[core] make actor self reference optional

2 files changed, 46 insertions, 3 deletions

diff --git a/include/mbgl/actor/actor.hpp b/include/mbgl/actor/actor.hpp
index 00e1bb82f8..74d5a66285 100644
--- a/include/mbgl/actor/actor.hpp
+++ b/include/mbgl/actor/actor.hpp
@@ -7,6 +7,7 @@
 
 #include <memory>
 #include <future>
+#include <type_traits>
 
 namespace mbgl {
 
@@ -47,10 +48,21 @@ namespace mbgl {
 template <class Object>
 class Actor : public util::noncopyable {
 public:
-    template <class... Args>
+
+    // Enabled for Objects with a constructor taking ActorRef<Object> as the first parameter
+    template <typename U = Object, class... Args,
+            typename std::enable_if<std::is_constructible<U, ActorRef<Object>, Args...>::value>::type...>
+    Actor(Scheduler& scheduler, Args&&... args_)
+            : mailbox(std::make_shared<Mailbox>(scheduler)),
+              object(self(), std::forward<Args>(args_)...) {
+    }
+
+    // Enabled for plain Objects
+    template <typename U = Object, class... Args,
+            typename std::enable_if<!std::is_constructible<U, ActorRef<Object>, Args...>::value>::type...>
     Actor(Scheduler& scheduler, Args&&... args_)
-        : mailbox(std::make_shared<Mailbox>(scheduler)),
-          object(self(), std::forward<Args>(args_)...) {
+            : mailbox(std::make_shared<Mailbox>(scheduler)),
+              object(std::forward<Args>(args_)...) {
     }
 
     ~Actor() {
diff --git a/test/actor/actor.test.cpp b/test/actor/actor.test.cpp
index 2b4c83f566..39d7ff81f4 100644
--- a/test/actor/actor.test.cpp
+++ b/test/actor/actor.test.cpp
@@ -305,3 +305,34 @@ TEST(Actor, Ask) {
     ASSERT_EQ(std::future_status::ready, status);
     ASSERT_EQ(2, result.get());
 }
+
+TEST(Actor, NoSelfActorRef) {
+    // Not all actors need a reference to self
+
+    // Trivially constructable
+    struct Trivial {};
+
+    ThreadPool pool { 2 };
+    Actor<Trivial> trivial(pool);
+
+
+    // With arguments
+    struct WithArguments {
+        std::promise<void> promise;
+
+        WithArguments(std::promise<void> promise_)
+                : promise(std::move(promise_)) {
+        }
+
+        void receive() {
+            promise.set_value();
+        }
+    };
+
+    std::promise<void> promise;
+    auto future = promise.get_future();
+    Actor<WithArguments> withArguments(pool, std::move(promise));
+
+    withArguments.invoke(&WithArguments::receive);
+    future.wait();
+}