diff --git a/core/reactor.hh b/core/reactor.hh
index 0501c77802..855add3a58 100644
--- a/core/reactor.hh
+++ b/core/reactor.hh
@@ -93,6 +93,8 @@ private:
     bool _queued = false;
     bool _expired = false;
 public:
+    timer() = default;
+    explicit timer(callback_t&& callback);
     ~timer();
     future<> expired();
     void set_callback(callback_t&& callback);
@@ -1443,6 +1445,11 @@ future<size_t> pollable_fd::sendto(socket_address addr, const void* buf, size_t
     });
 }
 
+template <typename Clock>
+inline
+timer<Clock>::timer(callback_t&& callback) : _callback(std::move(callback)) {
+}
+
 template <typename Clock>
 inline
 timer<Clock>::~timer() {