add copy-constructor version of Observer::Target::Notify

iface/common/dealer.hh

@@ -49,7 +49,7 @@ class Taker : public Dealer<T>, public Observer<T>::Target {
  public:
   explicit Taker(const DealerMeta& meta = {}) noexcept : Dealer<T>(meta) { }
 
-  void Take(const T& v) noexcept { Notify(v); }
+  void Take(const T& v) noexcept { Observer<T>::Target::Notify(v); }
 };
 
 }  // namespace nf7

iface/common/observer.hh

@@ -48,6 +48,15 @@ class Observer<T>::Target {
   Target& operator=(Target&&) = delete;
 
  protected:
+  void Notify(const T& v) noexcept {
+    assert(!calling_observer_ && "do not call Notify from observer callback");
+
+    calling_observer_ = true;
+    for (auto obs : obs_) {
+      obs->Notify(v);
+    }
+    calling_observer_ = false;
+  }
   void Notify(T&& v) noexcept {
     assert(!calling_observer_ && "do not call Notify from observer callback");
 
@@ -58,12 +67,7 @@ class Observer<T>::Target {
       calling_observer_ = false;
       return;
     }
-
+    Notify(v);
-    calling_observer_ = true;
-    for (auto obs : obs_) {
-      obs->Notify(v);
-    }
-    calling_observer_ = false;
   }
 
   bool observed(const T* = nullptr) const noexcept { return !obs_.empty(); }