From 2c8294b069f5df30d281e0f4f580f6f313a50eb2 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Mon, 4 Dec 2017 12:39:33 +0200
Subject: Add cast_empty() for value casting

---
 build2/variable.ixx | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

(limited to 'build2/variable.ixx')

diff --git a/build2/variable.ixx b/build2/variable.ixx
index 6126a4e..634e3dc 100644
--- a/build2/variable.ixx
+++ b/build2/variable.ixx
@@ -187,21 +187,46 @@ namespace build2
   }
 
   template <typename T>
-  inline T
-  cast_false (const value& v) {return v && cast<T> (v);}
+  inline const T&
+  cast_empty (const value& v)
+  {
+    return v ? cast<T> (v) : value_traits<T>::empty_instance;
+  }
+
+  template <typename T>
+  inline const T&
+  cast_empty (const lookup& l)
+  {
+    return l ? cast<T> (l) : value_traits<T>::empty_instance;
+  }
 
   template <typename T>
   inline T
-  cast_false (const lookup& l) {return l && cast<T> (l);}
+  cast_false (const value& v)
+  {
+    return v && cast<T> (v);
+  }
 
   template <typename T>
   inline T
-  cast_true (const value& v) {return !v || cast<T> (v);}
+  cast_false (const lookup& l)
+  {
+    return l && cast<T> (l);
+  }
 
   template <typename T>
   inline T
-  cast_true (const lookup& l) {return !l || cast<T> (l);}
+  cast_true (const value& v)
+  {
+    return !v || cast<T> (v);
+  }
 
+  template <typename T>
+  inline T
+  cast_true (const lookup& l)
+  {
+    return !l || cast<T> (l);
+  }
 
   template <typename T>
   inline void
-- 
cgit v1.1