Specializing macro with arguments

Question

Suppose I have the following macro:

#define LOG(L, ...) func(L, __VA_ARGS__);

Where L can be one of INFO, WARN, FATAL

Now I want to define it for FATAL differently.

#define LOG(FATAL, ...) {func(FATAL, __VA_ARGS__); exit(-1);}

How to accomplish this?

Edit: As a follow up to above, is there a better way to do it? i.e. by avoiding macros for example.

Answer 1

Macros are mostly a bad choice in C++ – essentially because of they are namespace agnostic and may take effect where it is unexpected.

That said – a sample for how OPs issue could be solved e.g. using token pasting:

#include <iostream>

#define LOG(LEVEL, ...) LOG_##LEVEL(__VA_ARGS__)

#define LOG_INFO(...) log(Info, __VA_ARGS__)
#define LOG_WARN(...) log(Warn, __VA_ARGS__)
#define LOG_FATAL(...) do { log(Error, __VA_ARGS__); std::cerr << "Program aborted!\n"; } while (false)

enum Level { Info, Warn, Error };

void log(Level level, const char *text)
{
  static const char *levelText[] = { "INFO", "WARNING", "ERROR" };
  std::cerr << levelText[level] << ": " << text << '\n';
}

int main()
{
  LOG(INFO, "Everything fine. :-)");
  LOG(WARN, "Not so fine anymore. :-|");
  LOG(FATAL, "Things became worst. :-(");
}

Output:

INFO: Everything fine. :-)
WARNING: Not so fine anymore. :-|
ERROR: Things became worst. :-(
Program aborted!

Live Demo on coliru

---

Another sample for the follow-up question - with variadic templates instead of macros:

#include <iostream>

enum Level { Info, Warn, Error, Fatal };

template <typename ...ARGS>
void log(Level level, ARGS&& ... args)
{
  static const char *levelText[] = { "INFO", "WARNING", "ERROR", "FATAL" };
  std::cerr << levelText[level] << ": ";
  (std::cerr << ... << args);
  std::cerr << '\n';
  if (level == Fatal) std::cerr << "Program aborted!";
}

int main()
{
  log(Info, "Everything fine.", ' ', ":-)");
  log(Warn, "Not so fine anymore.", ' ', ":-|");
  log(Error, "Things became bad.", ' ', ":-(");
  log(Fatal, "Things became worst.", ' ', "XXX");
}

Output:

INFO: Everything fine. :-)
WARNING: Not so fine anymore. :-|
ERROR: Things became bad. :-(
FATAL: Things became worst. XXX
Program aborted!

Live Demo on coliru

I must admit that I needed some help for the parameter unpacking – found here:
SO: What is the easiest way to print a variadic parameter pack using std::ostream?.