How to share the same file sink between different loggers (cateogories)

[gianmarcoodorizzi]

Possibly related to this, but I am not sure I grasped whether this is possible or it is not.

I know that the idea is to share a common sink between two loggers, but how can I share that sink between different classes? I would like to avoid having to pass it to every class (e.g.: in its constructor).

The goal is to have different classes writing to the same log file, but with different categories, for instance:

[2022-05-13 13:54:07.200] [ClassA] [info] Test entry
[2022-05-13 13:54:07.200] [ClassB] [info] Test entry
[2022-05-13 13:54:07.200] [ClassC] [info] Test entry

Any suggestion would be greatly appreciated, thank you!

[tt4g]

You can get sink from logger.

// get another_logger from registry.
auto another_logger = spdlog::get("another");

// get sink by `spdlog::logger::sinks()`.
auto sinks = another_logger->sinks():
// create new_logger.
auto new_logger = std::make_shared<spdlog::logger>("new_logger", sinks.begin(), sinks.end());

// clone logger (share sinks).
auto clone_logger = another_logger->clone("clone_logger"):

Wiki: https://github.com/gabime/spdlog/wiki/4.-Sinks#adding-sinks-to-the-logger-after-creation

[gianmarcoodorizzi]

Thanks! Do I have to specify flush_on policy after registering every new logger to the global registry? It seems like if I do not, log entries are not properly flushed.

[tt4g]

You can call spdlog::initizlie_logger() instead of spdlog::register_logger() to apply the flush policy you previously set with spdlog::flush_on().
Note spdlog::initizlie_logger() also applies spdlog global settings other than flash policy to the logger.

[gianmarcoodorizzi]

Thank you! I have one more question, though. I create multiple object instances for some of my classes, and registering (or initializing) the log with the same name throws an exception. I did not have that issue before, since I was not registering my logs to the global registry. Is there a way to work around this, i.e.: a way to register loggers with the same name? Thanks.

[tt4g]

No. Since spdlog identifies loggers by name and registers them in the global registry, loggers with the same name cannot be registered.

[gianmarcoodorizzi]

I see, thanks!