#define

[srinivas reddy]

I have defined some variables using #define preprocessor instruction.
And then later I checked whether I had defined a variable. Sometimes
even though a variable have been defined, #if !defined(var) construct
is returning true. I am using gcc3.0.1 on SunOS8. I would appreciate
if any body can tell me whether this is a bug or I am doing something
wrong.

tia,
Srinivas

[David White]

"Paul Mensonides" <le******@comca st.net> wrote in message
news:ZoHOa.1206 5$H17.3639@sccr nsc02...

David White wrote:

Incomprehensibl y, #if var1 == var2 simply converts var1 and var2 to
"0" (yes, "0", even though the preprocessor is a _text_ replacer) if
it hasn't come across definitions of them (something like Basic
assuming that any undefined variable it comes across must be an int;
and I thought C++ got rid of implicit this and implicit that because
they are thought unsafe). 0 == 0 is true, of course.

I can only assume that those with influence who wish to see the end
of the preprocessor altogether are trying to accelerate its death by
ensuring that it works as badly as possible.
It isn't the preprocessor that is bad--even the conversion to 0 that you

mention here.
Well, I think the conversion to 0 _is_ bad. Given that you can use #ifdef or
#if defined() for things such as:
#ifdef _cplusplus

how can the implicit conversion to 0 of an undefined symbol be a good thing?
Why is it better than issuing an error?
It is *misuse* of the preprocessor that is bad. The preprocessor is
actually a critical component of the C and C++ compilation process.
I agree. That's why I'd like it to work _safely_.
It makes it
possible to write code that works on multiple platforms, as well as write code that works on various current compilers (as opposed to the idealistic perfect C++ implementation) .

Yes, but I want to do it safely. I do not want the outcome of an #if to be
one of these two possibilities:
1. The result of the expression of previously defined symbols.
2. A programmer's mistake in forgetting to include the defined symbols.

This is inherently unsafe. The possibility of no. 2 is the reason that C++
insists on all function definitions being present and that there is a
suitable match for every argument. Does not one other person here think that
this is a problem?

DW

[Alexander Terekhov]

David White wrote:
[...]

This is inherently unsafe. The possibility of no. 2 is the reason that C++
insists on all function definitions being present and that there is a
suitable match for every argument. Does not one other person here think that
this is a problem?

http://groups.google.com/groups?thre...01%40hsv3.UUCP
(Subject: Using a define that hasn't been #defined)

regards,
alexander.

--
"Status quo, you know, that is Latin for ``the mess we're in.''"

-- Ronald Reagan

[Ron Natalie]

"David White" <[e-mail] ded> wrote in message news:Kb******** **********@nasa l.pacific.net.a u...

Doesn't this replace the symbol 'X' found anywhere in the source code with
the text 'Y'?
No, it doesn't. Pete is right, you seem not to understand the preprocessor.
Given that macros _do_ replace text, why should an undefined symbol become
'0' rather than ''?

They do not replace text, they replace tokens.

[Paul Mensonides]

Ron Natalie wrote:

"David White" <[e-mail] ded> wrote in message
news:Kb******** **********@nasa l.pacific.net.a u...

Doesn't this replace the symbol 'X' found anywhere in the source
code with
the text 'Y'?

No, it doesn't. Pete is right, you seem not to understand the
preprocessor.

Given that macros _do_ replace text, why should an undefined symbol
become '0' rather than ''?

They do not replace text, they replace tokens.

Even more specifically, they replace macro invocations:

#define X() Y

X // X

Regards,
Paul Mensonides

[Paul Mensonides]

David White wrote:

It isn't the preprocessor that is bad--even the conversion to 0 that
you mention here.
Well, I think the conversion to 0 _is_ bad. Given that you can use
#ifdef or #if defined() for things such as:
#ifdef _cplusplus

how can the implicit conversion to 0 of an undefined symbol be a good
thing? Why is it better than issuing an error?

Because it is a "reasonable default." Reasonable defaults make code less
verbose. This happens with templates also:

template<class T> void f(int reactorType)
{
// No definition of REACTOR_NEW_MOD EL given
if(reactorType == T::REACTOR_NEW_ MODEL)
{
// ...
}
}

The compiler will pass this with no problem even though it still parses the
expression, etc.. The reasonable default here is "non-type". The point being
that the language has to deal with unknown names and make assumptions about what
they mean in various places.

That is just the way it is. You know what the behavior is, so writing "safe"
code is up to you to use the language in safe ways. C and C++ certainly don't
protect you from unsafe usage many areas, why should they do that here?

If you changed the behavior to an error, how would you do this in a non-verbose
way:

# if !__cplusplus && __STDC_VERSION_ _ >= 199901L

You'd have to do something really annoying because you cannot use any
conditional test that uses the name outside the defined operator. You can't
even do this:

#if defined(__STDC_ VERSION__) && __STDC_VERSION_ _ >= 199901L

...because that constitutes an error under your model if __STDC_VERSION_ _ is not
defined. You'd have to separate the test for definition from the conditional
expression:

# if !defined __cplusplus && defined __STDC_VERSION_ _
# if __STDC_VERSION_ _ >= 199901L
# // 1
# else
# // 2
# endif
# else
# // 2
# endif

....and that is a code doubler for point 2.

If you changed the behavior to expanding to nil instead of 0, you'd have silent
changes in other ways. You'd also end of seeing a lot of "hacks" like this:

# if !(__cplusplus+0 ) && (__STDC_VERSION __+0) >= 199901L

In order to simulate the common scenario that we already have built into the
preprocessor.

It is *misuse* of the preprocessor that is bad. The preprocessor is
actually a critical component of the C and C++ compilation process.

I agree. That's why I'd like it to work _safely_.

It does work safely if used correctly. I said it before already: The #if and
#elif directives are not designed to implicitly perform the kind of verification
that you want--because that kind of verification (if done by default) is
downright annoying.

Further, the root problem here is 1) forgetting to include a file, or 2) design
error. Assuming that it is just a case of forgetting to include the file that
defines the symbols, there a many ways in which a program can silently change
meaning in C++ by not including a file (e.g. silently choosing different
function overloads or different template specializations ).

It makes it
possible to write code that works on multiple platforms, as well as
write code that works on various current compilers (as opposed to
the idealistic perfect C++ implementation) .

Yes, but I want to do it safely. I do not want the outcome of an #if
to be one of these two possibilities:
1. The result of the expression of previously defined symbols.

It is totally ill-conceived. You can do what you want reasonably, but you
cannot do what it already does reasonably. You already have the option to do
what you want:

#if defined(REACTOR _TYPE) \
&& defined(REACTOR _NEW_MODEL) \
&& REACTOR_TYPE == REACTOR_NEW_MOD EL

You can't go back the other way.
2. A programmer's mistake in forgetting to include the defined
symbols.

This is inherently unsafe.
No it isn't _inherently_ unsafe. It can be unsafe in certain contexts, and you
have to be aware of that when you write code. However, the alternative is much
worse. You can simulate what you want with a small amount of code; you cannot
simulate what it already does with a small amount of code.
The possibility of no. 2 is the reason
that C++ insists on all function definitions being present and that
there is a suitable match for every argument. Does not one other
person here think that this is a problem?

C++ does not insist on all function declarations that you've defined in a group
of files be present at each overload resolution--which can cause silent
differences in overload resolution, etc..

Regards,
Paul Mensonides

[David White]

Ron Natalie <ro*@sensor.com > wrote in message
news:3f******** *************** @news.newshosti ng.com...

"David White" <[e-mail] ded> wrote in message news:Kb******** **********@nasa l.pacific.net.a u...

Doesn't this replace the symbol 'X' found anywhere in the source code with the text 'Y'?

No, it doesn't. Pete is right, you seem not to understand the

preprocessor.

I do. I just didn't use precise enough language in this pedantic place. I am
aware that the 'X' in 'MAX' would not be replaced by 'Y'.

Given that macros _do_ replace text, why should an undefined symbol become '0' rather than ''?

They do not replace text, they replace tokens.

Okay. Why you didn't correct Stroustrup's loose language as well? :)

Getting off the track.

DW

[David White]

Alexander Terekhov <te******@web.d e> wrote in message
news:3F******** *******@web.de. ..

David White wrote:
[...]

This is inherently unsafe. The possibility of no. 2 is the reason that C++ insists on all function definitions being present and that there is a
suitable match for every argument. Does not one other person here think that this is a problem?

http://groups.google.com/groups?thre...01%40hsv3.UUCP
(Subject: Using a define that hasn't been #defined)

Thanks, Alexander. I am not alone after all.

DW

[Sam Holden]

On Thu, 10 Jul 2003 13:35:24 +1000, David White <[e-mail] ded> wrote:

Sam Holden <sh*****@flexal .cs.usyd.edu.au > wrote in message
news:slrnbgpln5 .fft.sh*****@fl exal.cs.usyd.ed u.au...

On Thu, 10 Jul 2003 12:31:36 +1000,
David White <[e-mail] ded> wrote:

>

Using #if in anything but a header file is madness in my opinion. I
guess using it to give some platform specific optimisation might be OK,
but I'd try and find a way to move the #if itself into a header.

You can't always use C++ itself. How, for example, do you make a declaration
depend on whether you are compiling a NORMAL or LITE version of the
software? Suppose you also have EXTRA_LITE and PREMIUM versions. And suppose
the following constant has a different value for each one:

#if MODEL == EXTRA_LITE
const int NrWheels = 2;
#elif MODEL == LITE
const int NrWheels = 4;
// etcetera

By moving those values into the header file, since they are obviously
in some way dependant on the MODEL and hence having them in one place
rather than scattered throughout many source files is nice (even
if they are in fact only used once - it unifies the location of all
such data).

#define MODEL_NRWHEELS 2
or
#define MODEL_NRWHEELS 4

which get set by the appropriate model specific header.

and then

const int NrWheels = MODEL_NRWHEELS;

Has the disadvantage that when the code is changed, sometimes
the model header files will have to be modified as well. But
has the advantage that when adding another model you will
hopefully just have to add the appropriate header file set
a the appropriate define and all the code now works - as opposed
to finding all those #ifs and adding #elif MODEL == NEW_MODEL.
Of course in practice things never work out and you have
to modify a bit of the code.

But I'd personally be using the C++ type system rather than
#defines to tell things apart. I've been bitten by inconsistant
settings in #defines in different compile runs or files in C
too many times.

--
Sam Holden

[Paul Mensonides]

David White wrote:

It's not going to change. No matter how much you complain about it.

Yes, I realized that very quickly. I can't even get anyone here to
agree that anything needs to be fixed.

It isn't that I don't agree that the kind of danger you refer to doesn't exist.
Instead, its that I don't consider it a big enough problem to prohibit the
utility of the implicit conversion to 0. IMO, the benefits outweigh the costs.

Regards,
Paul Mensonides

[Ron Natalie]

"David White" <[e-mail] ded> wrote in message news:%G******** *****@nasal.pac ific.net.au...

Ron Natalie <ro*@sensor.com > wrote in message I do. I just didn't use precise enough language in this pedantic place. I am
aware that the 'X' in 'MAX' would not be replaced by 'Y'.
There's much more to it than that.

They do not replace text, they replace tokens.

Okay. Why you didn't correct Stroustrup's loose language as well? :)

Because Stroustrup wasn't wrong. You were. The only reference to text
on the page you reference says "the preprocessor rearranges the text ..."
He doesn't say anything specific about macro replacment
Getting off the track.

You're the one driving us there.