g++ -O3 --std=c++17 `pkg-config --cflags gtkmm-3.0` -c -o src/window_main.o src/window_main.cpp In file included from /usr/include/sigc++-2.0/sigc++/signal_base.h:27, from /usr/include/sigc++-2.0/sigc++/signal.h:8, from /usr/include/sigc++-2.0/sigc++/sigc++.h:104, from /usr/include/glibmm-2.4/glibmm/thread.h:49, from /usr/include/glibmm-2.4/glibmm.h:88, from /usr/include/gtkmm-3.0/gtkmm.h:87, from src/window_main.h:21, from src/window_main.cpp:1: /usr/include/sigc++-2.0/sigc++/functors/slot.h: In instantiation of ‘static T_return sigc::internal::slot_call1::call_it(sigc::internal::slot_rep*, sigc::type_trait_take_t) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*; sigc::type_trait_take_t = _GdkEventKey* const&]’: /usr/include/sigc++-2.0/sigc++/functors/slot.h:155:37: required from ‘static void* (* sigc::internal::slot_call1::address())(void*) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*; sigc::internal::hook = void* (*)(void*)]’ /usr/include/sigc++-2.0/sigc++/functors/slot.h:657:90: required from ‘sigc::slot1::slot1(const T_functor&) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*]’ /usr/include/sigc++-2.0/sigc++/functors/slot.h:1821:26: required from ‘sigc::slot::slot(const T_functor&) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*]’ src/window_main.cpp:55:113: required from here /usr/include/sigc++-2.0/sigc++/functors/slot.h:148:20: error: void value not ignored as it ought to be (a_1); ^ In file included from /usr/include/sigc++-2.0/sigc++/functors/slot.h:7, from /usr/include/sigc++-2.0/sigc++/signal_base.h:27, from /usr/include/sigc++-2.0/sigc++/signal.h:8, from /usr/include/sigc++-2.0/sigc++/sigc++.h:104, from /usr/include/glibmm-2.4/glibmm/thread.h:49, from /usr/include/glibmm-2.4/glibmm.h:88, from /usr/include/gtkmm-3.0/gtkmm.h:87, from src/window_main.h:21, from src/window_main.cpp:1: /usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h: In instantiation of ‘typename sigc::adaptor_functor::deduce_result_type::type sigc::adaptor_functor::operator()(T_arg1) const [with T_arg1 = _GdkEventKey* const&; T_functor = sigc::bound_mem_functor0; typename sigc::adaptor_functor::deduce_result_type::type = void]’: /usr/include/sigc++-2.0/sigc++/functors/slot.h:148:20: required from ‘static T_return sigc::internal::slot_call1::call_it(sigc::internal::slot_rep*, sigc::type_trait_take_t) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*; sigc::type_trait_take_t = _GdkEventKey* const&]’ /usr/include/sigc++-2.0/sigc++/functors/slot.h:155:37: required from ‘static void* (* sigc::internal::slot_call1::address())(void*) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*; sigc::internal::hook = void* (*)(void*)]’ /usr/include/sigc++-2.0/sigc++/functors/slot.h:657:90: required from ‘sigc::slot1::slot1(const T_functor&) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*]’ /usr/include/sigc++-2.0/sigc++/functors/slot.h:1821:26: required from ‘sigc::slot::slot(const T_functor&) [with T_functor = sigc::bound_mem_functor0; T_return = bool; T_arg1 = _GdkEventKey*]’ src/window_main.cpp:55:113: required from here /usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:89:30: error: no match for call to ‘(sigc::bound_mem_functor0) (_GdkEventKey* const&)’ { return functor_(_A_arg1); } ^ In file included from /usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:9, from /usr/include/sigc++-2.0/sigc++/functors/slot.h:7, from /usr/include/sigc++-2.0/sigc++/signal_base.h:27, from /usr/include/sigc++-2.0/sigc++/signal.h:8, from /usr/include/sigc++-2.0/sigc++/sigc++.h:104, from /usr/include/glibmm-2.4/glibmm/thread.h:49, from /usr/include/glibmm-2.4/glibmm.h:88, from /usr/include/gtkmm-3.0/gtkmm.h:87, from src/window_main.h:21, from src/window_main.cpp:1: /usr/include/sigc++-2.0/sigc++/functors/mem_fun.h:1990:12: note: candidate: ‘T_return sigc::bound_mem_functor0::operator()() const [with T_return = void; T_obj = WindowMain]’ T_return operator()() const ^~~~~~~~ /usr/include/sigc++-2.0/sigc++/functors/mem_fun.h:1990:12: note: candidate expects 0 arguments, 1 provided In file included from /usr/include/sigc++-2.0/sigc++/functors/slot.h:7, from /usr/include/sigc++-2.0/sigc++/signal_base.h:27, from /usr/include/sigc++-2.0/sigc++/signal.h:8, from /usr/include/sigc++-2.0/sigc++/sigc++.h:104, from /usr/include/glibmm-2.4/glibmm/thread.h:49, from /usr/include/glibmm-2.4/glibmm.h:88, from /usr/include/gtkmm-3.0/gtkmm.h:87, from src/window_main.h:21, from src/window_main.cpp:1: /usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:89:30: error: return-statement with a value, in function returning ‘sigc::adaptor_functor >::deduce_result_type<_GdkEventKey* const&, void, void, void, void, void, void>::type’ {aka ‘void’} [-fpermissive] { return functor_(_A_arg1); } ^ make: *** [: src/window_main.o] Error 1