Why compiler complain about this macro declaration

Question

I write the following macro for debug convinience,

1 #ifndef DEF_H
2 #define DEF_H
3 #define DEBUG_MODE
4 #define DEBUG_INFO(message)     \
5         #ifdef DEBUG_MODE       \
6                 cout << message << endl; \
7         #endif                          \
8 #endif

but gcc complains as the following

def.h:4: error: '#' is not followed by a macro parameter
def.h:1: error: unterminated #ifndef

What's wrong with this piece of code? Do I miss some important points here?

score 25 · Accepted Answer · answered Apr 09 '12 at 14:16

25

You cannot have #ifdefs inside a macro definition. You need to turn it inside out:

#ifdef DEBUG_MODE
#define DEBUG_INFO(message) cout << (message) << endl
#else
#define DEBUG_INFO(message)
#endif

answered Apr 09 '12 at 14:16

Oliver Charlesworth

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569
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Thank you. So how can I achieve the goal using macro to control the debug switch. In the client end, I prefer to call just a single line like DEBUG_INFO("debug info") – speedmancs Apr 09 '12 at 14:18
+1. Also, there is a backslash after line 7, which causes the #endif in the last line to be part of line 4, and not match the #ifndef in the first line – Mr Lister Apr 09 '12 at 14:20

score 3 · Answer 2 · answered Apr 09 '12 at 14:20

You can't embed a preprocessor directive in another preprocessor directive (the #ifdef DEBUG_MODE inside the definition of DEBUG_INFO). Instead, do something like

#ifdef DEBUG_MODE
# define DEBUG_INFO(message) cout << message << endl
#else
# define DEBUG_INFO(message) 0
#endif

(This is still not ideal; defensive macro coding suggests something like

#ifdef DEBUG_MODE
# define DEBUG_INFO(message) do {cout << message << endl;} while (0)
#else
# define DEBUG_INFO(message) 0
#endif

Perhaps an inline function would work better.)

"You can't embed a preprocessor directive in another preprocessor directive ", this is not true for all cases, is it ? e.g. #if, #ifdef and #ifndef — Zoso, May 24 '18 at 15:02

score 0 · Answer 3 · answered Apr 09 '12 at 15:53

0

I geuss if you eat '\' at line 7, the piece of Code will work.

answered Apr 09 '12 at 15:53

Kevin

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Why compiler complain about this macro declaration

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