convert return value of wrapped C++ function with respect to class ptr_traits

A wrapped function may return a wrapped C++ object, so we need to use
appropriate converter specialization (e.g. `convert<std::shared_ptr<T>`) for
such a wrapped class `T`.

Reusing `call_from_v8_traits<F>::arg_converter<return_type, Traits>` meta-function
to get the converter type in `forward_ret()` for the certain function result type.

Extracted `call_from_v8_traits<F>::arg_converter<Arg, Traits>` to use a type
instead of argument index.

Added optional `Traits = raw_ptr_traits` template argument to `wrap_function()`
and `wrap_function_template()` in order to allow `forward_function()`
instantiation for the specified `Traits`

Work in context of issue #113

Author: pmed

v8pp/call_from_v8.hpp

@@ -39,15 +39,11 @@ struct call_from_v8_traits
 		using type = void;
 	};
 
-	template<size_t Index>
-	using arg_type = typename tuple_element<Index + is_mem_fun,
-		Index < (arg_count + Offset)>::type;
-
-	template<size_t Index, typename Traits, typename Arg = arg_type<Index>,
+	template<typename Arg, typename Traits,
 		typename T = typename std::remove_reference<Arg>::type,
 		typename U = typename std::remove_pointer<T>::type
 	>
-	using arg_convert = typename std::conditional<
+	using arg_converter = typename std::conditional<
 		is_wrapped_class<typename std::remove_cv<U>::type>::value,
 		typename std::conditional<
 			std::is_pointer<T>::value,
@@ -57,6 +53,13 @@ struct call_from_v8_traits
 		convert<typename std::remove_cv<T>::type>
 	>::type;
 
+	template<size_t Index>
+	using arg_type = typename tuple_element<Index + is_mem_fun,
+		Index < (arg_count + Offset)>::type;
+
+	template<size_t Index, typename Traits>
+	using arg_convert = arg_converter<arg_type<Index>, Traits>;
+
 	template<size_t Index, typename Traits>
 	static decltype(arg_convert<Index, Traits>::from_v8(std::declval<v8::Isolate*>(), std::declval<v8::Local<v8::Value>>()))
 	arg_from_v8(v8::FunctionCallbackInfo<v8::Value> const& args)

v8pp/function.hpp

@@ -153,7 +153,9 @@ void forward_ret(v8::FunctionCallbackInfo<v8::Value> const& args, std::true_type
 template<typename Traits, typename F>
 void forward_ret(v8::FunctionCallbackInfo<v8::Value> const& args, std::false_type /*is_void_return*/)
 {
-	args.GetReturnValue().Set(to_v8(args.GetIsolate(),
+	using return_type = typename function_traits<F>::return_type;
+	using converter = call_from_v8_traits<F>::arg_converter<return_type, Traits>;
+	args.GetReturnValue().Set(converter::to_v8(args.GetIsolate(),
 		invoke<Traits, F>(args, std::is_member_function_pointer<F>())));
 }
 
@@ -179,25 +181,25 @@ void forward_function(v8::FunctionCallbackInfo<v8::Value> const& args)
 } // namespace detail
 
 /// Wrap C++ function into new V8 function template
-template<typename F>
+template<typename F, typename Traits = raw_ptr_traits>
 v8::Local<v8::FunctionTemplate> wrap_function_template(v8::Isolate* isolate, F&& func)
 {
 	using F_type = typename std::decay<F>::type;
 	return v8::FunctionTemplate::New(isolate,
-		&detail::forward_function<raw_ptr_traits, F_type>,
+		&detail::forward_function<Traits, F_type>,
 		detail::set_external_data(isolate, std::forward<F_type>(func)));
 }
 
 /// Wrap C++ function into new V8 function
 /// Set nullptr or empty string for name
 /// to make the function anonymous
-template<typename F>
+template<typename F, typename Traits = raw_ptr_traits>
 v8::Local<v8::Function> wrap_function(v8::Isolate* isolate,
-	char const* name, F&& func)
+	char const* name, F && func)
 {
 	using F_type = typename std::decay<F>::type;
 	v8::Local<v8::Function> fn = v8::Function::New(isolate->GetCurrentContext(),
-		&detail::forward_function<raw_ptr_traits, F_type>,
+		&detail::forward_function<Traits, F_type>,
 		detail::set_external_data(isolate, std::forward<F_type>(func))).ToLocalChecked();
 	if (name && *name)
 	{