From 5af9070fda0dba591264ed675920efcfd62e81bc Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 8 Apr 2015 16:09:31 +0200
Subject: Distinguish between undefined and null variables

---
 build/variable | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

(limited to 'build/variable')

diff --git a/build/variable b/build/variable
index fc0ba2b..67ec33c 100644
--- a/build/variable
+++ b/build/variable
@@ -90,11 +90,19 @@ namespace build
 
   // value_proxy
   //
+  // A variable can be undefined, null, or contain some actual value.
+  //
   struct value_proxy
   {
     typedef build::scope scope_type;
 
-    explicit operator bool () const {return p != nullptr && *p != nullptr;}
+    bool
+    defined () const {return p != nullptr;}
+
+    bool
+    null () const {return *p == nullptr;}
+
+    explicit operator bool () const {return defined () && !null ();}
     explicit operator value_ptr& () const {return *p;}
 
     scope_type* scope;
-- 
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