Add basic support for variable value inheritance

Currently, only abs_dir_path inherits from dir_path.

4 files changed, 34 insertions, 7 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().
     //
diff --git a/build2/variable.cxx b/build2/variable.cxx
index db65e5b..c4e2721 100644
--- a/build2/variable.cxx
+++ b/build2/variable.cxx
@@ -313,6 +313,7 @@ namespace build2
   {
     "bool",
     sizeof (bool),
+    nullptr,                      // No base.
     nullptr,                      // No dtor (POD).
     nullptr,                      // No copy_ctor (POD).
     nullptr,                      // No copy_assign (POD).
@@ -350,6 +351,7 @@ namespace build2
   {
     "uint64",
     sizeof (uint64_t),
+    nullptr,                          // No base.
     nullptr,                          // No dtor (POD).
     nullptr,                          // No copy_ctor (POD).
     nullptr,                          // No copy_assign (POD).
@@ -433,6 +435,7 @@ namespace build2
   {
     "string",
     sizeof (string),
+    nullptr,                      // No base.
     &default_dtor<string>,
     &default_copy_ctor<string>,
     &default_copy_assign<string>,
@@ -475,6 +478,7 @@ namespace build2
   {
     "path",
     sizeof (path),
+    nullptr,                    // No base.
     &default_dtor<path>,
     &default_copy_ctor<path>,
     &default_copy_assign<path>,
@@ -515,6 +519,7 @@ namespace build2
   {
     "dir_path",
     sizeof (dir_path),
+    nullptr,                        // No base, or should it be path?
     &default_dtor<dir_path>,
     &default_copy_ctor<dir_path>,
     &default_copy_assign<dir_path>,
@@ -545,14 +550,15 @@ namespace build2
   {
     "abs_dir_path",
     sizeof (abs_dir_path),
+    &value_traits<dir_path>::value_type, // Assume direct cast works for both.
     &default_dtor<abs_dir_path>,
     &default_copy_ctor<abs_dir_path>,
     &default_copy_assign<abs_dir_path>,
-    &simple_assign<abs_dir_path, true>, // Allow empty paths.
+    &simple_assign<abs_dir_path, true>,  // Allow empty paths.
     &simple_append<abs_dir_path, true>,
-    nullptr,                            // No prepend.
+    nullptr,                             // No prepend.
     &simple_reverse<abs_dir_path>,
-    nullptr,                            // No cast (cast data_ directly).
+    nullptr,                             // No cast (cast data_ directly).
     &simple_compare<abs_dir_path>
   };
 
@@ -577,6 +583,7 @@ namespace build2
   {
     "name",
     sizeof (name),
+    nullptr,                    // No base.
     &default_dtor<name>,
     &default_copy_ctor<name>,
     &default_copy_assign<name>,
diff --git a/build2/variable.ixx b/build2/variable.ixx
index e52a150..9b29608 100644
--- a/build2/variable.ixx
+++ b/build2/variable.ixx
@@ -81,17 +81,27 @@ namespace build2
   inline const T&
   cast (const value& v)
   {
-    assert (!v.null () && v.type == &value_traits<T>::value_type);
+    assert (!v.null ());
+
+    // Find base if any.
+    //
+    const value_type* b (v.type);
+    for (; b != nullptr && b != &value_traits<T>::value_type; b = b->base) ;
+    assert (b != nullptr);
+
     return *static_cast<const T*> (v.type->cast == nullptr
                                    ? static_cast<const void*> (&v.data_)
-                                   : v.type->cast (v));
+                                   : v.type->cast (v, b));
   }
 
   template <typename T>
   inline T&
   cast (value& v)
   {
-    return const_cast<T&> (cast<T> (static_cast<const value&> (v)));
+    assert (!v.null () && v.type == &value_traits<T>::value_type);
+    return *static_cast<T*> (v.type->cast == nullptr
+                             ? static_cast<void*> (&v.data_)
+                             : const_cast<void*> (v.type->cast (v, v.type)));
   }
 
   template <typename T>
diff --git a/build2/variable.txx b/build2/variable.txx
index bd7a18b..99ea79b 100644
--- a/build2/variable.txx
+++ b/build2/variable.txx
@@ -254,6 +254,7 @@ namespace build2
   {
     value_traits<vector<T>>::type_name.c_str (),
     sizeof (vector<T>),
+    nullptr,                          // No base.
     &default_dtor<vector<T>>,
     &default_copy_ctor<vector<T>>,
     &default_copy_assign<vector<T>>,
@@ -392,6 +393,7 @@ namespace build2
   {
     value_traits<map<K, V>>::type_name.c_str (),
     sizeof (map<K, V>),
+    nullptr,             // No base.
     &default_dtor<map<K, V>>,
     &default_copy_ctor<map<K, V>>,
     &default_copy_assign<map<K, V>>,