diff --git a/include/xgboost/json_io.h b/include/xgboost/json_io.h
index d781ecd4f..2bde544ac 100644
--- a/include/xgboost/json_io.h
+++ b/include/xgboost/json_io.h
@@ -115,12 +115,10 @@ class JsonReader {
 class JsonWriter {
   static constexpr size_t kIndentSize = 2;
 
-  size_t n_spaces_;
   std::vector<char>* stream_;
 
  public:
-  explicit JsonWriter(std::vector<char>* stream) :
-      n_spaces_{0}, stream_{stream} {}
+  explicit JsonWriter(std::vector<char>* stream) : stream_{stream} {}
 
   virtual ~JsonWriter() = default;
 
diff --git a/src/c_api/c_api_utils.h b/src/c_api/c_api_utils.h
index 66825722b..f29c302a1 100644
--- a/src/c_api/c_api_utils.h
+++ b/src/c_api/c_api_utils.h
@@ -165,9 +165,9 @@ inline float GetMissing(Json const &config) {
 
 // Safe guard some global variables from being changed by XGBoost.
 class XGBoostAPIGuard {
+#if defined(XGBOOST_USE_CUDA)
   int32_t device_id_ {0};
 
-#if defined(XGBOOST_USE_CUDA)
   void SetGPUAttribute();
   void RestoreGPUAttribute();
 #else
diff --git a/src/tree/updater_quantile_hist.h b/src/tree/updater_quantile_hist.h
index 0fa680ffe..5fb357e32 100644
--- a/src/tree/updater_quantile_hist.h
+++ b/src/tree/updater_quantile_hist.h
@@ -166,11 +166,8 @@ class QuantileHistMaker: public TreeUpdater {
       builder_monitor_.Init("Quantile::Builder");
     }
     // update one tree, growing
-    virtual void Update(const GHistIndexMatrix& gmat,
-                        const ColumnMatrix& column_matrix,
-                        HostDeviceVector<GradientPair>* gpair,
-                        DMatrix* p_fmat,
-                        RegTree* p_tree);
+    void Update(const GHistIndexMatrix& gmat, const ColumnMatrix& column_matrix,
+                HostDeviceVector<GradientPair>* gpair, DMatrix* p_fmat, RegTree* p_tree);
 
     bool UpdatePredictionCache(const DMatrix* data,
                                linalg::VectorView<float> out_preds);