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')

include/wrenpp/detail/deref_ifn.hpp

@@ -0,0 +1,42 @@
+/* Copyright 2020-2024, Michele Santullo
+ * This file is part of wrenpp.
+ *
+ * Wrenpp is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Wrenpp is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with wrenpp.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <type_traits>
+
+namespace wren::detail {
+
+//Dereferences the given parameter if necessary, always returning an optionally
+//cv-qualified ref
+template <typename T>
+struct DerefIFN {
+	static typename std::remove_reference<T>::type& deref (
+		typename std::remove_reference<T>::type& param
+	) {
+		return param;
+	}
+};
+
+template <typename T>
+struct DerefIFN<T*> {
+	static T& deref (T* param) { return *param; }
+};
+
+//Add specialisations for std::unique_ptr & co?
+
+} //namespace wren::detail

include/wrenpp/detail/foreign_object.hpp

@@ -22,6 +22,30 @@
 
 namespace wren {
 
+class VM;
+
+//This is meant to be a short lived object representing a still-in-a-slot
+//foreign object. Upon casting it to ForeignObject it will create a handle to
+//the data in given slot
+template <typename T>
+class TempForeignObject {
+public:
+	TempForeignObject (VM* vm, T* object, int slot_num) :
+		m_vm(vm),
+		m_data(object),
+		m_slot_num(slot_num)
+	{ }
+
+	VM* vm() const { return m_vm; }
+	T* data() const { return m_data; }
+	int slot_num() const { return m_slot_num; }
+
+private:
+	VM* m_vm;
+	T* m_data;
+	int m_slot_num;
+};
+
 template <typename T>
 class ForeignObject {
 public:
@@ -29,6 +53,7 @@ public:
 	ForeignObject (std::nullptr_t) : ForeignObject() {}
 	ForeignObject (T* obj, Handle&& handle);
 	ForeignObject (ForeignObject&&) = default;
+	ForeignObject (const TempForeignObject<T>& temp_object);
 	~ForeignObject() noexcept = default;
 
 	ForeignObject& operator= (ForeignObject&&) = default;
@@ -58,6 +83,12 @@ inline ForeignObject<T>::ForeignObject (T* obj, Handle&& handle) :
 	m_object(obj)
 { }
 
+template <typename T>
+inline ForeignObject<T>::ForeignObject (const TempForeignObject<T>& temp_object) :
+	m_handle(temp_object.vm(), temp_object.slot_num()),
+	m_object(temp_object.data())
+{ }
+
 template <typename T>
 inline void ForeignObject<T>::set_to_slot (int num) {
 	m_handle.set_to_slot(num);

include/wrenpp/detail/is_kindof_handle.hpp

@@ -0,0 +1,41 @@
+/* Copyright 2020-2024, Michele Santullo
+ * This file is part of wrenpp.
+ *
+ * Wrenpp is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Wrenpp is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with wrenpp.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "deref_ifn.hpp"
+
+namespace wren::detail {
+
+template <typename T, typename TRaw>
+struct IsKindOfHandleHelper {
+	static constexpr bool value = false;
+};
+template <typename TRaw>
+struct IsKindOfHandleHelper<Handle, TRaw> {
+	static constexpr bool value = true;
+	static void set_to_slot (int slot_num, TRaw obj) {DerefIFN<TRaw>::deref(obj).set_to_slot(slot_num);}
+};
+template <typename U, typename TRaw>
+struct IsKindOfHandleHelper<ForeignObject<U>, TRaw> {
+	static constexpr bool value = true;
+	static void set_to_slot (int slot_num, TRaw obj) {DerefIFN<TRaw>::deref(obj).set_to_slot(slot_num);}
+};
+template <typename T>
+using IsKindOfHandle = IsKindOfHandleHelper<typename std::remove_pointer<typename std::remove_cvref<T>::type>::type, T>;
+
+} //namespace wren::detail

include/wrenpp/detail/setters_getters.hpp

@@ -19,7 +19,9 @@
 
 #include "../vm.hpp"
 #include "../handle.hpp"
+#include "foreign_object.hpp"
 #include "module_and_name.hpp"
+#include "is_kindof_handle.hpp"
 #include <string_view>
 #include <vector>
 #include <tuple>
@@ -34,8 +36,14 @@ namespace wren {
 		template<typename T>struct GetTypeToRetType{
 			static_assert(not std::is_fundamental_v<T>, "User type expected");
 			static_assert(not std::is_pointer_v<T>, "Unexpected pointer type");
-			typedef std::remove_cv_t<T>* type;
+			typedef TempForeignObject< std::remove_cv_t<T> > type;
 		};
+		template<typename T>struct GetTypeToRetType<ForeignObject<T>> {
+			//This doesn't make much sense, essentially you're asking wrenpp to
+			//give you a foreign object by vm/slot when you already have its
+			//handle
+		};
+		template<typename T>struct GetTypeToRetType<TempForeignObject<T>> : TType<TempForeignObject<T>>{};
 		template<>struct GetTypeToRetType<const char*> : TType<const char*>{};
 		template<>struct GetTypeToRetType<double> : TType<double>{};
 		template<>struct GetTypeToRetType<bool> : TType<bool>{};
@@ -166,16 +174,16 @@ namespace wren {
 
 	template <typename T>
 	inline detail::GetTypeToRetType_t<T> get (VM& vm, int slot_num) {
-		return foreign<T>(vm, slot_num);
+		return {&vm, foreign<T>(vm, slot_num), slot_num};
 	}
 
 	template<> const char* get<const char*> (VM& vm, int slot_num);
-	template <> double get<double> (VM& vm, int slot_num);
-	template <> bool get<bool> (VM& vm, int slot_num);
-	template <> std::pair<const char*, int> get<std::pair<const char*,int>> (VM& vm, int slot_num);
+	template<> double get<double> (VM& vm, int slot_num);
+	template<> bool get<bool> (VM& vm, int slot_num);
+	template<> std::pair<const char*, int> get<std::pair<const char*,int>> (VM& vm, int slot_num);
 	template<> int get<int> (VM& vm, int slot_num);
 	template<> std::size_t get<std::size_t> (VM& vm, int slot_num);
 	template<> std::string get<std::string> (VM& vm, int slot_num);
-	template <> std::string_view get<std::string_view> (VM& vm, int slot_num);
-	template <> std::vector<char> get<std::vector<char>> (VM& vm, int slot_num);
+	template<> std::string_view get<std::string_view> (VM& vm, int slot_num);
+	template<> std::vector<char> get<std::vector<char>> (VM& vm, int slot_num);
 } //namespace wren

include/wrenpp/handle.hpp

@@ -27,6 +27,7 @@ namespace wren {
 		Handle() noexcept;
 		Handle (const Handle&) = delete;
 		Handle (Handle&& other) noexcept;
+		Handle (VM* vm, int slot_num);
 		Handle (VM* vm, WrenHandle* handle) noexcept :
 			m_handle(handle),
 			m_vm(vm)

include/wrenpp/vm_fun.hpp

@@ -26,6 +26,7 @@
 #include "detail/construct_foreign_class.hpp"
 #include "detail/setters_getters.hpp"
 #include "detail/wren_class_name_from_type.hpp"
+#include "detail/is_kindof_handle.hpp"
 #include "detail/foreign_object.hpp"
 #include <string>
 #include <string_view>
@@ -62,6 +63,13 @@ namespace wren {
 	void abort_fiber_with_error (VM& vm, int slot, std::string_view message);
 
 	namespace detail {
+		template <typename T>
+		struct CleanParamForGet { typedef typename std::decay<T>::type type; };
+		template <typename T>
+		struct CleanParamForGet<ForeignObject<T>> { typedef T type; };
+		template <typename T>
+		using CleanParamForGetT = typename CleanParamForGet<T>::type;
+
 		//Sets arguments so wren is ready to perform a function call. It doesn't
 		//touch at slot 0, so eventual objects for method calls must be set
 		//manually
@@ -82,8 +90,8 @@ namespace wren {
 			template <int... Indices>
 			static R call_implem (std::integer_sequence<int, Indices...>, VM& vm) {
 				static_assert(sizeof...(Indices) == sizeof...(Args), "Mismatching argument count");
-				T* obj = foreign<T>(vm, 0);
-				return (obj->*Method)(get<std::decay_t<Args>>(vm, Indices + 1)...);
+				T* const obj = foreign<T>(vm, 0);
+				return (obj->*Method)(get<CleanParamForGetT<Args>>(vm, Indices + 1)...);
 			}
 		};
 		template <typename T, typename R, typename... Args, R(T::*ConstMethod)(Args...)const>
@@ -95,8 +103,8 @@ namespace wren {
 			template <int... Indices>
 			static R call_implem (std::integer_sequence<int, Indices...>, VM& vm) {
 				static_assert(sizeof...(Indices) == sizeof...(Args), "Mismatching argument count");
-				T* obj = foreign<T>(vm, 0);
-				return (obj->*ConstMethod)(get<std::decay_t<Args>>(vm, Indices + 1)...);
+				T* const obj = foreign<T>(vm, 0);
+				return (obj->*ConstMethod)(get<CleanParamForGetT<Args>>(vm, Indices + 1)...);
 			}
 		};
 		template <typename R, typename... Args, R(*Function)(Args...)>
@@ -108,7 +116,7 @@ namespace wren {
 			template <int... Indices>
 			static R call_implem (std::integer_sequence<int, Indices...>, VM& vm) {
 				static_assert(sizeof...(Indices) == sizeof...(Args), "Mismatching argument count");
-				return (*Function)(get<std::decay_t<Args>>(vm, Indices + 1)...);
+				return (*Function)(get<CleanParamForGetT<Args>>(vm, Indices + 1)...);
 			}
 		};
 
@@ -125,10 +133,14 @@ namespace wren {
 					else {
 						auto ret = CallImplemProvider<V>::call_implem(std::make_integer_sequence<int, argument_count>(), vm);
 						//TODO: check for errors
-						vm.ensure_slots(1);
+						assert(vm.slot_count() >= 1); //vm.ensure_slots(1);
 						if constexpr (std::is_fundamental<R>::value) {
 							set(vm, 0, ret);
 						}
+						else if constexpr (IsKindOfHandle<R>::value) {
+							IsKindOfHandle<R>::set_to_slot(0, ret);
+							assert(vm.slot_type(0) == SlotType::Foreign);
+						}
 						else {
 							ModuleAndName wren_name = wren_class_name_from_type<R>(vm);
 							ForeignObject<R> new_object = make_wren_object<R>(vm, wren_name, std::move(ret));

src/handle.cpp

@@ -21,6 +21,11 @@
 #include <ciso646>
 
 namespace wren {
+	Handle::Handle (VM* vm, int slot_num) :
+		Handle(vm->slot_handle(slot_num))
+	{
+	}
+
 	Handle::~Handle() noexcept {
 		release();
 	}

src/meson.build

@@ -19,6 +19,7 @@ wrenpp = library(meson.project_name(),
   'class_manager.cpp',
   'wren_class_name_from_type.cpp',
   'module_and_name.cpp',
+  'setters_getters.cpp',
   dependencies: [wren_dep, crc32_dep],
   include_directories: public_incl,
   install: (not meson.is_subproject() or get_option('default_library')=='shared'),

src/setters_getters.cpp

@@ -0,0 +1,21 @@
+/* Copyright 2020-2024, Michele Santullo
+ * This file is part of wrenpp.
+ *
+ * Wrenpp is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Wrenpp is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with wrenpp.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "wrenpp/detail/setters_getters.hpp"
+
+namespace wren {
+} //namespace wren