refactor: Do not allocate memory if the surface is connected with a detector el…

Core/include/Acts/Surfaces/Surface.hpp

@@ -1,6 +1,6 @@
 // This file is part of the Acts project.
 //
-// Copyright (C) 2016-2020 CERN for the benefit of the Acts project
+// Copyright (C) 2016-2024 CERN for the benefit of the Acts project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -479,7 +479,7 @@ class Surface : public virtual GeometryObject,
  protected:
   /// Transform3 definition that positions
   /// (translation, rotation) the surface in global space
-  Transform3 m_transform = Transform3::Identity();
+  std::unique_ptr<const Transform3> m_transform{};
 
   /// Pointer to the a DetectorElementBase
   const DetectorElementBase* m_associatedDetElement{nullptr};

Core/src/Geometry/CylinderLayer.cpp

@@ -37,7 +37,8 @@ Acts::CylinderLayer::CylinderLayer(
   auto cVolumeBounds = std::make_shared<CylinderVolumeBounds>(
       *CylinderSurface::m_bounds, thickness);
   // @todo rotate around x for the avePhi if you have a sector
-  m_representingVolume = std::make_unique<Volume>(m_transform, cVolumeBounds);
+  Transform3 volTrf = m_transform ? *m_transform : Transform3::Identity();
+  m_representingVolume = std::make_unique<Volume>(volTrf, cVolumeBounds);
 
   // associate the layer to the surface
   CylinderSurface::associateLayer(*this);

Core/src/Geometry/DiscLayer.cpp

@@ -39,7 +39,9 @@ Acts::DiscLayer::DiscLayer(const Transform3& transform,
     auto rVolumeBounds =
         std::make_shared<CylinderVolumeBounds>(*rBounds, thickness);
     // @todo rotate around x for the avePhi if you have a sector
-    m_representingVolume = std::make_unique<Volume>(m_transform, rVolumeBounds);
+    Transform3 volTrf = m_transform ? *m_transform : Transform3::Identity();
+    m_representingVolume =
+        std::make_unique<Volume>(std::move(volTrf), rVolumeBounds);
   }
   // associate the layer to the layer surface itself
   DiscSurface::associateLayer(*this);

Core/src/Surfaces/PlaneSurface.cpp

@@ -38,7 +38,8 @@ Acts::PlaneSurface::PlaneSurface(const GeometryContext& gctx,
 
 Acts::PlaneSurface::PlaneSurface(const Vector3& center, const Vector3& normal)
     : RegularSurface(), m_bounds(nullptr) {
-  m_transform = CurvilinearSurface(center, normal).transform();
+  m_transform = std::make_unique<Transform3>(
+      CurvilinearSurface(center, normal).transform());
 }
 
 Acts::PlaneSurface::PlaneSurface(std::shared_ptr<const PlanarBounds> pbounds,

Core/src/Surfaces/Surface.cpp

@@ -22,21 +22,26 @@ std::array<std::string, Acts::Surface::SurfaceType::Other>
         "Cone", "Cylinder", "Disc", "Perigee", "Plane", "Straw", "Curvilinear"};
 
 Acts::Surface::Surface(const Transform3& transform)
-    : GeometryObject(), m_transform(transform) {}
+    : GeometryObject(), m_transform(std::make_unique<Transform3>(transform)) {}
 
 Acts::Surface::Surface(const DetectorElementBase& detelement)
     : GeometryObject(), m_associatedDetElement(&detelement) {}
 
 Acts::Surface::Surface(const Surface& other)
     : GeometryObject(other),
       std::enable_shared_from_this<Surface>(),
-      m_transform(other.m_transform),
-      m_surfaceMaterial(other.m_surfaceMaterial) {}
+
+      m_associatedDetElement(other.m_associatedDetElement),
+      m_surfaceMaterial(other.m_surfaceMaterial) {
+  if (other.m_transform) {
+    m_transform = std::make_unique<Transform3>(*other.m_transform);
+  }
+}
 
 Acts::Surface::Surface(const GeometryContext& gctx, const Surface& other,
                        const Transform3& shift)
     : GeometryObject(),
-      m_transform(shift * other.transform(gctx)),
+      m_transform(std::make_unique<Transform3>(shift * other.transform(gctx))),
       m_surfaceMaterial(other.m_surfaceMaterial) {}
 
 Acts::Surface::~Surface() = default;
@@ -156,7 +161,11 @@ Acts::Surface& Acts::Surface::operator=(const Surface& other) {
   if (&other != this) {
     GeometryObject::operator=(other);
     // detector element, identifier & layer association are unique
-    m_transform = other.m_transform;
+    if (other.m_transform) {
+      m_transform = std::make_unique<Transform3>(*other.m_transform);
+    } else {
+      m_transform.reset();
+    }
     m_associatedLayer = other.m_associatedLayer;
     m_surfaceMaterial = other.m_surfaceMaterial;
     m_associatedDetElement = other.m_associatedDetElement;
@@ -182,7 +191,8 @@ bool Acts::Surface::operator==(const Surface& other) const {
     return false;
   }
   // (e) compare transform values
-  if (!m_transform.isApprox(other.m_transform, 1e-9)) {
+  if (m_transform && other.m_transform &&
+      !m_transform->isApprox((*other.m_transform), 1e-9)) {
     return false;
   }
   // (f) compare material
@@ -240,7 +250,7 @@ const Acts::Transform3& Acts::Surface::transform(
   if (m_associatedDetElement != nullptr) {
     return m_associatedDetElement->transform(gctx);
   }
-  return m_transform;
+  return (*m_transform);
 }
 
 bool Acts::Surface::insideBounds(const Vector2& lposition,
@@ -337,7 +347,7 @@ void Acts::Surface::assignDetectorElement(
   m_associatedDetElement = &detelement;
   // resetting the transform as it will be handled through the detector element
   // now
-  m_transform = Transform3::Identity();
+  m_transform.reset();
 }
 
 void Acts::Surface::assignSurfaceMaterial(