optional_last_value, which does not throw, has replaced last_value as
Boosts default combiner. Besides being better supported, it also doesn't
trigger gcc's -Wmaybe-unitialized warning, presumably because exceptions no
longer bubble-up out of signals:
In file included from ui_interface.cpp:9:
/bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp: In member function 'boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::result_type boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::operator()(Args ...) [with Combiner = boost::signals2::last_value<bool>; Group = int; GroupCompare = std::less<int>; SlotFunction = boost::function<bool(const bilingual_str&, const std::__cxx11::basic_string<char>&, unsigned int)>; ExtendedSlotFunction = boost::function<bool(const boost::signals2::connection&, const bilingual_str&, const std::__cxx11::basic_string<char>&, unsigned int)>; Mutex = boost::signals2::mutex; R = bool; Args = {const bilingual_str&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned int}]':
/bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:54:36: warning: '*((void*)& value +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
if(value) return value.get();
^
/bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:43:21: note: '*((void*)& value +1)' was declared here
optional<T> value;
^~~~~
/bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp: In member function 'boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::result_type boost::signals2::detail::signal_impl<R(Args ...), Combiner, Group, GroupCompare, SlotFunction, ExtendedSlotFunction, Mutex>::operator()(Args ...) [with Combiner = boost::signals2::last_value<bool>; Group = int; GroupCompare = std::less<int>; SlotFunction = boost::function<bool(const bilingual_str&, const std::__cxx11::basic_string<char>&, const std::__cxx11::basic_string<char>&, unsigned int)>; ExtendedSlotFunction = boost::function<bool(const boost::signals2::connection&, const bilingual_str&, const std::__cxx11::basic_string<char>&, const std::__cxx11::basic_string<char>&, unsigned int)>; Mutex = boost::signals2::mutex; R = bool; Args = {const bilingual_str&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, unsigned int}]':
/bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:54:36: warning: '*((void*)& value +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
if(value) return value.get();
^
/bitcoin/depends/x86_64-pc-linux-gnu/share/../include/boost/signals2/last_value.hpp:43:21: note: '*((void*)& value +1)' was declared here
optional<T> value;
^~~~~
The change in default happened in Boost 1.39.0 (along with the introduction of the Signals2 library.
More information is also available here https://www.boost.org/doc/libs/1_73_0/doc/html/signals2/rationale.html#id-1.3.36.9.4:
The default combiner for Boost.Signals2 has changed from the last_value combiner used by default in the original Boost.Signals library. This is because last_value requires that at least 1 slot be connected to the signal when it is invoked (except for the last_value<void> specialization). In a multi-threaded environment where signal invocations and slot connections and disconnections may be happening concurrently, it is difficult to fulfill this requirement. When using optional_last_value, there is no requirement for slots to be connected when a signal is invoked, since in that case the combiner may simply return an empty boost::optional.
