std::function target must be copy-constructible

[rodgull]

Hi,
I used your gnat project on an ESP32 3 years ago and got it running ok.

I have now updated to the latest ESP IDF and I can't compile the server.h file.
I have renamed server.h to MQTTBroker.h to improve clarity in my codebase, but have not changed the fundamental operation.

I tried changing the std::function to std::move_only_function but that gave a different set of errors.

I really don't understand c++ well enough to properly debug this and don't know how to fix it.

Maybe the issue is with the unique_ptr in the DataStoreEntry?

The full error is:

main/include/gnat/mqtt_broker.h:172:15: required from 'gnat::Status gnat::MQTTBroker<DataStore, Clock>::HandleMessage(gnat::Packet*) [with ClientConnection = AcTcpConnection; DataStore = gnat::DataStore<std::__cxx11::basic_string >; Clock = ClockWrapper]'
main/ac_mqtt_connection.cpp:73:42: required from here
/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20240530/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/bits/std_function.h:439:69: error: static assertion failed: std::function target must be copy-constructible
439 | static_assert(is_copy_constructible<__decay_t<_Functor>>::value,
| ^~~~~
/.espressif/tools/xtensa-esp-elf/esp-13.2.0_20240530/xtensa-esp-elf/xtensa-esp-elf/include/c++/13.2.0/bits/std_function.h:439:69: note: 'std::integral_constant<bool, false>::value' evaluates to false
[19/25] Building CXX object esp-idf/main/CMakeFiles/__idf_main.dir/ac_mqtt_server.cpp.obj