Can `#ifdef` be used inside a macro? - c++

Question

I only found this related question, which isn't quite what I am looking for.
I used to have macros defined inside an #ifdef statement:
#ifdef DEBUG
# define PRINT_IF_DEBUGGING(format) printf(format);
# define PRINTF_IF_DEBUGGING(format, ...) printf(format, __VA_ARGS__);
#else
# define PRINT_IF_DEBUGGING(...)
# define PRINTF_IF_DEBUGGING(...)
#endif
Now, I want to do the inverse, to have the #ifdef statements inside the macros. Something like this:
#define PRINT_IF_DEBUGGING(format, ...) \
#if defined(DEBUG) print(format); #endif
#define PRINTF_IF_DEBUGGING(format, ...) \
#if defined(DEBUG) printf(format, __VA_ARGS__); #endif
However, I am having an issue using __VA_ARGS__ inside the #ifdef defined.
error: '#' is not followed by a macro parameter
#define PRINT_IF_DEBUGGING(format, ...)
error: '#' is not followed by a macro parameter
#define PRINTF_IF_DEBUGGING(format, ...)
warning: __VA_ARGS__ can only appear in the expansion of a C++11 variadic macro
#if defined(DEBUG) printf(format, __VA_ARGS__); #endif
Is this possible?

Answer 1

This should really be a comment, but I can't format that in a way that will allow me to say what I want to say, so I'm answering instead.
Anyway, just change this:
#if defined(DEBUG) print(format); #endif
to this:
#if defined(DEBUG)
print(format);
#endif
and so on, and that should fix it.

Answer 2

You can't use #ifdef inside of #define , so no, this is not possible. The first code you showed is the correct solution.

Answer 3

Using #ifdef inside of #define is not possible. But there are still ways you can detect wether or not a macro has been defined within a macro definition.
1. Solution
godbolt
#define CAT(a, b) CAT_IMPL(a, b)
#define CAT_IMPL(a, b) a ## b
#define IS_DEBUG_DEFINED() CHECK((CAT(CHECK_,DEBUG), 0, 1))
#define CHECK_DEBUG ~,~
#define CHECK(tup) CHECK_IMPL tup
#define CHECK_IMPL(a, b, c, ...) c
#define IIF(condition, true_value, false_value) CAT(IIF_,condition)(true_value, false_value)
#define IIF_0(true_value, false_value) false_value
#define IIF_1(true_value, false_value) true_value
#define PRINT_IF_DEBUGGING(format) IIF(IS_DEBUG_DEFINED(), PRINT_DEBUGGING, PRINT_NOT_DEBUGGING)(format)
// this will be used if DEBUG is defined:
#define PRINT_DEBUGGING(format) debugPrint(format)
// this will be used if DEBUG is NOT defined:
#define PRINT_NOT_DEBUGGING(format) print(format)
PRINT_IF_DEBUGGING(foo) will expand to:
if DEBUG is defined: debugPrint(foo)
if DEBUG is not defined: print(foo)
Example:
PRINT_IF_DEBUGGING("test1");
#define DEBUG
PRINT_IF_DEBUGGING("test2");
#undef DEBUG
PRINT_IF_DEBUGGING("test3");
would result in:
print("test1");
debugPrint("test2");
print("test3");
2. How IS_DEBUG_DEFINED() works
The fundamental trick behind this is to use concatenation - if the macro was defined it will be expanded, otherwise the token will be left unmodified by the preprocessor:
godbolt
#define CAT(a, b) CAT_IMPL(a, b)
#define CAT_IMPL(a, b) a ## b
// DEBUG NOT DEFINED:
CAT(CHECK_,DEBUG) // will expand to CHECK_DEBUG
// DEBUG DEFINED:
#define DEBUG 1234
CAT(CHECK_,DEBUG) // will expand to CHECK_1234
The first CAT will expand to CHECK_DEBUG, because DEBUG was not defined.
The second CAT however will expand to CHECK_1234, because DEBUG was defined and expanded to 1234 before the concatenation with CHECK_.
By defining a macro named CHECK_DEBUG we can change the result if the macro was not defined, e.g.:
godbolt
#define TEST CAT(CHECK_,DEBUG), 0, 1
#define CHECK_DEBUG ~,~
If DEBUG is not defined the result will be ~, ~, 0, 1 (4 comma-separated tokens)
If DEBUG is defined the result will be CHECK_, 0, 1 (3 comma-separated tokens)
Notice how we got 4 tokens in the first case, but only 3 tokens in the second.
Now all we need to do is take the 3rd token from that sequence (which will be 0 if DEBUG is not defined and 1 otherwise), for example with a simple macro that always returns the 3rd argument:
#define CHECK(a, b, c, ...) c
Putting it all together, this is what a full IS_DEBUG_DEFINED() could look like:
godbolt
#define CAT(a, b) CAT_IMPL(a, b)
#define CAT_IMPL(a, b) a ## b
#define IS_DEBUG_DEFINED() CHECK((CAT(CHECK_,DEBUG), 0, 1))
#define CHECK_DEBUG ~,~
#define CHECK(tup) CHECK_IMPL tup
#define CHECK_IMPL(a, b, c, ...) c
IS_DEBUG_DEFINED() will expand to 0 if DEBUG is not defined, and 1 if it is, e.g.:
IS_DEBUG_DEFINED() // -> 0
#define DEBUG
IS_DEBUG_DEFINED() // -> 1
#undef DEBUG
IS_DEBUG_DEFINED() // -> 0
With IS_DEBUG_DEFINED() you can then use a standard preprocessor IIF to change the behaviour of your macro depending on wether DEBUG is defined or not.
Example: godbolt
#define IIF(condition, true_value, false_value) CAT(IIF_,condition)(true_value, false_value)
#define IIF_0(true_value, false_value) false_value
#define IIF_1(true_value, false_value) true_value
#define PRINT_IF_DEBUGGING(format) IIF(IS_DEBUG_DEFINED(), PRINT_DEBUGGING, PRINT_NOT_DEBUGGING)(format)
// this will be used if DEBUG is defined:
#define PRINT_DEBUGGING(format) debugPrint(format)
// this will be used if DEBUG is NOT defined:
#define PRINT_NOT_DEBUGGING(format) print(format)
PRINT_IF_DEBUGGING("test"); // -> print("test");
#define DEBUG
PRINT_IF_DEBUGGING("test"); // -> debugPrint("test");
#undef DEBUG
PRINT_IF_DEBUGGING("test"); // -> print("test");
3. Caveats
One small caveat with this is that if DEBUG is defined it must expand to a valid preprocessing token (so it must only contain letters, digits and underscores) - otherwise the concatenation will result in an error.
So this would not work:
#define DEBUG ()
#define DEBUG +