diff --git a/src/timer.cpp b/src/timer.cpp
index 5cbca57..1ca03b0 100644
--- a/src/timer.cpp
+++ b/src/timer.cpp
@@ -9,12 +9,12 @@ std::mutex sc::timer::mutex;
 
 void sc::timer::callback(union sigval sv) {
     timer* self = reinterpret_cast<timer*>(sv.sival_ptr);
-    self->_expired_func(self);
+    self->_expired_func(*self);
 }
 
 sc::timer::timer(double time,
                  bool repeat,
-                 void(*expired_func)(timer*),
+                 void(*expired_func)(const timer&),
                  void* context)
     : _id {}, _time {time}, _repeat {repeat},
       _expired_func {expired_func}, _context {context}
diff --git a/src/timer.hpp b/src/timer.hpp
index 8f96da5..3eed608 100644
--- a/src/timer.hpp
+++ b/src/timer.hpp
@@ -14,7 +14,7 @@ namespace sc {
         uint64_t _id {};
         double _time {};
         bool _repeat {};
-        void (*_expired_func)(timer* self);
+        void (*_expired_func)(const timer& self);
         void* _context {};
 
         static void callback(union sigval);
@@ -22,7 +22,7 @@ namespace sc {
         public:
             timer(double time,
                   bool repeat,
-                  void(*expired_func)(timer*),
+                  void(*expired_func)(const timer&),
                   void* context = nullptr);
             ~timer();