CPP: avoiding macro expansion of a macro function parameter - c++

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?

Answer 1

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

Answer 2

If you don't need the expanded lib aliases (i.e. '1' & '2') in your cpp macros, you could also use an enum instead of defined values.

Answer 3

I don't think you can. What you could do, though, is add a layer of macro for it to unpeel in its place:
#define WRAP(x) x
#define LOG_ERROR(ptr, lib, str) LOG_ERROR_SIMPLE(ptr, lib, WHERE WRAP(str))

Answer 4

You almost had it. Use
#define LOG_ERROR(ptr, lib, str) LOG_ERROR_SIMPLE(ptr, ##lib, WHERE str)
On gcc
LOG_ERROR(this, LIB_OTHER, "some error")
yields
this->log ("ERROR " "at file #a, line #l, function #f: " "some error" " at library " "LIB_OTHER");
I would also remove the trailing ';' from your macro so that your code would look like:
LOG_ERROR(this, LIB_OTHER, "some error");