diff --git a/eqpalg/utility/expression_engine.cpp b/eqpalg/utility/expression_engine.cpp
index 4687336..e4ad0c8 100644
--- a/eqpalg/utility/expression_engine.cpp
+++ b/eqpalg/utility/expression_engine.cpp
@@ -35,12 +35,6 @@ ExpressionEngine::ExpressionEngine(std::map<std::string, double>& mm_vars,
   }
 }
 
-// 惰性初始化 VarsCache（首次变量刷新时调用）
-static void ensureVarCache(VarsCache& vc, size_t tag_count) {
-  if (vc.tag_num == 0 && tag_count > 0) {
-    vc.init(tag_count, 6);
-  }
-}
 
 int ExpressionEngine::registerExpression(const std::string& name,
                                           const std::string& raw_exp_str) {
diff --git a/eqpalg/utility/expression_engine.h b/eqpalg/utility/expression_engine.h
index 26b0957..2b3edc0 100644
--- a/eqpalg/utility/expression_engine.h
+++ b/eqpalg/utility/expression_engine.h
@@ -118,6 +118,12 @@ private:
   VarsCache var_cache_;
   static constexpr size_t PV_NUM = 6;
 
+  static void ensureVarCache(VarsCache& vc, size_t tag_count) {
+    if (vc.tag_num == 0 && tag_count > 0) {
+      vc.init(tag_count, 6);
+    }
+  }
+
   // 内部辅助
   int initHoldExpStr(const std::string& exp_str);
 };