From 8545f9e03519e472c9172b9e76dfd5b0378fbecc Mon Sep 17 00:00:00 2001
From: Steve Baer <steve@mcneel.com>
Date: Tue, 1 Feb 2022 09:27:42 -0800
Subject: [PATCH] keep upstream change

---
 opennurbs_mesh.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/opennurbs_mesh.cpp b/opennurbs_mesh.cpp
index c7c78cb2..44428ae8 100644
--- a/opennurbs_mesh.cpp
+++ b/opennurbs_mesh.cpp
@@ -14831,7 +14831,9 @@ bool ON_MeshCache::Transform(
     ON_Mesh* mesh = item->m_mesh_sp.get();
     if (nullptr == mesh || mesh->IsEmpty())
       continue;
-    if (false == item->m_mesh_sp.unique())
+    // NOTE: use_count() == 1 is an approximation in multi-threaded environments
+    // https:// en.cppreference.com/w/cpp/memory/shared_ptr/unique
+    if (item->m_mesh_sp.use_count() != 1)
     {
       // make a copy and transform the copy
       std::shared_ptr<ON_Mesh>(new ON_Mesh(*mesh)).swap(item->m_mesh_sp);