Add basic support for variable value inheritance

Currently, only abs_dir_path inherits from dir_path.

1 files changed, 9 insertions, 1 deletions

diff --git a/build2/variable b/build2/variable
index 14bf7af..ebf0f02 100644
--- a/build2/variable
+++ b/build2/variable
@@ -28,6 +28,14 @@ namespace build2
     const char* name;  // Type name for diagnostics.
     const size_t size; // Type size in value::data_ (only used for PODs).
 
+    // Base type, if any. We have very limited support for inheritance: a
+    // const value (but not non-const) can be cast to the base type. In
+    // particular, a derived/base value cannot be assigned to base/derived.
+    // If not NULL, then the cast function below is expected to return the
+    // base pointer if its second argument points to the base's value_type.
+    //
+    const value_type* base;
+
     // Destroy the value. If it is NULL, then the type is assumed to be POD
     // with a trivial destructor.
     //
@@ -59,7 +67,7 @@ namespace build2
     // static_cast to const T*. If it is NULL, then cast data_ directly. Note
     // that this function is used for both const and non-const values.
     //
-    const void* (*const cast) (const value&);
+    const void* (*const cast) (const value&, const value_type*);
 
     // If NULL, then the types are compared as PODs using memcmp().
     //