diff --git a/src/libslic3r/ExtrusionEntityCollection.cpp b/src/libslic3r/ExtrusionEntityCollection.cpp
index 9ae116c47..70c2348af 100644
--- a/src/libslic3r/ExtrusionEntityCollection.cpp
+++ b/src/libslic3r/ExtrusionEntityCollection.cpp
@@ -11,7 +11,7 @@ ExtrusionEntityCollection::ExtrusionEntityCollection(const ExtrusionPaths &paths
     this->append(paths);
 }
 
-ExtrusionEntityCollection& ExtrusionEntityCollection::operator= (const ExtrusionEntityCollection &other)
+ExtrusionEntityCollection& ExtrusionEntityCollection::operator=(const ExtrusionEntityCollection &other)
 {
     this->entities      = other.entities;
     for (size_t i = 0; i < this->entities.size(); ++i)
@@ -175,20 +175,20 @@ size_t ExtrusionEntityCollection::items_count() const
 }
 
 // Returns a single vector of pointers to all non-collection items contained in this one.
-void ExtrusionEntityCollection::flatten(ExtrusionEntityCollection* retval) const
-{
-    for (const ExtrusionEntity *entity : this->entities)
-        if (entity->is_collection())
-            retval->append(static_cast<const ExtrusionEntityCollection*>(entity)->flatten().entities);
-        else
-            retval->append(*entity);
-}
-
 ExtrusionEntityCollection ExtrusionEntityCollection::flatten() const
 {
-    ExtrusionEntityCollection coll;
-    this->flatten(&coll);
-    return coll;
+	struct Flatten {
+		ExtrusionEntityCollection out;
+		void recursive_do(const ExtrusionEntityCollection &collection) {
+			for (const ExtrusionEntity* entity : collection.entities)
+				if (entity->is_collection())
+					this->recursive_do(*static_cast<const ExtrusionEntityCollection*>(entity));
+				else
+					out.append(*entity);
+		}
+	} flatten;
+	flatten.recursive_do(*this);
+    return flatten.out;
 }
 
 double ExtrusionEntityCollection::min_mm3_per_mm() const
diff --git a/src/libslic3r/ExtrusionEntityCollection.hpp b/src/libslic3r/ExtrusionEntityCollection.hpp
index 4fe964ee1..221afc453 100644
--- a/src/libslic3r/ExtrusionEntityCollection.hpp
+++ b/src/libslic3r/ExtrusionEntityCollection.hpp
@@ -85,7 +85,6 @@ public:
     Polygons polygons_covered_by_spacing(const float scaled_epsilon = 0.f) const
         { Polygons out; this->polygons_covered_by_spacing(out, scaled_epsilon); return out; }
     size_t items_count() const;
-    void flatten(ExtrusionEntityCollection* retval) const;
     ExtrusionEntityCollection flatten() const;
     double min_mm3_per_mm() const;
     double total_volume() const override { double volume=0.; for (const auto& ent : entities) volume+=ent->total_volume(); return volume; }