feature: Introduce maybe_owning_ptr

Author: ChristianTackeGSI

fairroot/tools/CMakeLists.txt

@@ -20,6 +20,10 @@ set(sources
 
 fair_change_extensions_if_exists(.cxx .h FILES "${sources}" OUTVAR headers)
 
+list(APPEND headers
+  FairMemory.h
+)
+
 add_library(${target} SHARED ${sources} ${headers})
 fairroot_library_settings(${target})
 # Keep old filesystem name

fairroot/tools/FairMemory.h

@@ -0,0 +1,67 @@
+/********************************************************************************
+ *    Copyright (C) 2023 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH    *
+ *                                                                              *
+ *              This software is distributed under the terms of the             *
+ *              GNU Lesser General Public Licence (LGPL) version 3,             *
+ *                  copied verbatim in the file "LICENSE"                       *
+ ********************************************************************************/
+
+#ifndef FAIRROOT_MEMORY_H_
+#define FAIRROOT_MEMORY_H_
+
+#include <memory>
+#include <optional>
+
+/**
+ * \brief Internal implementation details
+ *
+ * Any declarations in here are strictly for internal usage.
+ * They may never enter public APIs.
+ *
+ * \note Anything in here can change at any time without notice!
+ */
+namespace fairroot::detail
+{
+
+template<typename T, typename Deleter = std::default_delete<T>>
+struct optional_deleter
+{
+    std::optional<Deleter> deleter{std::in_place};
+
+    optional_deleter() = default;
+    optional_deleter(const Deleter& x)
+        : deleter(std::in_place, x)
+    {}
+    optional_deleter(std::nullopt_t)
+        : deleter()
+    {}
+
+    void operator()(T* ptr) const
+    {
+        if (deleter) {
+            (*deleter)(ptr);
+        }
+    }
+};
+
+/**
+ * \brief unique_ptr that alternatively can hold a reference
+ *
+ * In rare cases we need a unique_ptr that can alternatively
+ * contain a reference (and thus not delete its content
+ * on destruction).
+ *
+ * \note Think twice before using this!
+ *
+ * It's mostly meant for cases, where FairRoot has a public API
+ * that enforces this behaviour. This should be deprecated and
+ * removed sooner than later.
+ */
+template<typename T, typename Deleter = std::default_delete<T>>
+using maybe_owning_ptr = std::unique_ptr<T, optional_deleter<T, Deleter>>;
+
+inline constexpr auto non_owning = std::nullopt;
+
+}   // namespace fairroot::detail
+
+#endif