DPL: finally use concepts to separate the make method

In the past, substituting SFINAE with constexpr made sense because
understanding what was going on in case of failure was extremely complicated.

Now that we have constrains and cocepts, it makes sense to split again the
method since the diagnostics are good and the compiler is limited to optimise
the code in question, without actually having to instanciate all the templates.

Author: ktf

Framework/Core/include/Framework/DataAllocator.h

@@ -57,14 +57,6 @@ namespace o2::framework
 {
 struct ServiceRegistry;
 
-#define ERROR_STRING                                    \
-  "data type T not supported by API, "                  \
-  "\n specializations available for"                    \
-  "\n - trivially copyable, non-polymorphic structures" \
-  "\n - arrays of those"                                \
-  "\n - TObject with additional constructor arguments"  \
-  "\n - std containers of those"
-
 /// Helper to allow framework managed objecs to have a callback
 /// when they go out of scope. For example, this could
 /// be used to serialize a message into a buffer before the
@@ -130,6 +122,10 @@ struct LifetimeHolder {
   }
 };
 
+template <typename T>
+concept VectorOfMessageableTypes = is_specialization_v<T, std::vector> &&
+                                   is_messageable<typename T::value_type>::value;
+
 /// This allocator is responsible to make sure that the messages created match
 /// the provided spec and that depending on how many pipelined reader we
 /// have, messages get created on the channel for the reader of the current
@@ -163,93 +159,115 @@ class DataAllocator
   // and with subspecification 0xdeadbeef.
   void cookDeadBeef(const Output& spec);
 
-  /// Generic helper to create an object which is owned by the framework and
-  /// returned as a reference to the own object.
-  /// Note: decltype(auto) will deduce the return type from the expression and it
-  /// will be lvalue reference for the framework-owned objects. Instances of local
-  /// variables like shared_ptr will be returned by value/move/return value optimization.
-  /// Objects created this way will be sent to the channel specified by @spec
   template <typename T, typename... Args>
+    requires(is_specialization_v<T, UninitializedVector>)
   decltype(auto) make(const Output& spec, Args... args)
   {
     auto& timingInfo = mRegistry.get<TimingInfo>();
     auto& context = mRegistry.get<MessageContext>();
 
-    if constexpr (is_specialization_v<T, UninitializedVector>) {
-      auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
-      // plain buffer as polymorphic spectator std::vector, which does not run constructors / destructors
-      using ValueType = typename T::value_type;
-
-      // Note: initial payload size is 0 and will be set by the context before sending
-      fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodNone, 0);
-      return context.add<MessageContext::VectorObject<ValueType, MessageContext::ContainerRefObject<std::vector<ValueType, o2::pmr::NoConstructAllocator<ValueType>>>>>(
-                      std::move(headerMessage), routeIndex, 0, std::forward<Args>(args)...)
-        .get();
-    } else if constexpr (is_specialization_v<T, std::vector> && has_messageable_value_type<T>::value) {
-      auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
-      // this catches all std::vector objects with messageable value type before checking if is also
-      // has a root dictionary, so non-serialized transmission is preferred
-      using ValueType = typename T::value_type;
-
-      // Note: initial payload size is 0 and will be set by the context before sending
-      fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodNone, 0);
-      return context.add<MessageContext::VectorObject<ValueType>>(std::move(headerMessage), routeIndex, 0, std::forward<Args>(args)...).get();
-    } else if constexpr (has_root_dictionary<T>::value == true && is_messageable<T>::value == false) {
-      auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
-      // Extended support for types implementing the Root ClassDef interface, both TObject
-      // derived types and others
-      if constexpr (enable_root_serialization<T>::value) {
-        fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodROOT, 0);
-
-        return context.add<typename enable_root_serialization<T>::object_type>(std::move(headerMessage), routeIndex, std::forward<Args>(args)...).get();
-      } else {
-        static_assert(enable_root_serialization<T>::value, "Please make sure you include RootMessageContext.h");
-      }
-      // Note: initial payload size is 0 and will be set by the context before sending
-    } else if constexpr (std::is_base_of_v<std::string, T>) {
-      auto* s = new std::string(args...);
-      adopt(spec, s);
-      return *s;
-    } else if constexpr (requires { static_cast<struct TableBuilder>(std::declval<std::decay_t<T>>()); }) {
-      auto tb = std::move(LifetimeHolder<TableBuilder>(new std::decay_t<T>(args...)));
-      adopt(spec, tb);
-      return tb;
-    } else if constexpr (requires { static_cast<struct TreeToTable>(std::declval<std::decay_t<T>>()); }) {
-      auto t2t = std::move(LifetimeHolder<TreeToTable>(new std::decay_t<T>(args...)));
-      adopt(spec, t2t);
-      return t2t;
-    } else if constexpr (sizeof...(Args) == 0) {
-      if constexpr (is_messageable<T>::value == true) {
-        return *reinterpret_cast<T*>(newChunk(spec, sizeof(T)).data());
-      } else {
-        static_assert(always_static_assert_v<T>, ERROR_STRING);
-      }
-    } else if constexpr (sizeof...(Args) == 1) {
-      using FirstArg = typename std::tuple_element<0, std::tuple<Args...>>::type;
-      if constexpr (std::is_integral_v<FirstArg>) {
-        if constexpr (is_messageable<T>::value == true) {
-          auto [nElements] = std::make_tuple(args...);
-          auto size = nElements * sizeof(T);
-          auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
-
-          fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodNone, size);
-          return context.add<MessageContext::SpanObject<T>>(std::move(headerMessage), routeIndex, 0, nElements).get();
-        }
-      } else if constexpr (std::is_same_v<FirstArg, std::shared_ptr<arrow::Schema>>) {
-        if constexpr (std::is_base_of_v<arrow::ipc::RecordBatchWriter, T>) {
-          auto [schema] = std::make_tuple(args...);
-          std::shared_ptr<arrow::ipc::RecordBatchWriter> writer;
-          create(spec, &writer, schema);
-          return writer;
-        }
-      } else {
-        static_assert(always_static_assert_v<T>, ERROR_STRING);
-      }
+    auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
+    // plain buffer as polymorphic spectator std::vector, which does not run constructors / destructors
+    using ValueType = typename T::value_type;
+
+    // Note: initial payload size is 0 and will be set by the context before sending
+    fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodNone, 0);
+    return context.add<MessageContext::VectorObject<ValueType, MessageContext::ContainerRefObject<std::vector<ValueType, o2::pmr::NoConstructAllocator<ValueType>>>>>(
+                    std::move(headerMessage), routeIndex, 0, std::forward<Args>(args)...)
+      .get();
+  }
+
+  template <typename T, typename... Args>
+    requires VectorOfMessageableTypes<T>
+  decltype(auto) make(const Output& spec, Args... args)
+  {
+    auto& timingInfo = mRegistry.get<TimingInfo>();
+    auto& context = mRegistry.get<MessageContext>();
+
+    auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
+    // this catches all std::vector objects with messageable value type before checking if is also
+    // has a root dictionary, so non-serialized transmission is preferred
+    using ValueType = typename T::value_type;
+
+    // Note: initial payload size is 0 and will be set by the context before sending
+    fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodNone, 0);
+    return context.add<MessageContext::VectorObject<ValueType>>(std::move(headerMessage), routeIndex, 0, std::forward<Args>(args)...).get();
+  }
+
+  template <typename T, typename... Args>
+    requires(!VectorOfMessageableTypes<T> && has_root_dictionary<T>::value == true && is_messageable<T>::value == false)
+  decltype(auto) make(const Output& spec, Args... args)
+  {
+    auto& timingInfo = mRegistry.get<TimingInfo>();
+    auto& context = mRegistry.get<MessageContext>();
+
+    auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
+    // Extended support for types implementing the Root ClassDef interface, both TObject
+    // derived types and others
+    if constexpr (enable_root_serialization<T>::value) {
+      fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodROOT, 0);
+
+      return context.add<typename enable_root_serialization<T>::object_type>(std::move(headerMessage), routeIndex, std::forward<Args>(args)...).get();
     } else {
-      static_assert(always_static_assert_v<T>, ERROR_STRING);
+      static_assert(enable_root_serialization<T>::value, "Please make sure you include RootMessageContext.h");
     }
   }
 
+  template <typename T, typename... Args>
+    requires(std::is_base_of_v<std::string, T>)
+  decltype(auto) make(const Output& spec, Args... args)
+  {
+    auto* s = new std::string(args...);
+    adopt(spec, s);
+    return *s;
+  }
+
+  template <typename T, typename... Args>
+    requires(requires { static_cast<struct TableBuilder>(std::declval<std::decay_t<T>>()); })
+  decltype(auto) make(const Output& spec, Args... args)
+  {
+    auto tb = std::move(LifetimeHolder<TableBuilder>(new std::decay_t<T>(args...)));
+    adopt(spec, tb);
+    return tb;
+  }
+
+  template <typename T, typename... Args>
+    requires(requires { static_cast<struct TreeToTable>(std::declval<std::decay_t<T>>()); })
+  decltype(auto) make(const Output& spec, Args... args)
+  {
+    auto t2t = std::move(LifetimeHolder<TreeToTable>(new std::decay_t<T>(args...)));
+    adopt(spec, t2t);
+    return t2t;
+  }
+
+  template <typename T>
+    requires(is_messageable<T>::value == true)
+  decltype(auto) make(const Output& spec)
+  {
+    return *reinterpret_cast<T*>(newChunk(spec, sizeof(T)).data());
+  }
+
+  template <typename T>
+    requires(is_messageable<T>::value)
+  decltype(auto) make(const Output& spec, std::integral auto nElements)
+  {
+    auto& timingInfo = mRegistry.get<TimingInfo>();
+    auto& context = mRegistry.get<MessageContext>();
+    auto routeIndex = matchDataHeader(spec, timingInfo.timeslice);
+
+    fair::mq::MessagePtr headerMessage = headerMessageFromOutput(spec, routeIndex, o2::header::gSerializationMethodNone, nElements * sizeof(T));
+    return context.add<MessageContext::SpanObject<T>>(std::move(headerMessage), routeIndex, 0, nElements).get();
+  }
+
+  template <typename T, typename Arg>
+    requires(std::is_same_v<Arg, std::shared_ptr<arrow::Schema>>)
+  decltype(auto) make(const Output& spec, Arg schema)
+  {
+    std::shared_ptr<arrow::ipc::RecordBatchWriter> writer;
+    create(spec, &writer, schema);
+    return writer;
+  }
+
   /// Adopt a string in the framework and serialize / send
   /// it to the consumers of @a spec once done.
   void