diff --git a/examples/dialog.cpp b/examples/dialog.cpp
index eef1e272..cbd80b65 100644
--- a/examples/dialog.cpp
+++ b/examples/dialog.cpp
@@ -26,13 +26,13 @@ using finalcut::FPoint;
 using finalcut::FSize;
 
 // function prototype
-void cb_quit (finalcut::FApplication&);
+void cb_quit (const finalcut::FApplication&);
 
 
 //----------------------------------------------------------------------
 // callback function
 //----------------------------------------------------------------------
-void cb_quit (finalcut::FApplication& app)
+void cb_quit (const finalcut::FApplication& app)
 {
   app.quit();
 }
diff --git a/examples/fullwidth-character.cpp b/examples/fullwidth-character.cpp
index b375e210..3e358379 100644
--- a/examples/fullwidth-character.cpp
+++ b/examples/fullwidth-character.cpp
@@ -105,7 +105,7 @@ int main (int argc, char* argv[])
 
   // Callback lambda expressions
   auto cb_exit = \
-      [] (finalcut::FApplication& a)
+      [] (const finalcut::FApplication& a)
       {
         a.quit();
       };
diff --git a/examples/input-dialog.cpp b/examples/input-dialog.cpp
index b1438d8b..e977d60d 100644
--- a/examples/input-dialog.cpp
+++ b/examples/input-dialog.cpp
@@ -26,20 +26,21 @@ using finalcut::FPoint;
 using finalcut::FSize;
 
 // function prototypes
-void cb_quit (finalcut::FApplication&);
-void cb_publish (finalcut::FCheckBox&, finalcut::FCheckBox&);
+void cb_quit (const finalcut::FApplication&);
+void cb_publish (const finalcut::FCheckBox&, finalcut::FCheckBox&);
 
 
 //----------------------------------------------------------------------
 // callback functions
 //----------------------------------------------------------------------
-void cb_quit (finalcut::FApplication& app)
+void cb_quit (const finalcut::FApplication& app)
 {
   app.quit();
 }
 
 //----------------------------------------------------------------------
-void cb_publish (finalcut::FCheckBox& cbox1, finalcut::FCheckBox& cbox2)
+void cb_publish ( const finalcut::FCheckBox& cbox1
+                , finalcut::FCheckBox& cbox2 )
 {
   if ( cbox1.isChecked() )
     cbox2.setEnable();
diff --git a/examples/menu.cpp b/examples/menu.cpp
index 934c3916..216c630f 100644
--- a/examples/menu.cpp
+++ b/examples/menu.cpp
@@ -61,7 +61,7 @@ class Menu final : public finalcut::FDialog
     void onClose (finalcut::FCloseEvent*) override;
 
     // Callback method
-    void cb_message (finalcut::FMenuItem*);
+    void cb_message (const finalcut::FMenuItem*);
 
     // Data members
     finalcut::FString        line{13, fc::BoxDrawingsHorizontal};
@@ -305,7 +305,7 @@ void Menu::onClose (finalcut::FCloseEvent* ev)
 }
 
 //----------------------------------------------------------------------
-void Menu::cb_message (finalcut::FMenuItem* menuitem)
+void Menu::cb_message (const finalcut::FMenuItem* menuitem)
 {
   auto text = menuitem->getText();
   text = text.replace('&', "");
diff --git a/examples/opti-move.cpp b/examples/opti-move.cpp
index a4950f5e..70b1cb05 100644
--- a/examples/opti-move.cpp
+++ b/examples/opti-move.cpp
@@ -217,12 +217,12 @@ int main (int argc, char* argv[])
   // Force terminal initialization without calling show()
   term_app.initTerminal();
 
-  // Pointer to the global virtual terminal object
-  app = &term_app;
-
   if ( finalcut::FApplication::isQuit() )
     return 0;
 
+  // Pointer to the global virtual terminal object
+  app = &term_app;
+
   // Get screen dimension
   int xmax = int(term_app.getDesktopWidth() - 1);
   int ymax = int(term_app.getDesktopHeight() - 1);
diff --git a/examples/ui.cpp b/examples/ui.cpp
index 6676d9ca..bd86c144 100644
--- a/examples/ui.cpp
+++ b/examples/ui.cpp
@@ -304,12 +304,13 @@ class MyDialog final : public finalcut::FDialog
     void cb_switchTheme (const finalcut::FCheckMenuItem*) const;
     void cb_input2buttonText ( finalcut::FButton&
                              , const finalcut::FLineEdit& ) const;
-    void cb_setTitlebar (finalcut::FLineEdit&);
+    void cb_setTitlebar (const finalcut::FLineEdit&);
     void cb_showProgressBar();
-    void cb_updateNumber (finalcut::FListBox&, finalcut::FLabel&) const;
-    void cb_activateButton ( finalcut::FRadioButton&
+    void cb_updateNumber ( const finalcut::FListBox&
+                         , finalcut::FLabel& ) const;
+    void cb_activateButton ( const finalcut::FRadioButton&
                            , finalcut::FButton& ) const;
-    void cb_view (finalcut::FMenuItem*);
+    void cb_view (const finalcut::FMenuItem*);
     void cb_setInput (finalcut::FListBox&, finalcut::FLineEdit&) const;
 
     // Data members
@@ -951,7 +952,7 @@ void MyDialog::cb_input2buttonText ( finalcut::FButton& button
 }
 
 //----------------------------------------------------------------------
-void MyDialog::cb_setTitlebar (finalcut::FLineEdit& lineedit)
+void MyDialog::cb_setTitlebar (const finalcut::FLineEdit& lineedit)
 {
   finalcut::FString title{};
   lineedit >> title;
@@ -968,7 +969,7 @@ void MyDialog::cb_showProgressBar()
 }
 
 //----------------------------------------------------------------------
-void MyDialog::cb_updateNumber ( finalcut::FListBox& list
+void MyDialog::cb_updateNumber ( const finalcut::FListBox& list
                                , finalcut::FLabel& num) const
 {
   const auto count = list.getCount();
@@ -984,7 +985,7 @@ void MyDialog::cb_updateNumber ( finalcut::FListBox& list
 }
 
 //----------------------------------------------------------------------
-void MyDialog::cb_activateButton ( finalcut::FRadioButton& rb
+void MyDialog::cb_activateButton ( const finalcut::FRadioButton& rb
                                  , finalcut::FButton& button) const
 {
   if ( rb.isChecked() )
@@ -996,7 +997,7 @@ void MyDialog::cb_activateButton ( finalcut::FRadioButton& rb
 }
 
 //----------------------------------------------------------------------
-void MyDialog::cb_view (finalcut::FMenuItem* item)
+void MyDialog::cb_view (const finalcut::FMenuItem* item)
 {
   finalcut::FString file{};
 
diff --git a/src/fbuttongroup.cpp b/src/fbuttongroup.cpp
index 0a917bba..1ae70041 100644
--- a/src/fbuttongroup.cpp
+++ b/src/fbuttongroup.cpp
@@ -542,7 +542,7 @@ void FButtonGroup::directFocus()
 }
 
 //----------------------------------------------------------------------
-void FButtonGroup::cb_buttonToggled (FToggleButton* button) const
+void FButtonGroup::cb_buttonToggled (const FToggleButton* button) const
 {
   if ( (button && ! button->isChecked()) || buttonlist.empty() )
     return;
diff --git a/src/ffiledialog.cpp b/src/ffiledialog.cpp
index 7b18e59f..66a4c70f 100644
--- a/src/ffiledialog.cpp
+++ b/src/ffiledialog.cpp
@@ -201,7 +201,7 @@ void FFileDialog::setPath (const FString& dir)
     return;
   }
 
-  if ( fsystem->realpath(dir.c_str(), resolved_path) != nullptr )
+  if ( fsystem && fsystem->realpath(dir.c_str(), resolved_path) != nullptr )
     r_dir.setString(resolved_path);
   else
     r_dir.setString(dir);
diff --git a/src/fmenu.cpp b/src/fmenu.cpp
index c249f008..5ecfd668 100644
--- a/src/fmenu.cpp
+++ b/src/fmenu.cpp
@@ -339,7 +339,7 @@ void FMenu::cb_menuitemDisabled()
 }
 
 //----------------------------------------------------------------------
-void FMenu::cb_menuitemToggled (FMenuItem* m_item) const
+void FMenu::cb_menuitemToggled (const FMenuItem* m_item) const
 {
   if ( ! (has_checkable_items && m_item && m_item->isChecked()) )
     return;
diff --git a/src/fmenubar.cpp b/src/fmenubar.cpp
index ac6bc01d..c1ceecfd 100644
--- a/src/fmenubar.cpp
+++ b/src/fmenubar.cpp
@@ -228,7 +228,7 @@ void FMenuBar::onAccel (FAccelEvent* ev)
 }
 
 //----------------------------------------------------------------------
-void FMenuBar::cb_itemDeactivated (FMenuItem* menuitem) const
+void FMenuBar::cb_itemDeactivated (const FMenuItem* menuitem) const
 {
   if ( ! menuitem->hasMenu() )
     return;
diff --git a/src/fmessagebox.cpp b/src/fmessagebox.cpp
index 6896ced0..4d0396be 100644
--- a/src/fmessagebox.cpp
+++ b/src/fmessagebox.cpp
@@ -108,7 +108,7 @@ FMessageBox& FMessageBox::operator = (const FMessageBox& mbox)
   }
   else
   {
-    for (uInt n{0}; n < num_buttons; n++)
+    for (uInt n{0}; n < num_buttons && button[n]; n++)
       delete button[n];
 
     if ( mbox.getParentWidget() )
@@ -138,7 +138,7 @@ void FMessageBox::setHeadline (const FString& headline)
   headline_text.setString(headline);
   setHeight(getHeight() + 2, true);
 
-  for (uInt n{0}; n < num_buttons; n++)
+  for (uInt n{0}; n < num_buttons && button[n]; n++)
     button[n]->setY (int(getHeight()) - 4, false);
 
   const std::size_t column_width = getColumnWidth(headline_text);
@@ -268,7 +268,7 @@ inline void FMessageBox::allocation()
 //----------------------------------------------------------------------
 inline void FMessageBox::deallocation()
 {
-  for (uInt n{0}; n < num_buttons; n++)
+  for (uInt n{0}; n < num_buttons && button[n]; n++)
     delete button[n];
 }
 
@@ -388,7 +388,7 @@ void FMessageBox::resizeButtons() const
   std::size_t len[3]{};
   std::size_t max_size{};
 
-  for (std::size_t n{0}; n < num_buttons; n++)
+  for (std::size_t n{0}; n < num_buttons && button[n]; n++)
   {
     len[n] = button[n]->getText().getLength();
 
@@ -410,7 +410,7 @@ void FMessageBox::resizeButtons() const
   if ( max_size < 7 )
     max_size = 7;
 
-  for (std::size_t n{0}; n < num_buttons; n++)
+  for (std::size_t n{0}; n < num_buttons && button[n]; n++)
     button[n]->setWidth(max_size + 3, false);
 }
 
@@ -420,7 +420,7 @@ void FMessageBox::adjustButtons()
   static constexpr std::size_t gap = 4;
   std::size_t btn_width{0};
 
-  for (std::size_t n{0}; n < num_buttons; n++)
+  for (std::size_t n{0}; n < num_buttons && button[n]; n++)
   {
     if ( n == num_buttons - 1 )
       btn_width += button[n]->getWidth();
@@ -439,7 +439,7 @@ void FMessageBox::adjustButtons()
 
   const int btn_x = int((getWidth() - btn_width) / 2);
 
-  for (std::size_t n{0}; n < num_buttons; n++)
+  for (std::size_t n{0}; n < num_buttons && button[n]; n++)
   {
     if ( n == 0 )
       button[n]->setX(btn_x);
diff --git a/src/fstatusbar.cpp b/src/fstatusbar.cpp
index 0dab686f..b4b0bd3e 100644
--- a/src/fstatusbar.cpp
+++ b/src/fstatusbar.cpp
@@ -488,7 +488,7 @@ void FStatusBar::onMouseMove (FMouseEvent* ev)
 }
 
 //----------------------------------------------------------------------
-void FStatusBar::cb_statuskey_activated (FStatusKey* statuskey)
+void FStatusBar::cb_statuskey_activated (const FStatusKey* statuskey)
 {
   if ( ! statuskey )
     return;
diff --git a/src/fstring.cpp b/src/fstring.cpp
index 1216fa32..fff47f25 100644
--- a/src/fstring.cpp
+++ b/src/fstring.cpp
@@ -387,7 +387,10 @@ std::size_t FString::getUTF8length() const
   const char* s = c_str();
 
   while ( *s )
-    len += std::size_t((*s++ & 0xc0) != 0x80);
+  {
+    len += std::size_t((*s & 0xc0) != 0x80);
+    s++;
+  }
 
   return len;
 }
@@ -1449,29 +1452,26 @@ inline const char* FString::_to_cstring (const wchar_t s[]) const
   if ( c_string )
     delete[](c_string);
 
-  const int size = int(std::wcslen(s)) + 1;
-  const int dest_size = size * int(CHAR_SIZE);
   const wchar_t* src = s;
-  std::mbstate_t state{};
-  std::memset (&state, '\0', sizeof(mbstate_t));
+  auto state = std::mbstate_t();
+  auto size = std::wcsrtombs(nullptr, &src, 0, &state) + 1;
 
   try
   {
-    c_string = new char[std::size_t(dest_size)]();
+    c_string = new char[size]();
 
     // pre-initialiaze the whole string with '\0'
-    std::memset (c_string, '\0', std::size_t(dest_size));
+    std::memset (c_string, '\0', size);
   }
   catch (const std::bad_alloc&)
   {
-    badAllocOutput ("char[std::size_t(dest_size)]");
+    badAllocOutput ("char[size]");
     return nullptr;
   }
 
-  const int mblength = \
-      int(std::wcsrtombs (c_string, &src, std::size_t(dest_size), &state));
+  const auto mblength = std::wcsrtombs (c_string, &src, size, &state);
 
-  if ( mblength == -1 && errno != EILSEQ )
+  if ( mblength == static_cast<std::size_t>(-1) && errno != EILSEQ )
   {
     delete[](c_string);
     c_string = nullptr;
@@ -1501,29 +1501,26 @@ inline const wchar_t* FString::_to_wcstring (const char s[]) const
     }
   }
 
-  const int size = int(std::strlen(s)) + 1;
-  const int dest_size = size * int(CHAR_SIZE);
   const char* src = s;
   wchar_t* dest{};
-  std::mbstate_t state{};
-  std::memset (&state, '\0', sizeof(mbstate_t));
+  auto state = std::mbstate_t();
+  auto size = std::mbsrtowcs(nullptr, &src, 0, &state) + 1;
 
   try
   {
-    dest = new wchar_t[std::size_t(size)]();
+    dest = new wchar_t[size]();
     // pre-initialiaze the whole string with '\0'
-    std::wmemset (dest, L'\0', std::size_t(size));
+    std::wmemset (dest, L'\0', size);
   }
   catch (const std::bad_alloc&)
   {
-    badAllocOutput ("wchar_t[std::size_t(size)]");
+    badAllocOutput ("wchar_t[size]");
     return nullptr;
   }
 
-  const int wclength = \
-      int(std::mbsrtowcs (dest, &src, std::size_t(dest_size), &state));
+  const auto wclength = std::mbsrtowcs (dest, &src, size, &state);
 
-  if ( wclength == -1 )
+  if ( wclength == static_cast<std::size_t>(-1) )
   {
     if ( src != s )
       return dest;
diff --git a/src/include/final/fapplication.h b/src/include/final/fapplication.h
index 7c61a8f9..fc7c0b76 100644
--- a/src/include/final/fapplication.h
+++ b/src/include/final/fapplication.h
@@ -139,7 +139,7 @@ class FApplication : public FWidget
     static void           closeConfirmationDialog (FWidget*, FCloseEvent*);
 
     // Callback method
-    void                  cb_exitApp (FWidget*);
+    void                  cb_exitApp (FWidget*) const;
 
   protected:
     virtual void          processExternalUserEvent();
@@ -235,7 +235,7 @@ inline char** FApplication::getArgv() const
 { return app_argv; }
 
 //----------------------------------------------------------------------
-inline void FApplication::cb_exitApp (FWidget* w)
+inline void FApplication::cb_exitApp (FWidget* w) const
 { w->close(); }
 
 }  // namespace finalcut
diff --git a/src/include/final/fbuttongroup.h b/src/include/final/fbuttongroup.h
index 878ff340..f59e05fd 100644
--- a/src/include/final/fbuttongroup.h
+++ b/src/include/final/fbuttongroup.h
@@ -130,7 +130,7 @@ class FButtonGroup : public FScrollView
     void                directFocus();
 
     // Callback method
-    void                cb_buttonToggled (FToggleButton*) const;
+    void                cb_buttonToggled (const FToggleButton*) const;
 
     // Data members
     FString        text{};
diff --git a/src/include/final/fcallback.h b/src/include/final/fcallback.h
index d6ec1cad..1afa8339 100644
--- a/src/include/final/fcallback.h
+++ b/src/include/final/fcallback.h
@@ -62,43 +62,6 @@ struct FCallbackData
     , cb_function(c)
   { }
 
-  FCallbackData (const FCallbackData& c)  // copy constructor
-    : cb_signal(c.cb_signal)
-    , cb_instance(c.cb_instance)
-    , cb_function_ptr(c.cb_function_ptr)
-    , cb_function(c.cb_function)
-  { }
-
-  FCallbackData (FCallbackData&& c) noexcept  // move constructor
-    : cb_signal(std::move(c.cb_signal))
-    , cb_instance(std::move(c.cb_instance))
-    , cb_function_ptr(std::move(c.cb_function_ptr))
-    , cb_function(std::move(c.cb_function))
-  { }
-
-  // Destructor
-  ~FCallbackData()
-  { }
-
-  // Overloaded operators
-  FCallbackData& operator = (const FCallbackData& c)
-  {
-    cb_signal = c.cb_signal;
-    cb_instance = c.cb_instance;
-    cb_function_ptr = c.cb_function_ptr;
-    cb_function = c.cb_function;
-    return *this;
-  }
-
-  FCallbackData& operator = (FCallbackData&& c) noexcept
-  {
-    cb_signal = std::move(c.cb_signal);
-    cb_instance = std::move(c.cb_instance);
-    cb_function_ptr = std::move(c.cb_function_ptr);
-    cb_function = std::move(c.cb_function);
-    return *this;
-  }
-
   // Data members
   FString   cb_signal{};
   FWidget*  cb_instance{};
@@ -132,7 +95,7 @@ class FCallback
       return "FCallback";
     }
 
-    std::size_t getCallbackCount()
+    std::size_t getCallbackCount() const
     {
       return callback_objects.size();
     }
@@ -235,10 +198,9 @@ class FCallback
       // Add a function as callback
 
       auto ptr = reinterpret_cast<void*>(&cb_function);
-      auto fn = std::bind ( std::forward<Function>(cb_function)
-                          , std::forward<Args>(args)... );
+      auto fn = std::bind (cb_function, std::forward<Args>(args)...);
       FCallbackData obj{ cb_signal, nullptr, ptr, fn };
-      callback_objects.push_back(obj);;
+      callback_objects.push_back(obj);
     }
 
     template<typename Function
@@ -278,8 +240,7 @@ class FCallback
     {
       // Add a non-union class type as callback
 
-      auto fn = std::bind ( std::forward<Function>(cb_function)
-                          , std::forward<Args>(args)... );
+      auto fn = std::bind (cb_function, std::forward<Args>(args)...);
       FCallbackData obj{ cb_signal, nullptr, nullptr, fn };
       callback_objects.push_back(obj);
     }
@@ -419,7 +380,7 @@ class FCallback
     }
 
 
-    void emitCallback (const FString& emit_signal)
+    void emitCallback (const FString& emit_signal) const
     {
       // Initiate callback for the given signal
 
diff --git a/src/include/final/fmenu.h b/src/include/final/fmenu.h
index 89fce8b8..3613694c 100644
--- a/src/include/final/fmenu.h
+++ b/src/include/final/fmenu.h
@@ -127,7 +127,7 @@ class FMenu : public FWindow, public FMenuList
     // Callback method
     void                cb_menuitemEnabled();
     void                cb_menuitemDisabled();
-    void                cb_menuitemToggled (FMenuItem*) const;
+    void                cb_menuitemToggled (const FMenuItem*) const;
 
   private:
     // Constants
diff --git a/src/include/final/fmenubar.h b/src/include/final/fmenubar.h
index adcaed0e..40961246 100644
--- a/src/include/final/fmenubar.h
+++ b/src/include/final/fmenubar.h
@@ -99,7 +99,7 @@ class FMenuBar : public FWindow, public FMenuList
     void          onAccel (FAccelEvent*) override;
 
     // Callback methods
-    void          cb_itemDeactivated (FMenuItem*) const;
+    void          cb_itemDeactivated (const FMenuItem*) const;
 
   private:
     // Constants
diff --git a/src/include/final/fstatusbar.h b/src/include/final/fstatusbar.h
index 566549c5..65ca48e5 100644
--- a/src/include/final/fstatusbar.h
+++ b/src/include/final/fstatusbar.h
@@ -227,7 +227,7 @@ class FStatusBar : public FWindow
     void                onMouseMove (FMouseEvent*) override;
 
     // Callback method
-    void                cb_statuskey_activated (FStatusKey*);
+    void                cb_statuskey_activated (const FStatusKey*);
 
   private:
     // Typedef
diff --git a/src/include/final/fstring.h b/src/include/final/fstring.h
index c3c5bcf4..3021143c 100644
--- a/src/include/final/fstring.h
+++ b/src/include/final/fstring.h
@@ -245,7 +245,6 @@ class FString
     // Constants
     static constexpr uInt FWDBUFFER = 15;
     static constexpr uInt INPBUFFER = 200;
-    static constexpr uInt CHAR_SIZE = sizeof(wchar_t);  // bytes per character
 
     // Methods
     void           _initLength (std::size_t);
diff --git a/src/include/final/fwidget.h b/src/include/final/fwidget.h
index dfb27999..b16a488a 100644
--- a/src/include/final/fwidget.h
+++ b/src/include/final/fwidget.h
@@ -317,10 +317,10 @@ class FWidget : public FVTerm, public FObject
     virtual bool             close();
     void                     clearStatusbarMessage();
     template<typename... Args>
-    void                     addCallback (FString&&, Args&&...);
+    void                     addCallback (const FString&, Args&&...);
     template<typename... Args>
     void                     delCallback (Args&&...);
-    void                     emitCallback (FString&&);
+    void                     emitCallback (const FString&);
     void                     addAccelerator (FKey);
     virtual void             addAccelerator (FKey, FWidget*);
     void                     delAccelerator ();
@@ -984,10 +984,9 @@ inline void FWidget::clearStatusbarMessage()
 
 //----------------------------------------------------------------------
 template<typename... Args>
-inline void FWidget::addCallback (FString&& cb_signal, Args&&... args)
+inline void FWidget::addCallback (const FString& cb_signal, Args&&... args)
 {
-  callback_impl.addCallback ( std::forward<FString>(cb_signal)
-                            , std::forward<Args>(args)... );
+  callback_impl.addCallback (cb_signal, std::forward<Args>(args)...);
 }
 
 //----------------------------------------------------------------------
@@ -998,9 +997,9 @@ inline void FWidget::delCallback (Args&&... args)
 }
 
 //----------------------------------------------------------------------
-inline void FWidget::emitCallback (FString&& emit_signal)
+inline void FWidget::emitCallback (const FString& emit_signal)
 {
-  callback_impl.emitCallback(std::forward<FString>(emit_signal));
+  callback_impl.emitCallback(emit_signal);
 }
 
 //----------------------------------------------------------------------
diff --git a/test/fcallback-test.cpp b/test/fcallback-test.cpp
index 65aef882..51d25371 100644
--- a/test/fcallback-test.cpp
+++ b/test/fcallback-test.cpp
@@ -38,20 +38,20 @@ class Widget
 {
   public:
     template<typename... Args>
-    void addCallback (finalcut::FString&& cb_signal, Args&&... args)
+    void addCallback (const finalcut::FString& cb_signal, Args&&... args)
     {
-      cb.addCallback(std::forward<finalcut::FString>(cb_signal), std::forward<Args>(args)...);
+      cb.addCallback (cb_signal, std::forward<Args>(args)...);
     }
 
     template<typename... Args>
     void delCallback (Args&&... args)
     {
-      cb.delCallback(std::forward<Args>(args)...);
+      cb.delCallback (std::forward<Args>(args)...);
     }
 
-    void emitCallback (finalcut::FString&& emit_signal)
+    void emitCallback (const finalcut::FString& emit_signal)
     {
-      cb.emitCallback(std::forward<finalcut::FString>(emit_signal));
+      cb.emitCallback (emit_signal);
     }
 
   private: