How to verify a preprocessor macros defined with no value

[congot]

How to verify a preprocessor macros defined with no value , and only if defined with a value should the code within that be exectued.

#ifdef ABC
int a;
#endif

int a is available only if a value of ABC is available.

ABC is defined as
#define ABC

Some Old threads related to the same were observed but none gave correct working answers.

[Tassos Souris]

Try this:

#if defined( ABC ) && ABC == VALUE_YOU_WANT
...
#endif
 

[Banfa]

You should note that if ABC is defined as shown in the OP

#define ABC

the preprocessor automatically assigns it the value 0. That means that you can not differentiate between

#define ABC

and

#define ABC 0

[manjuks]

Hi All,

#define ABC

When I tried to print this one i got an compilation error

But It got printed if I define like as below
#define ABC 100

Last post says that #define ABC will automatically assign 0 to ABC

Please clarify me

Thanks,
Manjunath

[Banfa]

Sorry my mistake it should say that a defined preprocessor symbol that is defined without a value such as

#define ABC

is treated as it it had the value 0 in other pre-processor commands such as

#if ABC

Within the body of the C/C++ code a normal text substitution will occur so ABC would be replace with nothing which might lead to syntax errors such as

printf("%d\n", ABC);

would be preprocessed to

printf("%d\n", );

which is a syntax error.

[congot]

@Tassos Souris

On using the above method, I received parse error.
Aslo, the value of the macro is not a numeric value. Then is it possible to check like the above??

[Banfa]

Err, no the preprocessor is a simple thing and only knows about numeric values.

What is it the value it is defined to?

[congot]

@Banfa
it is defined as a function:

#define ABC foo()

[Banfa]

Your problem is that you have defined a function like macro but you haven't used function like macro syntax. You should be using one of the 2 definitions

#define ABC() foo()

or

#define ABC foo

The first is a function like macro that in fact calls a function (foo). The second is a text replacement of one function name for another.

Both may be used in code like this

ABC();

I think the second one will work better for you because if ABC is defined as

#define ABC

the code becomes

();

which is a syntax error. And if you want the macro to expand to nothing that is easily achieved like this

#define ABC DO_NOTHING
#define DO_NOTHING()

or this

#define ABC()