feature request: better logging plugin options

[berndbischl]

llogging.h

https://github.com/CMA-ES/libcmaes/blob/master/include/libcmaes/llogging.h

Looks like this:

#ifndef LLOGGING_H
#define LLOGGING_H

#include <libcmaes/libcmaes_config.h>

#ifdef HAVE_GLOG // HAVE_LIB_GLOG
#include <glog/logging.h>
#else
#include <iostream>

namespace libcmaes
{
  static std::string INFO="INFO";
  static std::string WARNING="WARNING";
  static std::string ERROR="ERROR";
  static std::string FATAL="FATAL";

  static std::ostream nullstream(0);

inline std::ostream& LOG(const std::string &severity,std::ostream &out=std::cout)
{
  out << severity << " - ";
  return out;
}

inline std::ostream& LOG_IF(const std::string &severity,const bool &condition,std::ostream &out=std::cout)
{
  if (condition)
    return LOG(severity,out);
  else return nullstream;
}
}
#endif
#endif

I am writing here as I currently connect libcmaes to R, see #252.
(Probably a normal usecase for such a lib)

The problem is: I have to redirect output. I cannot have code that directly prints to stderr / stdout.
Otherwise my package gets "flagged" on CRAN (package distribution server for R).
I think, a general and good pattern here is to allow ppl like me, to hook in their own code for "LOG" -- without touching your code.

---

Side comment: I also found (at least) one instance in cmaparamters.cc where you dont go thru your own logger funs, but directly print to cerr:
std::cerr << "[Warning]: set_vd on non VD algorithm " << this->_algo << ". Not activating VD update\n";

---

If you currently dont have a lot of time: Its not the most pressing isse for me, I can manually edit your code in my vendored version.
But: not ideal IMHO. OTOH that is really easy to change on your side IMHO, and I also could do a PR.

---

Probably something like this would be enough

#ifndef LOG
  // Default to libcmaes' own LOG(...) if client didn't override
  #define LOG(sev) ::libcmaes::LOG(sev)
#endif

#ifndef LOG_IF
  // Derive LOG_IF from LOG so clients don't need to provide a second macro
  #define LOG_IF(sev, cond) ((cond) ? LOG(sev) : ::libcmaes::nullstream)
#endif

Clients can then do this in an extra header:

#pragma once
#define LOG(sev) bla

and in cmake

DCMAKE_CXX_FLAGS="-include /abs/path/my_log_provider.h"

?

[beniz]

Hi, thanks for your thorough review, yes you can PR what suits you, as it will certainly suit others.