Distinguish between undefined and null variables

3 files changed, 51 insertions, 27 deletions

diff --git a/build/config/operation.cxx b/build/config/operation.cxx
index b85a5c7..ef83e47 100644
--- a/build/config/operation.cxx
+++ b/build/config/operation.cxx
@@ -99,13 +99,13 @@ namespace build
           //
           if (auto gval = (*global_scope)[var])
           {
-            if (!pval || !pval->compare (gval.as<const value&> ()))
+            if (pval == nullptr || !pval->compare (gval.as<const value&> ()))
               warn << "variable " << var.name << " configured value "
                    << "differs from command line value" <<
                 info << "reconfigure the project to use command line value";
           }
 
-          if (pval)
+          if (pval != nullptr)
           {
             //@@ TODO: assuming list
             //
@@ -116,8 +116,8 @@ namespace build
           }
           else
           {
-            ofs << var.name << " =" << endl; // @@ TODO: [undefined]
-            //text << var.name << " = [undefined]";
+            ofs << var.name << " = #[null]" << endl; // @@ TODO: [null]
+            //text << var.name << " = [null]";
           }
         }
       }
diff --git a/build/cxx/module.cxx b/build/cxx/module.cxx
index 2bfdc41..e29d311 100644
--- a/build/cxx/module.cxx
+++ b/build/cxx/module.cxx
@@ -104,41 +104,57 @@ namespace build
       //
       // These are optional so all we need to do is "import" them
       // into the root scope if they were specified on the command
-      // line and set them to empty if unspecified (the last part
+      // line and set them to NULL if unspecified (the last part
       // is important to distinguish between the "configured as
-      // undefined" and "not configured" cases).
+      // unspecified" and "not configured" cases).
       //
-      if (auto val = root["config.cxx.poptions"])
       {
-        if (val.scope == global_scope)
-          root.variables["config.cxx.poptions"] = val;
+        auto v (root["config.cxx.poptions"]);
+
+        if (v.defined ())
+        {
+          if (v.scope == global_scope)
+            root.variables["config.cxx.poptions"] = v;
+        }
+        else
+          root.variables["config.cxx.poptions"]; // Set to NULL.
       }
-      else
-        root.variables["config.cxx.poptions"]; // Undefined.
 
-      if (auto val = root["config.cxx.coptions"])
       {
-        if (val.scope == global_scope)
-          root.variables["config.cxx.coptions"] = val;
+        auto v (root["config.cxx.coptions"]);
+
+        if (v.defined ())
+        {
+          if (v.scope == global_scope)
+            root.variables["config.cxx.coptions"] = v;
+        }
+        else
+          root.variables["config.cxx.coptions"]; // Set to NULL.
       }
-      else
-        root.variables["config.cxx.coptions"]; // Undefined.
 
-      if (auto val = root["config.cxx.loptions"])
       {
-        if (val.scope == global_scope)
-          root.variables["config.cxx.loptions"] = val;
+        auto v (root["config.cxx.loptions"]);
+
+        if (v.defined ())
+        {
+          if (v.scope == global_scope)
+            root.variables["config.cxx.loptions"] = v;
+        }
+        else
+          root.variables["config.cxx.loptions"]; // Set to NULL.
       }
-      else
-        root.variables["config.cxx.loptions"]; // Undefined.
 
-      if (auto val = root["config.cxx.libs"])
       {
-        if (val.scope == global_scope)
-          root.variables["config.cxx.libs"] = val;
+        auto v (root["config.cxx.libs"]);
+
+        if (v.defined ())
+        {
+          if (v.scope == global_scope)
+            root.variables["config.cxx.libs"] = v;
+        }
+        else
+          root.variables["config.cxx.libs"]; // Set to NULL.
       }
-      else
-        root.variables["config.cxx.libs"]; // Undefined.
     }
   }
 }
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;