CPP: avoiding macro expansion of a macro function parameter

Question

what I'd like to do (for logging purposes) is something like this:

This code has been written to show my problem, actual code is complex and yes, I have good reasons to use macros even on C++ =)

# define LIB_SOME 1
# define LIB_OTHER 2

# define WHERE "at file #a, line #l, function #f: "
// (look for syntax hightlighting error at SO xd)
# define LOG_ERROR_SIMPLE(ptr, lib, str) ptr->log ("ERROR " str \
                                                   " at library " #lib);
# define LOG_ERROR(ptr, lib, str) LOG_ERROR_SIMPLE(ptr, lib, WHERE str)

LOG_ERROR_SIMPLE (this, LIB_SOME, "doing something")
LOG_ERROR (this, LIB_OTHER, "doing something else")

LOG_ERROR_SIMPLE() writes the stringification of the lib parameter (a macro name surrounded by " ")

but then LOG_ERROR writes the stringification of the macro already expanded ("2"). this is expected, since lib got its expansion before expanding and calling LOG_ERROR_SIMPLE. but this is not what I need.

Basically my question is this: how to avoid macro expansion of a macro function parameter when calling another macro function?

There is a trick I use that avoids macro expansion:

  LOG_ERROR(ptr, lib, str, x) LOG_ERROR_SIMPLE(ptr, x##lib, WHERE str)

  LOG_ERROR(this, LIB_OTHER, "some error",)

(pasting x and lib produces LIB_OTHER and this value is used to call LOG_ERROR_SIMPLE, its not macro expanded before that call)

There is some way to obtain this same behaviour without using a trick?

Gregory Pakosz Gregory Pakosz · Accepted Answer · 2009-12-10T13:33:21

I'm doing:

#include <cstdio>

#define FOO 1
#define BAR 2

#define LOG_SIMPLE(ptr, lib, str) printf("%s\n", #lib);
#define LOG(ptr, lib, str) LOG_SIMPLE(ptr, ##lib, str)

int main()
{
  LOG_SIMPLE(0, FOO, "some error");
  LOG(0, BAR, "some other error");
}

which prints out:

FOO
BAR

Works with MSVC2005 but not with gcc/g++.

EDIT: to make it work with gcc/g++ you can abuse variadic macros:

#include <stdio.h>

#define FOO 1
#define BAR 2

#define LOG_SIMPLE(ptr, str, lib) printf("%s\n", #lib);
#define LOG(ptr, str, lib, ...) LOG_SIMPLE(ptr, str, lib##__VA_ARGS__)

int main()
{
  LOG_SIMPLE(0, "some error", FOO);
  LOG(0, "some other error", BAR);
  LOG(0, "some other error", FOO, BAR);
}

However, it's your discipline not to use the macro with too many parameters. MSVC2005 prints out

FOO
BAR
FOO2

while gcc prints out

FOO
BAR
FOOBAR

CPP: avoiding macro expansion of a macro function parameter

4 Answers