CODEWITHSUNDEEP
c++macros
user3794186
user3794186

Reputation: 709

C++ Combining #define and #ifndef macro

I was wondering if there's a way to combine #define and #ifndef macro..

What this means is that I want to use #ifndef macro within the #define macro..

Since it's kind of hard to explain,, this is an example of what I want to do:

#define RUN_IF_DEBUG                \
    #ifndef DEBUG_MODE              \
        ;      // do nothing        \
    #else                           \
        cout << "Run!" << endl;     \
    #endif

int main() {
    RUN_IF_DEBUG
}

So I want the RUN_IF_DEBUG macro to run ONLY IF the DEBUG_MODE is defined...

Is there a way to do this?

Upvotes: 2

Views: 1050

Answers (3)

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

Simply do 

#ifndef DEBUG_MODE
    #define RUN_IF_DEBUG ;      // do nothing
#else
    #define RUN_IF_DEBUG cout << "Run!" << endl;
#endif

You can't put other preprocessor statements within a macro's body.

As from the c++ standards definitionsdraft section

16 Preprocessing directives

...
control-line:
...
# define identifier replacement-list new-line
# define identifier lparen identifier-listopt) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list, ... ) replacement-list new-line

These are the allowed syntax variants for#define statements.

`

Upvotes: 2

Some programmer dude
Some programmer dude

Reputation: 409176

The problem is the line-continuation in the macro. What they do is put everything on a single line, so the expanded macro will look something like

int main() {
    #ifndef DEBUG_MODE ; #else cout ...; #endif
}

This will not work very well with the preprocessor or the compiler.

Instead you should switch the nesting, and use #ifndef first, and #define the macros in the inner level.

Upvotes: 0

ltjax
ltjax

Reputation: 15997

It is usually done the other way around:

#ifndef DEBUG_MODE
#  define RUN_IF_DEBUG ;
#else
#  define RUN_IF_DEBUG cout << "Run!" << endl;
#endif

Upvotes: 5

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