Determining whether a macro has been defined

[BigMan]

Are these two lines equivalent?

#ifdef MACRO

#if MACRO==0

 

[Andrey Tarasevich]

Are these two lines equivalent?

#ifdef MACRO

#if MACRO==0

No. Why would they be? If 'MACRO' is defined as

#define MACRO 2

the first variant will "evaluate" to 'true', while the second one - to
'false'.

 

[gb]

No. The first one evalutes to true only if MACRO has been defined
(regardless of its definition). The second evalutes to true if either
of these is true:

MACRO is undefined
MACRO is defined but evalutes at compile time to 0.

See section 16.1.4 in the standard.

Gregg

 

[BigMan]

Ooops, I've put it the wrong way. Here's what I mean: does it matter
which line I use to check if a macro has been defined.

 

[Default User]

Ooops, I've put it the wrong way. Here's what I mean: does it matter
which line I use to check if a macro has been defined.

Yes, it matters. Didn't you read the other posts? The first way is
correct.


Brian

 

[Ron Natalie]

Yes, it matters. Didn't you read the other posts? The first way is
correct.

If he doesn't want to worry about the case where it is defined and is zero.
It doesn't matter.

 

[Default User]

If he doesn't want to worry about the case where it is defined and is zero.
It doesn't matter.

I don't recall that in his problem description.


Brian

 

[Andrey Tarasevich]

Ooops, I've put it the wrong way. Here's what I mean: does it matter
which line I use to check if a macro has been defined.

Yes it does. The former method (with '#ifdef') checks whether it has
been defined. The latter one (with '#if') checks whether it is
substituted with integral zero (if well-formed). This is not the same.

For example, defining 'MACRO' as either '0' or '1' will give you the
same results from the former method (i.e. '#ifdef' will say that it is
defined in both cases), but different results from the latter method.

Defining 'MACRO' as follows

#define MACRO

will work with the former method, but it makes the latter one ill-formed.

 

[BigMan]

Thanks to all of you! I think I got it.