Implement ROOT read rules for PortableHostObject types

Author: fwyzard

DataFormats/Portable/interface/PortableHostObjectReadRules.h

@@ -0,0 +1,76 @@
+#ifndef DataFormats_Portable_interface_PortableHostObjectReadRules_h
+#define DataFormats_Portable_interface_PortableHostObjectReadRules_h
+
+#include <TGenericClassInfo.h>
+#include <TVirtualObject.h>
+
+#include "DataFormats/Portable/interface/PortableHostObject.h"
+#include "FWCore/Utilities/interface/concatenate.h"
+#include "FWCore/Utilities/interface/stringize.h"
+
+// read function for PortableHostObject, called for every event
+template <typename T>
+static void readPortableHostObject_v1(char *target, TVirtualObject *from_buffer) {
+  // extract the actual types
+  using Object = T;
+  using Product = typename Object::Product;
+
+  // valid only for PortableHostObject<T>
+  static_assert(std::is_same_v<Object, PortableHostObject<Product>>);
+
+  // proxy for the object being read from file
+  struct OnFile {
+    Product *product_;
+  };
+
+  // address in memory of the buffer containing the object being read from file
+  char *address = static_cast<char *>(from_buffer->GetObject());
+  // offset of the "product_" data member
+  static ptrdiff_t product_offset = from_buffer->GetClass()->GetDataMemberOffset("product_");
+  // pointer to the Product object being read from file
+  OnFile onfile = {*(Product **)(address + product_offset)};
+
+  // pointer to the Object object being constructed in memory
+  Object *newObj = (Object *)target;
+
+  // move the data from the on-file layout to the newly constructed object
+  Object::ROOTReadStreamer(newObj, *onfile.product_);
+}
+
+// put set_PortableHostObject_read_rules in the ROOT namespace to let it forward declare GenerateInitInstance
+namespace ROOT {
+
+  // set the read rules for PortableHostObject<T>;
+  // this is called only once, when the dictionary is loaded.
+  template <typename T>
+  static bool set_PortableHostObject_read_rules(std::string const &type) {
+    // forward declaration
+    TGenericClassInfo *GenerateInitInstance(T const *);
+
+    // build the read rules
+    std::vector<ROOT::Internal::TSchemaHelper> readrules(1);
+    ROOT::Internal::TSchemaHelper &rule = readrules[0];
+    rule.fTarget = "buffer_,product_";
+    rule.fSourceClass = type;
+    rule.fSource = type + "::Product* product_;";
+    rule.fCode = type + "::ROOTReadStreamer(newObj, *onfile.product_)";
+    rule.fVersion = "[1-]";
+    rule.fChecksum = "";
+    rule.fInclude = "";
+    rule.fEmbed = false;
+    rule.fFunctionPtr = reinterpret_cast<void *>(::readPortableHostObject_v1<T>);
+    rule.fAttributes = "";
+
+    // set the read rules
+    TGenericClassInfo *instance = GenerateInitInstance((T const *)nullptr);
+    instance->SetReadRules(readrules);
+
+    return true;
+  }
+}  // namespace ROOT
+
+#define SET_PORTABLEHOSTOBJECT_READ_RULES(OBJECT)                                                      \
+  static bool EDM_CONCATENATE(set_PortableHostObject_read_rules_done_at_, __LINE__) [[maybe_unused]] = \
+      ROOT::set_PortableHostObject_read_rules<OBJECT>(EDM_STRINGIZE(OBJECT))
+
+#endif  // DataFormats_Portable_interface_PortableHostObjectReadRules_h

DataFormats/PortableTestObjects/src/classes.cc

@@ -1,4 +1,7 @@
 #include "DataFormats/Portable/interface/PortableHostCollectionReadRules.h"
+#include "DataFormats/Portable/interface/PortableHostObjectReadRules.h"
 #include "DataFormats/PortableTestObjects/interface/TestHostCollection.h"
+#include "DataFormats/PortableTestObjects/interface/TestHostObject.h"
 
 SET_PORTABLEHOSTCOLLECTION_READ_RULES(portabletest::TestHostCollection);
+SET_PORTABLEHOSTOBJECT_READ_RULES(portabletest::TestHostObject);