diff --git a/ostd/format.hh b/ostd/format.hh
index 3cf0888..41760c2 100644
--- a/ostd/format.hh
+++ b/ostd/format.hh
@@ -887,7 +887,7 @@ namespace detail {
                         return -1;
                 }
                 if (spec.arg_width()) {
-                    if (argpos <= (argprec + 1)) {
+                    if (argpos <= (Size(argprec) + 1)) {
                         assert(false && "argument width not given");
                         return -1;
                     }
diff --git a/ostd/signal.hh b/ostd/signal.hh
index b49ed3a..98e83f1 100644
--- a/ostd/signal.hh
+++ b/ostd/signal.hh
@@ -11,73 +11,119 @@
 
 namespace ostd {
 
+namespace detail {
+    template<typename C, typename ...A>
+    struct EventBase {
+        EventBase(C *cl): p_class(cl), p_funcs(nullptr), p_nfuncs(0) {}
+
+        EventBase(const EventBase &ev): p_class(ev.p_class),
+                                        p_nfuncs(ev.p_nfuncs) {
+            using Func = Function<void(C &, A...)>;
+            Func *nbuf = (Func *)new byte[sizeof(Func) * p_nfuncs];
+            for (Size i = 0; i < p_nfuncs; ++i)
+                new (&nbuf[i]) Func(ev.p_funcs[i]);
+            p_funcs = nbuf;
+        }
+
+        EventBase(EventBase &&ev): p_class(ev.p_class), p_funcs(ev.p_funcs),
+                                   p_nfuncs(ev.p_nfuncs) {
+            ev.p_class = nullptr;
+            ev.p_funcs = nullptr;
+            ev.p_nfuncs = 0;
+        }
+
+        void clear() {
+            for (Size i = 0; i < p_nfuncs; ++i)
+                p_funcs[i].~Function<void(C &, A...)>();
+            delete[] (byte *)p_funcs;
+            p_funcs = nullptr;
+            p_nfuncs = 0;
+        }
+
+        template<typename F>
+        Size connect(F &&func) {
+            using Func = Function<void(C &, A...)>;
+            Func *nbuf = (Func *)new byte[sizeof(Func) * (p_nfuncs + 1)];
+            for (Size i = 0; i < p_nfuncs; ++i) {
+                new (&nbuf[i]) Func(move(p_funcs[i]));
+                p_funcs[i].~Func();
+            }
+            new (&nbuf[p_nfuncs]) Func(forward<F>(func));
+            delete[] (byte *)p_funcs;
+            p_funcs = nbuf;
+            return p_nfuncs++;
+        }
+
+        bool disconnect(Size idx) {
+            if ((idx >= p_nfuncs) || !p_funcs[idx]) return false;
+            p_funcs[idx] = nullptr;
+            return true;
+        }
+
+        template<typename ...Args>
+        void emit(Args &&...args) const {
+            if (!p_class) return;
+            for (Size i = 0; i < p_nfuncs; ++i)
+                if (p_funcs[i]) p_funcs[i](*p_class, args...);
+        }
+
+    private:
+        C *p_class;
+        Function<void(C &, A...)> *p_funcs;
+        Size p_nfuncs;
+    };
+} /* namespace detail */
+
 template<typename C, typename ...A>
 struct Event {
-    Event(C *cl = nullptr): p_class(cl), p_funcs(nullptr), p_nfuncs(0) {}
+private:
+    using Base = detail::EventBase<C, A...>;
+    Base p_base;
+public:
+    Event(C *cl): p_base(cl) {}
+    Event(const Event &ev): p_base(ev.p_base) {}
+    Event(Event &&ev): p_base(move(ev.p_base)) {}
 
-    Event(const Event &ev): p_class(ev.p_class), p_nfuncs(ev.p_nfuncs) {
-        using Func = Function<void(C &, A...)>;
-        Func *nbuf = (Func *)new byte[sizeof(Func) * p_nfuncs];
-        for (Size i = 0; i < p_nfuncs; ++i)
-            new (&nbuf[i]) Func(ev.p_funcs[i]);
-        p_funcs = nbuf;
-    }
+    ~Event() { clear(); }
 
-    Event(Event &&ev): p_class(ev.p_class), p_funcs(ev.p_funcs),
-                       p_nfuncs(ev.p_nfuncs) {
-        ev.p_class = nullptr;
-        ev.p_funcs = nullptr;
-        ev.p_nfuncs = 0;
-    }
-
-    ~Event() {
-        clear();
-    }
-
-    void clear() {
-        for (Size i = 0; i < p_nfuncs; ++i)
-            p_funcs[i].~Function<void(C &, A...)>();
-        delete[] (byte *)p_funcs;
-        p_funcs = nullptr;
-        p_nfuncs = 0;
-    }
+    void clear() { p_base.clear(); }
 
     template<typename F>
-    Size connect(F func) {
-        using Func = Function<void(C &, A...)>;
-        Func *nbuf = (Func *)new byte[sizeof(Func) * (p_nfuncs + 1)];
-        for (Size i = 0; i < p_nfuncs; ++i) {
-            new (&nbuf[i]) Func(move(p_funcs[i]));
-            p_funcs[i].~Func();
-        }
-        new (&nbuf[p_nfuncs]) Func(func);
-        delete[] (byte *)p_funcs;
-        p_funcs = nbuf;
-        return p_nfuncs++;
-    }
+    Size connect(F &&func) { return p_base.connect(forward<F>(func)); }
 
-    bool disconnect(Size idx) {
-        if ((idx >= p_nfuncs) || !p_funcs[idx]) return false;
-        p_funcs[idx] = nullptr;
-        return true;
-    }
+    bool disconnect(Size idx) { return p_base.disconnect(idx); }
 
     template<typename ...Args>
-    void emit(Args &&...args) const {
-        if (!p_class) return;
-        for (Size i = 0; i < p_nfuncs; ++i)
-            if (p_funcs[i]) p_funcs[i](*p_class, args...);
-    }
+    void emit(Args &&...args) { p_base.emit(forward<Args>(args)...); }
 
     template<typename ...Args>
-    void operator()(Args &&...args) const {
-        emit(forward<Args>(args)...);
-    }
+    void operator()(Args &&...args) { emit(forward<Args>(args)...); }
+};
 
+template<typename C, typename ...A>
+struct Event<const C, A...> {
 private:
-    C *p_class;
-    Function<void(C &, A...)> *p_funcs;
-    Size p_nfuncs;
+    using Base = detail::EventBase<const C, A...>;
+    Base p_base;
+public:
+    Event(C *cl): p_base(cl) {}
+    Event(const Event &ev): p_base(ev.p_base) {}
+    Event(Event &&ev): p_base(move(ev.p_base)) {}
+
+    ~Event() { clear(); }
+
+    void clear() { p_base.clear(); }
+
+    template<typename F>
+    Size connect(F &&func) { return p_base.connect(forward<F>(func)); }
+
+    bool disconnect(Size idx) { return p_base.disconnect(idx); }
+
+    template<typename ...Args>
+    void emit(Args &&...args) const { p_base.emit(forward<Args>(args)...); }
+
+    template<typename ...Args>
+    void operator()(Args &&...args) const { emit(forward<Args>(args)...); }
 };
 
 } /* namespace ostd */