undefined reference for std::function when not using file_event_handlers in daily_file_sink constructor call

[kslattery]

I have the following code to create a daily_file_sink with an overridden calc_filename():

    spdlog::init_thread_pool(4096,1);
    _thread_pool = spdlog::thread_pool();
    std::vector<spdlog::sink_ptr> sinks;
    sinks.push_back(std::make_shared<spdlog::sinks::daily_file_sink<std::mutex, custom_file_name_calculator>>(_output_dir, 0, 0));
    auto logger = std::make_shared<spdlog::async_logger>("mydailylogger", begin(sinks), end(sinks), std::move(_thread_pool), spdlog::async_overflow_policy::block);
    spdlog::register_logger(logger);

When this is linked, I get the following errors:

CMakeFiles/my_project.dir/src/RuntimeFileLogger.cpp.o: In function `void __gnu_cxx::new_allocator<spdlog::sinks::daily_file_sink<std::mutex, custom_file_name_calculator> >::construct<spdlog::sinks::daily_file_sink<std::mutex, custom_file_name_calculator>, std::string&, int, int>(spdlog::sinks::daily_file_sink<std::mutex, custom_file_name_calculator>*, std::string&, int&&, int&&)':
/usr/include/c++/5/ext/new_allocator.h:120: undefined reference to `std::function<void (std::string const&, _IO_FILE*)>::function()'
/usr/include/c++/5/ext/new_allocator.h:120: undefined reference to `std::function<void (std::string const&, _IO_FILE*)>::function()'
collect2: error: ld returned 1 exit status

Note that I'm not specifying every parameter for the daily_file_sink constructor, but relying on the default values specified here:

    daily_file_sink(filename_t base_filename, int rotation_hour, int rotation_minute, bool truncate = false, uint16_t max_files = 0,
        const file_event_handlers &event_handlers = {})

The problem seems to come from the fact that in places like file_helper::open(), we have code like this:

    if (event_handlers_.before_open)
    {
        event_handlers_.before_open(filename_);
    }

So although the before_open() call would never be made when using the default event_handlers = {}, the linker is still looking for the symbol which simply does not exist.

I worked around this by changing my code to be this:

    spdlog::init_thread_pool(4096,1);
    _thread_pool = spdlog::thread_pool();
    spdlog::file_event_handlers handlers;
    handlers.after_open = [](spdlog::filename_t, std::FILE*) {};
    handlers.before_close = [](spdlog::filename_t, std::FILE*) {};
    std::vector<spdlog::sink_ptr> sinks;
    sinks.push_back(std::make_shared<spdlog::sinks::daily_file_sink<std::mutex, custom_file_name_calculator>>(_output_dir, 0, 0, false, 0, handlers));
    auto logger = std::make_shared<spdlog::async_logger>("mydailylogger", begin(sinks), end(sinks), std::move(_thread_pool), spdlog::async_overflow_policy::block);
    spdlog::register_logger(logger);

I have no idea why I was getting the undefined reference errors for after_open() and before_close(), but not for before_open() and after_close(). Something to do with having two parameters instead of one?

One possible fix would be to change the defaulted value for the constructor:

    daily_file_sink(filename_t base_filename, int rotation_hour, int rotation_minute, bool truncate = false, uint16_t max_files = 0,
        const file_event_handlers &event_handlers = {[](spdlog::filename_t) {}, [](spdlog::filename_t, std::FILE*) {},[](spdlog::filename_t, std::FILE*) {},[](spdlog::filename_t) {}})

[gabime]

In which compiler version does it happen?

[kslattery]

$ gcc --version
gcc (Ubuntu 5.5.0-12ubuntu1~14.04) 5.5.0 20171010
Copyright (C) 2015 Free Software Foundation, Inc.

$ ld --version
GNU ld (GNU Binutils for Ubuntu) 2.24

I'm compiling with -std=c++14.

[gabime]

Thanks. If you happen to find a fix for this please share open a pr.