Add LHCb::make_obj_propagating_allocator<T>( old, args... ) helper.

Kernel/LHCbKernel/Kernel/AllocatorUtils.h

+/*****************************************************************************\
+* (c) Copyright 2020 CERN for the benefit of the LHCb Collaboration           *
+*                                                                             *
+* This software is distributed under the terms of the GNU General Public      *
+* Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
+*                                                                             *
+* In applying this licence, CERN does not waive the privileges and immunities *
+* granted to it by virtue of its status as an Intergovernmental Organization  *
+* or submit itself to any jurisdiction.                                       *
+\*****************************************************************************/
+#pragma once
+#include "GaudiKernel/detected.h"
+
+#include <memory>
+#include <type_traits>
+
+namespace LHCb {
+  namespace details_au {
+    template <typename T>
+    using has_get_allocator_ = decltype( std::declval<T>().get_allocator() );
+
+    template <typename T>
+    inline constexpr bool has_get_allocator_v = Gaudi::cpp17::is_detected_v<has_get_allocator_, T>;
+  } // namespace details_au
+
+  /** Construct a new object of type T using the given parameter pack and an allocator obtained from 'old' if possible.
+   *
+   * The body of this helper could be simplified with C++20's std::make_obj_using_allocator.
+   */
+  template <typename T, typename Old, typename... Args>
+  std::decay_t<T> make_obj_propagating_allocator( Old const& old, Args&&... args ) {
+    // Check if old.get_allocator() is valid
+    if constexpr ( details_au::has_get_allocator_v<Old> ) {
+      // Figure out if the allocator we can get is useful
+      using Alloc = std::decay_t<decltype( old.get_allocator() )>;
+      if constexpr ( std::uses_allocator_v<std::decay_t<T>, Alloc> ) {
+        // We should be able to use it!
+        if constexpr ( std::is_constructible_v<std::decay_t<T>, std::allocator_arg_t, Alloc const&, Args...> ) {
+          return std::decay_t<T>{std::allocator_arg, old.get_allocator(), std::forward<Args>( args )...};
+        } else {
+          static_assert( std::is_constructible_v<std::decay_t<T>, Args..., Alloc const&>,
+                         "std::uses_allocator_v<T, Alloc> contract violated" );
+          return std::decay_t<T>{std::forward<Args>( args )..., old.get_allocator()};
+        }
+      }
+    }
+    // Couldn't get a valid allocator, proceed without one
+    return std::decay_t<T>{std::forward<Args>( args )...};
+  }
+} // namespace LHCb
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