diff --git a/containers_cpp/test/object_pool_test.cc b/containers_cpp/test/object_pool_test.cc
index 76da782..50fa605 100644
--- a/containers_cpp/test/object_pool_test.cc
+++ b/containers_cpp/test/object_pool_test.cc
@@ -32,7 +32,7 @@ ObjectPoolTestStruct::ObjectPoolTestStruct(int thread_id)
   : thread_id(thread_id) {
 }
 
-int ObjectPoolTestStruct::GetThreadId() {
+int ObjectPoolTestStruct::GetThreadId() const {
   return thread_id;
 }
 } // namespace test
diff --git a/containers_cpp/test/object_pool_test.h b/containers_cpp/test/object_pool_test.h
index 9902888..ebb7861 100644
--- a/containers_cpp/test/object_pool_test.h
+++ b/containers_cpp/test/object_pool_test.h
@@ -39,7 +39,7 @@ class ObjectPoolTestStruct {
   int thread_id;
  public:
   explicit ObjectPoolTestStruct(int thread_id);
-  int GetThreadId();
+  int GetThreadId() const;
 };
 
 template<typename ValuePool>