diff --git a/internal/ceres/casts.h b/internal/ceres/casts.h
index d137071..af944520 100644
--- a/internal/ceres/casts.h
+++ b/internal/ceres/casts.h
@@ -1,5 +1,5 @@
 // Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2015 Google Inc. All rights reserved.
+// Copyright 2023 Google Inc. All rights reserved.
 // http://ceres-solver.org/
 //
 // Redistribution and use in source and binary forms, with or without
@@ -32,14 +32,13 @@
 #define CERES_INTERNAL_CASTS_H_
 
 #include <cassert>
-#include <cstddef>  // For NULL.
 
 namespace ceres {
 
 // Identity metafunction.
 template <class T>
 struct identity_ {
-  typedef T type;
+  using type = T;
 };
 
 // Use implicit_cast as a safe version of static_cast or const_cast
@@ -86,6 +85,7 @@
 //    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
 // You should design the code some other way not to need this.
 
+// TODO(sameeragarwal): Modernize this.
 template <typename To, typename From>  // use like this: down_cast<T*>(foo);
 inline To down_cast(From* f) {         // so we only accept pointers
   // Ensures that To is a sub-type of From *.  This test is here only
@@ -95,11 +95,11 @@
 
   // TODO(csilvers): This should use COMPILE_ASSERT.
   if (false) {
-    implicit_cast<From*, To>(NULL);
+    implicit_cast<From*, To>(nullptr);
   }
 
   // uses RTTI in dbg and fastbuild. asserts are disabled in opt builds.
-  assert(f == NULL || dynamic_cast<To>(f) != NULL);  // NOLINT
+  assert(f == nullptr || dynamic_cast<To>(f) != nullptr);  // NOLINT
   return static_cast<To>(f);
 }