wrapper-traits.hxx
@@ -0,0 +1,153 @@
+// file      : odb/wrapper-traits.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_WRAPPER_TRAITS_HXX
+#define ODB_WRAPPER_TRAITS_HXX
+
+#include <odb/pre.hxx>
+
+#include <memory> // std::auto_ptr
+
+#include <odb/nullable.hxx>
+
+namespace odb
+{
+  template <typename T>
+  class wrapper_traits;
+
+  // Sample specialization for raw pointers. It is not enabled by default
+  // since it makes many assumptions that may not always hold true (such
+  // as that instances are allocated with new and freed with delete).
+  // This makes it too dangerous to be enable unconditionally. If you
+  // need this functionality, you can copy the below code into your
+  // application. Also consider changing it to only specialize for
+  // specific types instead of for any pointer (it will almost always
+  // do the wrong thing for char*).
+  //
+#if 0
+  template <typename T>
+  class wrapper_traits<T*>
+  {
+  public:
+    typedef T wrapped_type;
+    typedef T* wrapper_type;
+
+    static const bool null_handler = true;
+    static const bool null_default = false;
+
+    static bool
+    get_null (const wrapper_type& p)
+    {
+      return p == 0;
+    }
+
+    static void
+    set_null (wrapper_type& p)
+    {
+      delete p;
+      p = 0;
+    }
+
+    static const type&
+    get_ref (const wrapper_type& p)
+    {
+      return *p;
+    }
+
+    static type&
+    set_ref (wrapper_type& p)
+    {
+      if (p == 0)
+        p = new type;
+
+      return *p;
+    }
+  };
+#endif
+
+  // Specialization for std::auto_ptr.
+  //
+  template <typename T>
+  class wrapper_traits< std::auto_ptr<T> >
+  {
+  public:
+    typedef T wrapped_type;
+    typedef std::auto_ptr<T> wrapper_type;
+
+    static const bool null_handler = true;
+    static const bool null_default = false;
+
+    static bool
+    get_null (const wrapper_type& p)
+    {
+      return p.get () == 0;
+    }
+
+    static void
+    set_null (wrapper_type& p)
+    {
+      p.reset ();
+    }
+
+    static const wrapped_type&
+    get_ref (const wrapper_type& p)
+    {
+      return *p;
+    }
+
+    static wrapped_type&
+    set_ref (wrapper_type& p)
+    {
+      if (p.get () == 0)
+        p.reset (new wrapped_type);
+
+      return *p;
+    }
+  };
+
+  // Specialization for odb::nullable.
+  //
+  template <typename T>
+  class wrapper_traits< nullable<T> >
+  {
+  public:
+    typedef T wrapped_type;
+    typedef nullable<T> wrapper_type;
+
+    static const bool null_handler = true;
+    static const bool null_default = true;
+
+    static bool
+    get_null (const wrapper_type& n)
+    {
+      return n.null ();
+    }
+
+    static void
+    set_null (wrapper_type& n)
+    {
+      n.reset ();
+    }
+
+    static const wrapped_type&
+    get_ref (const wrapper_type& n)
+    {
+      return *n;
+    }
+
+    static wrapped_type&
+    set_ref (wrapper_type& n)
+    {
+      if (n.null ())
+        n = T ();
+
+      return *n;
+    }
+  };
+}
+
+#include <odb/post.hxx>
+
+#endif // ODB_WRAPPER_TRAITS_HXX
-- 
cgit v1.1