conditional expansion of a macro

[vivekgarg]

Hi,
Can I use some conditional operator to select a part of code to be expanded.
What I meant was... Can I do something like this..

#define func(arg1) \
if arg1 == abc\ \* this line should not expand but just test*\
some code\
else\ \* this line should not expand but just test*\
some other code


Thanks

[donbock]

The macro will be replaced by its definition (after suitable argument replacement), so consider whether the expanded code is what you want to see there.

A common concern with conditional code is what happens if it is invoked from within another conditional block. For example:

#define demo(arg) if (arg > 0) blink()
if (this < that)
   demo(that);
else
   demo(this);
 ... expands to ...
if (this < that)
   if (that > 0) blink();
else
   if (this > 0) blink();

Notice that the 'else' ends up being associated with if(that>0) instead of if(this<that) as you would have expected from reading the source code.

The typical idiom used to prevent this from happening is to encapsulate any multi-line macro definition as follows:

#define demo(arg) \
do { \
if (arg > 0) blink(); \
} while(0)

Notice that all of the source lines of the macro definition are enclosed within a set of braces, making them act like a single instruction. This prevents the else confusion described above; it also prevents you from falling out early if the macro is invoked within a non-bracketed if statement.
Notice that there is no semi-colon after while(0). This forces the programmer to terminate the macro invocation with a semi-colon.

[vivekgarg]

Yeah Donbock what you said was certainly correct in it's own context but I guess I was not clear enough in my question.
Basically what I am asking is, why this peice of code dosen't work and what is the possible solution.

#ifdef ABC
#define TEST /
cout<<"abc"
#else
#define TEST
cout<<"xyz"
#endif

int main()
{
TEST;
#define ABC
TEST
#undef ABC
TEST
return 0;
}
=============
Output:xyzxyzyxz

Thanks

[JosAH]

That's the way cpp (the C Macro PreProcessor) works: (re)defining ABC after
macro TEST has been defined doesn't (re)define macro TEST. A macro processor
simply runs through text, from top to bottom, and it does what it has to do.

kind regards,

Jos