Specializing macro with arguments - c++

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?.

Answer 2

Extending Scheffs answer a bit, this one passes the log level as a template argument, allowing the use of if constexpr (as JVApen mentioned in his comment). You'd need to know the respective level for each log output at compile time, but I think that won't be an issue.
enum Level { Info, Warn, Error, Fatal };
template<Level L, typename ...ARGS>
void log(ARGS&& ... args)
{
static const char *levelText[] = { "INFO", "WARNING", "ERROR", "FATAL" };
std::cerr << levelText[L] << ": ";
(std::cerr << ... << args);
std::cerr << '\n';
if constexpr (L == Fatal)
std::cerr << "Program aborted\n";
}