Pass a marker struct instead of an empty array

src/common/communication.cpp

@@ -18,7 +18,7 @@ intptr_t send_event(boost::asio::local::stream_protocol::socket& socket,
         if (c_string[0] != 0) {
             payload = std::string(c_string);
         } else {
-            payload = std::array<char, max_string_length>();
+            payload = NeedsBuffer{};
         }
     }
 

src/common/communication.h

@@ -164,14 +164,15 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
                          event.payload, event.option);
     }
 
+    // Some buffer for the event to write to if needed. We only pass around a
+    // marker struct to indicate that this is indeed the case.
+    std::array<char, max_string_length> buffer;
     void* data = std::visit(
         overload{[&](const std::nullptr_t&) -> void* { return nullptr; },
                  [&](const std::string& s) -> void* {
                      return const_cast<char*>(s.c_str());
                  },
-                 [&](std::array<char, max_string_length>& buffer) -> void* {
+                 [&](NeedsBuffer&) -> void* { return buffer.data(); }},
-                     return buffer.data();
-                 }},
         event.payload);
     const intptr_t return_value = callback(plugin, event.opcode, event.index,
                                            event.value, data, event.option);
@@ -181,8 +182,7 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
     //      because it was not zeroed out by the host) for an event that should
     //      report some data back?
     const auto response_data =
-        std::holds_alternative<std::array<char, max_string_length>>(
+        std::holds_alternative<NeedsBuffer>(event.payload)
-            event.payload)
             ? std::optional(std::string(static_cast<char*>(data)))
             : std::nullopt;
 

src/common/logging.cpp

@@ -145,9 +145,7 @@ void Logger::log_event(bool is_dispatch,
             overload{
                 [&](const std::nullptr_t&) { message << "<nullptr>"; },
                 [&](const std::string& s) { message << "\"" << s << "\""; },
-                [&](const std::array<char, max_string_length>&) {
+                [&](const NeedsBuffer&) { message << "<writable_buffer>"; }},
-                    message << "<writeable_buffer>";
-                }},
             payload);
 
         message << ")";

src/common/serialization.h

@@ -47,6 +47,13 @@ struct overload : Ts... {
 template <class... Ts>
 overload(Ts...)->overload<Ts...>;
 
+/**
+ * Marker struct to indicate that that the event requires some buffer to write
+ * its results into. This is to prevent us from having to unnecessarily sending
+ * around empty arrays.
+ */
+struct NeedsBuffer {};
+
 /**
  * VST events are passed a void pointer that can contain a variety of different
  * data types depending on the event's opcode. This is typically either:
@@ -69,14 +76,8 @@ overload(Ts...)->overload<Ts...>;
  * - Some empty buffer for the plugin to write its own data to, for instance for
  *   a plugin to report its name or the label for a certain parameter. We'll
  *   assume that this is the default if none of the above options apply.
- *
- *   TODO: As a simple optimization we of course wouldn't have to send an entire
- *         empty array here, this should be replaced by some kind of marker
- *         struct. This would require some minor modifications in
- *         `passthrough_event()`.
  */
-using EventPayload = std::
+using EventPayload = std::variant<std::nullptr_t, std::string, NeedsBuffer>;
-    variant<std::nullptr_t, std::string, std::array<char, max_string_length>>;
 
 /**
  * An event as dispatched by the VST host. These events will get forwarded to
@@ -126,9 +127,7 @@ struct Event {
                   [](S& s, std::string& string) {
                       s.text1b(string, max_string_length);
                   },
-                  [](S& s, std::array<char, max_string_length>& buffer) {
+                  [](S&, NeedsBuffer&) {}});
-                      s.container1b(buffer);
-                  }});
     }
 };