Add support for returning a pre-existing foreign object from a foreign method

This is potentially subject to change in the near
future. This commit adds support for returning
an instance (currently wrapped in ForeignObject<>)
of a foreign object that is already instantiated.
The object returned this way simply gets selected
by handle into slot 0 of C Wren.
Conversely, returning just a naked object causes
wrenpp to make a Wren-accessible copy of it.

examples/calendar/main.cpp

@@ -128,7 +128,7 @@ int main() {
 
 	vm.interpret("main", g_test_script);
 
-	Calendar* const cale = std::get<0>(wren::variables<Calendar>(vm, MN<"main", "cale">));
+	Calendar* const cale = std::get<0>(wren::variables<Calendar>(vm, MN<"main", "cale">)).data();
 	cale->print_appointments();
 
 	return 0;

examples/cpp_calls/main.cpp

@@ -52,10 +52,19 @@ foreign class ClassB {
 	}
 
 	foreign greeting_message(first_name, family_name, emoji)
+	foreign set_obj_c (obj)
+	foreign get_obj_c()
 }
 )script";
 
-//This is three examples in one:
+constexpr char g_module_c[] =
+"foreign class ClassC {" R"script(
+	construct new (msg) {}
+	foreign print_message()
+}
+)script";
+
+//This is several examples in one:
 //1. instantiate ClassA in Wren and pass it to do_action() which is a
 //   non-foreign method in wren
 //2. instantiate ClassA in C++, have C++ invoke the non-foreign method
@@ -64,14 +73,21 @@ foreign class ClassB {
 //3. instantiate ClassB in C++, have C++ invoke the non-foreign method
 //   do_action() on it and pass the same ClassA instance from the previous
 //   step to it
+//4. show how to return a pre-existing instance of ClassC from ClassB both in
+//   C++ and Wren
 constexpr char g_script[] = ""
 R"script(
 import "module_a" for ClassA
 import "module_b" for ClassB
+import "module_c" for ClassC
 
 var obj_b = ClassB.new("TheWren")
 obj_b.do_action(ClassA.new("instanciated from script"), 2)
 obj_b.greeting_message("Jane", "Doe", "ʕ·͡ᴥ·ʔ")
+
+var obj_c = ClassC.new("Message of ClassC object instantiated in Wren")
+obj_b.set_obj_c(obj_c)
+obj_b.get_obj_c().print_message()
 )script";
 
 class MyWrenConfig : public wren::DefConfiguration {
@@ -81,6 +97,8 @@ public:
 			return copied(g_module_a, sizeof(g_module_a));
 		else if (module_name == "module_b")
 			return copied(g_module_b, sizeof(g_module_b));
+		else if (module_name == "module_c")
+			return copied(g_module_c, sizeof(g_module_c));
 		return nullptr;
 	}
 
@@ -112,6 +130,8 @@ struct ClassA {
 	std::string m_message;
 };
 
+class ClassC;
+
 class ClassB {
 public:
 	explicit ClassB (std::string nickname) : m_nickname(nickname) {
@@ -130,8 +150,23 @@ public:
 			m_nickname << "' " << family_name << "\" " << emoji << '\n';
 	}
 
+	void set_obj_c (wren::ForeignObject<ClassC> obj) {
+		m_obj_c = std::move(obj);
+	}
+	wren::ForeignObject<ClassC>* get_obj_c() { return &m_obj_c; }
+
 private:
 	std::string m_nickname;
+	wren::ForeignObject<ClassC> m_obj_c;
+};
+
+class ClassC {
+public:
+	ClassC (std::string msg) : m_message(msg) {}
+	void print_message() const { std::cout << m_message << '\n'; }
+
+private:
+	std::string m_message;
 };
 } //unnamed namespace
 
@@ -149,10 +184,14 @@ int main() {
 	vm.callback_manager()
 		.add_callback(false, "module_a", "ClassA", "say_hi()", make_function_bindable<&ClassA::say_hi>)
 		.add_callback(false, "module_b", "ClassB", "greeting_message(_,_,_)", make_function_bindable<&ClassB::greeting_message>)
+		.add_callback(false, "module_b", "ClassB", "set_obj_c(_)", make_function_bindable<&ClassB::set_obj_c>)
+		.add_callback(false, "module_b", "ClassB", "get_obj_c()", make_function_bindable<&ClassB::get_obj_c>)
+		.add_callback(false, "module_c", "ClassC", "print_message()", make_function_bindable<&ClassC::print_message>)
 	;
 	vm.class_manager()
 		.add_class_maker("module_b", "ClassB", make_foreign_class<ClassB, std::string>)
 		.add_class_maker("module_a", "ClassA", make_foreign_class<ClassA, std::string>)
+		.add_class_maker("module_c", "ClassC", make_foreign_class<ClassC, std::string>)
 	;
 
 	//Example 1: invoke obj_b.do_action() from Wren passing an obj_a that's
@@ -169,6 +208,11 @@ int main() {
 	auto obj_b = make_wren_object<ClassB>(vm, wren::MN<"module_b", "ClassB">, "TheCpp");
 	wren::call<void>(vm, obj_b, "do_action", obj_a, 4);
 
+	//Example 4: set obj_c on obj_c and query it back
+	wren::ForeignObject<ClassC> obj_c = make_wren_object<ClassC>(vm, wren::MN<"module_c", "ClassC">, "Message of ClassC object instantiated in C++");
+	obj_b.object().set_obj_c(std::move(obj_c));
+	obj_b.object().get_obj_c()->object().print_message();
+
 	std::cout << "Quitting in 1 sec" << std::endl;
 	std::this_thread::sleep_for(std::chrono::milliseconds(1000));
 	return 0;

examples/cpp_calls/meson.build

@@ -1,4 +1,4 @@
-executable('call_cpp',
+executable('cpp_calls',
   'main.cpp',
   dependencies: wrenpp_dep,
   install: false,

examples/meson.build

@@ -2,4 +2,4 @@ subdir('dieroll')
 subdir('greet')
 subdir('calendar')
 subdir('math_vector')
-subdir('call_cpp')
+subdir('cpp_calls')