I am creating a telemetry server using websocketpp, and have followed the example here. My application will be running as a linux daemon which starts on boot, and therefore I won't be able to write logs to standard out. I would therefore like to add a customer logger using spdlog, and understand that it can be done based on what's on this page. Looks like I need to use the websocketpp::log::stub interface to create my own customer logger. The issue is, the documentation on this is quite limited regarding logging, and I am not sure where to begin and how to incorporate it in the context of the telemetry server example linked above. I am not sure how to specify the logger when I define my server: typedef websocketpp::server<websocketpp::config::asio> server;.
How do I go about extending the stub class, and how do I initialize my server with this customer logger?
The only sample code I could find is in this thread here, but based on the linked comment this code is no longer relevant after V 0.3.x+.
Building a custom logger has two steps. First, write a policy class with the appropriate interface then create a custom config that uses that policy.
To write the policy class websocketpp::log::stub is a minimal implementation that doesn't actually do anything (it is primarily used for stubbing out logging in the unit tests) but it demonstrates and documents the interface that a logging class needs to implement. The logging class does not need to be a subclass of websocketpp::log::stub. You can look at other examples in the websocketpp/logger/* folder. The syslog logger in particular might be interesting as an example of a logging policy that outputs to something other than standard out.
To set up the custom config you will create a config class. It can be standalone or a subclass of one of the standard ones, like websocketpp::config::asio, that just overrides a small number of things. Your config might only override the loggers, for example. Once created, you will pass your config class into the endpoint template parameter instead of websocketpp::config::asio.
More details about what you can override at compile time via this config system can be found at https://docs.websocketpp.org/reference_8config.html. There is an example on this page that shows a custom config that replaces the default logger (among other changes).