Fix #201: Suggestion: class Inheritance (#641)

* Fix #201: Add class inheritance support with Derived : Base syntax

Classes can now inherit methods and attributes from a base class using
C++-style syntax: `class Derived : Base { ... }`. Base class methods and
attributes are automatically available on derived objects. Derived classes
can override base methods by defining a method with the same name.
Inheritance relationships are tracked to support proper type matching
in the dispatch system.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Address review: use implicit derived-to-base matching instead of copying base class functions

Instead of copying all base class methods/attributes into derived classes,
make the type matching system recognize inheritance relationships. Base class
methods now naturally match derived objects through dynamic_object_typename_match,
and dispatch ordering ensures derived overrides are preferred over base methods.

This is simpler (net -25 lines) and avoids duplicating function registrations.

Requested by @lefticus in PR #641 review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Add tests for passing derived objects to functions expecting Base

Tests cover: free functions calling base methods on derived objects,
polymorphic dispatch through containers, base attribute access on
derived objects, and multi-level inheritance (GrandChild : Derived : Base).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Add typed parameter tests for class inheritance

Use typed function signatures (e.g., `def call_do_something(Base obj)`)
instead of untyped parameters to test that derived objects are accepted
by functions expecting a base type, with correct polymorphic dispatch.

Requested by @lefticus in PR #641 review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: leftibot <leftibot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/dispatchkit/dispatchkit.hpp

@@ -1060,6 +1060,19 @@ namespace chaiscript {
           return true;
         }
 
+        // Sort more-derived Dynamic_Object types before base types so that
+        // overridden methods in derived classes are tried first during dispatch
+        const auto &lhs_dotn = lhs->dynamic_object_type_name();
+        const auto &rhs_dotn = rhs->dynamic_object_type_name();
+        if (!lhs_dotn.empty() && !rhs_dotn.empty() && lhs_dotn != rhs_dotn) {
+          if (dispatch::Dynamic_Object::type_matches(lhs_dotn, rhs_dotn)) {
+            return true; // lhs is derived from rhs, so lhs is more specific
+          }
+          if (dispatch::Dynamic_Object::type_matches(rhs_dotn, lhs_dotn)) {
+            return false; // rhs is derived from lhs, so rhs is more specific
+          }
+        }
+
         const auto &lhsparamtypes = lhs->get_param_types();
         const auto &rhsparamtypes = rhs->get_param_types();
 

include/chaiscript/dispatchkit/dynamic_object.hpp

@@ -44,6 +44,28 @@ namespace chaiscript {
 
       Dynamic_Object() = default;
 
+      /// Register that derived_name inherits from base_name
+      static void register_inheritance(const std::string &derived_name, const std::string &base_name) {
+        inheritance_map()[derived_name] = base_name;
+      }
+
+      /// Check if type_name is, or inherits from, base_name
+      static bool type_matches(const std::string &type_name, const std::string &base_name) noexcept {
+        if (type_name == base_name) {
+          return true;
+        }
+        const auto &m = inheritance_map();
+        auto it = m.find(type_name);
+        while (it != m.end()) {
+          if (it->second == base_name) {
+            return true;
+          }
+          it = m.find(it->second);
+        }
+        return false;
+      }
+
+
       bool is_explicit() const noexcept { return m_option_explicit; }
 
       void set_explicit(const bool t_explicit) noexcept { m_option_explicit = t_explicit; }
@@ -87,6 +109,11 @@ namespace chaiscript {
       std::map<std::string, Boxed_Value> get_attrs() const { return m_attrs; }
 
     private:
+      static std::map<std::string, std::string> &inheritance_map() {
+        static std::map<std::string, std::string> s_map;
+        return s_map;
+      }
+
       const std::string m_type_name = "";
       bool m_option_explicit = false;
 

include/chaiscript/dispatchkit/dynamic_object_detail.hpp

@@ -72,6 +72,8 @@ namespace chaiscript {
 
         bool is_attribute_function() const noexcept override { return m_is_attribute; }
 
+        const std::string &dynamic_object_type_name() const noexcept override { return m_type_name; }
+
         bool call_match(const chaiscript::Function_Params &vals, const Type_Conversions_State &t_conversions) const noexcept override {
           if (dynamic_object_typename_match(vals, m_type_name, m_ti, t_conversions)) {
             return m_func->call_match(vals, t_conversions);
@@ -112,7 +114,7 @@ namespace chaiscript {
           if (bv.get_type_info().bare_equal(m_doti)) {
             try {
               const Dynamic_Object &d = boxed_cast<const Dynamic_Object &>(bv, &t_conversions);
-              return name == "Dynamic_Object" || d.get_type_name() == name;
+              return name == "Dynamic_Object" || Dynamic_Object::type_matches(d.get_type_name(), name);
             } catch (const std::bad_cast &) {
               return false;
             }

include/chaiscript/dispatchkit/proxy_functions.hpp

@@ -120,7 +120,7 @@ namespace chaiscript {
             if (bv.get_type_info().bare_equal(dynamic_object_type_info)) {
               try {
                 const Dynamic_Object &d = boxed_cast<const Dynamic_Object &>(bv, &t_conversions);
-                if (!(name == "Dynamic_Object" || d.get_type_name() == name)) {
+                if (!(name == "Dynamic_Object" || Dynamic_Object::type_matches(d.get_type_name(), name))) {
                   return std::make_pair(false, false);
                 }
               } catch (const std::bad_cast &) {
@@ -233,6 +233,12 @@ namespace chaiscript {
         }
       }
 
+      /// Returns the Dynamic_Object type name this function is bound to, or empty string if not a Dynamic_Object function
+      virtual const std::string &dynamic_object_type_name() const noexcept {
+        static const std::string empty;
+        return empty;
+      }
+
       virtual bool compare_first_type(const Boxed_Value &bv, const Type_Conversions_State &t_conversions) const noexcept {
         /// TODO is m_types guaranteed to be at least 2??
         return compare_type_to_param(m_types[1], bv, t_conversions);

include/chaiscript/language/chaiscript_eval.hpp

@@ -828,11 +828,23 @@ namespace chaiscript {
       Boxed_Value eval_internal(const chaiscript::detail::Dispatch_State &t_ss) const override {
         chaiscript::eval::detail::Scope_Push_Pop spp(t_ss);
 
+        const auto &class_name = this->children[0]->text;
+
         /// \todo do this better
         // put class name in current scope so it can be looked up by the attrs and methods
-        t_ss.add_object("_current_class_name", const_var(this->children[0]->text));
+        t_ss.add_object("_current_class_name", const_var(class_name));
 
-        this->children[1]->eval(t_ss);
+        const bool has_base_class = (this->children.size() == 3);
+        const auto &block = has_base_class ? this->children[2] : this->children[1];
+
+        // Register inheritance before evaluating the class body so that
+        // function dispatch ordering can account for the relationship
+        if (has_base_class) {
+          const auto &base_name = this->children[1]->text;
+          dispatch::Dynamic_Object::register_inheritance(class_name, base_name);
+        }
+
+        block->eval(t_ss);
 
         return void_var();
       }

include/chaiscript/language/chaiscript_parser.hpp

@@ -1908,6 +1908,13 @@ namespace chaiscript {
 
           const auto class_name = m_match_stack.back()->text;
 
+          // Optionally parse ': BaseClassName' for inheritance
+          if (Char(':')) {
+            if (!Id(true)) {
+              throw exception::eval_error("Missing base class name in definition", File_Position(m_position.line, m_position.col), *m_filename);
+            }
+          }
+
           while (Eol()) {
           }
 

unittests/class_inheritance.chai

@@ -0,0 +1,137 @@
+
+class Base
+{
+  attr x
+
+  def Base()
+  {
+    this.x = 10
+  }
+
+  def do_something()
+  {
+    return this.x * 2
+  }
+}
+
+class Derived : Base
+{
+  attr y
+
+  def Derived()
+  {
+    this.x = 10
+    this.y = 20
+  }
+
+  def do_other()
+  {
+    return this.y * 3
+  }
+}
+
+// Test basic inheritance - derived can call base methods
+auto d = Derived()
+assert_equal(10, d.x)
+assert_equal(20, d.y)
+assert_equal(20, d.do_something())
+assert_equal(60, d.do_other())
+
+// Test base class still works independently
+auto b = Base()
+assert_equal(10, b.x)
+assert_equal(20, b.do_something())
+
+// Test method override
+class Derived2 : Base
+{
+  def Derived2()
+  {
+    this.x = 5
+  }
+
+  def do_something()
+  {
+    return this.x * 100
+  }
+}
+
+auto d2 = Derived2()
+assert_equal(500, d2.do_something())
+
+// Test passing a derived object to an untyped free function
+def call_do_something_untyped(obj) {
+  return obj.do_something()
+}
+
+auto d3 = Derived()
+assert_equal(20, call_do_something_untyped(d3))
+assert_equal(20, call_do_something_untyped(Base()))
+
+// Test typed functions: parameter declared as Base, accepts derived objects
+def call_do_something(Base obj) {
+  return obj.do_something()
+}
+
+assert_equal(20, call_do_something(Base()))
+assert_equal(20, call_do_something(d3))
+
+// Test typed function accessing base attributes on a derived object
+def get_x(Base obj) {
+  return obj.x
+}
+
+assert_equal(10, get_x(d3))
+assert_equal(10, get_x(Base()))
+
+// Test polymorphic dispatch through typed function: derived override is called
+auto d4 = Derived2()
+assert_equal(500, call_do_something(d4))
+
+// Test mixing base and derived in a container, calling base methods
+var objects = [Base(), Derived(), Derived2()]
+assert_equal(20, objects[0].do_something())
+assert_equal(20, objects[1].do_something())
+assert_equal(500, objects[2].do_something())
+
+// Test that derived objects still report correct type
+auto d5 = Derived()
+assert_true(d5.is_type("Derived"))
+
+// Test multi-level inheritance
+class GrandChild : Derived
+{
+  attr z
+
+  def GrandChild()
+  {
+    this.x = 1
+    this.y = 2
+    this.z = 3
+  }
+
+  def do_grandchild()
+  {
+    return this.z * 4
+  }
+}
+
+auto gc = GrandChild()
+assert_equal(1, gc.x)
+assert_equal(2, gc.y)
+assert_equal(3, gc.z)
+assert_equal(2, gc.do_something())   // Base method
+assert_equal(6, gc.do_other())       // Derived method
+assert_equal(12, gc.do_grandchild()) // Own method
+
+// Test passing grandchild to typed Base function (multi-level inheritance)
+assert_equal(2, call_do_something(gc))
+assert_equal(1, get_x(gc))
+
+// Test typed function expecting mid-level type
+def call_do_other(Derived obj) {
+  return obj.do_other()
+}
+
+assert_equal(6, call_do_other(gc))
+assert_equal(60, call_do_other(Derived()))