application of ## operator in C

[mail2sandeepnl]

hi all

what are the applications / examples that requires concatenation of 2
pre-processor variables.
i.e applications of " ## " operators in C/C++
i think its only use is in Compiler construction, but how i don't
know,
pls any one help me.
bye
sandeep

[karthikbalaguru]

On Aug 21, 6:38 pm, "[e-mail]"
<[e-mail]:

hi all

what are the applications / examples that requires concatenation of 2
pre-processor variables.
i.e applications of " ## " operators in C/C++
i think its only use is in Compiler construction, but how i don't
know,
pls any one help me.
bye
sandeep

It is used for the Macro replacement within a String Literal .
It is also called as stringizing operator .
It is used to avoid the warning message like
"warning : macro replacement within string literal"

#define MYPRINTF(var,fmt) \
printf("MYPRINTF Prints "#var" = "#fmt" \n",var)

so invocations like - MYPRINTF(i,%d);
will be expanded to - printf("MYPRINTF Prints i = %d \n",i);

Hope this helps.

Best Regards,
Karthik Balaguru

[Robert Gamble]

On Aug 21, 9:56 am, karthikbalaguru <[e-mail]>
wrote:

On Aug 21, 6:38 pm, "[e-mail]"

<[e-mail]:

hi all

what are the applications / examples that requires concatenation of 2
pre-processor variables.
i.e applications of " ## " operators in C/C++
i think its only use is in Compiler construction, but how i don't
know,
pls any one help me.
bye
sandeep

It is used for the Macro replacement within a String Literal .
It is also called as stringizing operator .

You are thinking of the # operator, the OP was asking about the ##
operator which is used for token pasting.

Robert Gamble

[Mark Bluemel]

ma************@gmail.com wrote:

hi all

what are the applications / examples that requires concatenation of 2
pre-processor variables.
i.e applications of " ## " operators in C/C++

Try the C FAQ, http://c-faq.com/cpp/index.html is probably relevant
around question 10.20 onwards.

[santosh]

karthikbalaguru wrote:

On Aug 21, 6:38 pm, "[e-mail]"
<[e-mail]:

>hi all

what are the applications / examples that requires concatenation of 2
pre-processor variables.
i.e applications of " ## " operators in C/C++
i think its only use is in Compiler construction, but how i don't
know,
pls any one help me.
bye
sandeep

It is used for the Macro replacement within a String Literal .
It is also called as stringizing operator .
It is used to avoid the warning message like
"warning : macro replacement within string literal"

#define MYPRINTF(var,fmt) \
printf("MYPRINTF Prints "#var" = "#fmt" \n",var)

so invocations like - MYPRINTF(i,%d);
will be expanded to - printf("MYPRINTF Prints i = %d \n",i);

Hope this helps.

You've explained the uses of the # operator, not the ## operator.

To OP: It is used for concatenation of adjacent tokens.

[Keith Thompson]

karthikbalaguru <ka***************@gmail.comwrites:
[...]

It is used to avoid the warning message like
"warning : macro replacement within string literal"

(The OP asked about ##; you answer was about #, but others have
already pointed that out.)

When do you get such a warning? Macros aren't replaced within string
literals, though I think they were in some pre-ANSI versions of the
language.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[karthikbalaguru]

On Aug 22, 12:16 am, Keith Thompson <[e-mail]:

karthikbalaguru <[e-mail]:

[...]

It is used to avoid the warning message like
"warning : macro replacement within string literal"

(The OP asked about ##; you answer was about #, but others have
already pointed that out.)

When do you get such a warning? Macros aren't replaced within string
literals, though I think they were in some pre-ANSI versions of the
language.

--
Keith Thompson (The_Other_Keith) [e-mail] <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

Apologies.... I happened to see it as # rather than ##.

Karthik Balaguru

[CBFalconer]

"ma************@gmail.com" wrote:

>
what are the applications / examples that requires concatenation
of 2 pre-processor variables. i.e applications of " ## " operators
in C/C++ i think its only use is in Compiler construction, but how
i don't know,

There is no such language as C/C++, making the question
meaningless.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[CBFalconer]

karthikbalaguru wrote:

Keith Thompson <[e-mail]:

.... snip ...

>>
When do you get such a warning? Macros aren't replaced within
string literals, though I think they were in some pre-ANSI
versions of the language.

--
Keith Thompson (The_Other_Keith) [e-mail] <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

Apologies.... I happened to see it as # rather than ##.

Last chance before PLONKing - snip all signatures, i.e. all that
follows the "-- " marker line.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[Ark Khasin]

CBFalconer wrote:

"ma************@gmail.com" wrote:

>what are the applications / examples that requires concatenation
of 2 pre-processor variables. i.e applications of " ## " operators
in C/C++ i think its only use is in Compiler construction, but how
i don't know,

There is no such language as C/C++, making the question
meaningless.

AFAIK, the preprocessor _language_ of C90 and C++ is one and the same,
making the question meaningful.
--Ark

[mail2sandeepnl]

On Aug 22, 9:04 am, Ark Khasin <[e-mail]:

CBFalconer wrote:

"[e-mail]" wrote:

what are the applications / examples that requires concatenation
of 2 pre-processor variables. i.e applications of " ## " operators
in C/C++ i think its only use is in Compiler construction, but how
i don't know,

There is no such language as C/C++, making the question
meaningless.

AFAIK, the preprocessor _language_ of C90 and C++ is one and the same,
making the question meaningful.
--Ark

I think ## (token pasting operator ) can be used in parsing
configuration files, may in used in compiler designing,
is there any other applications of ## other than these 2?

Note: C/C++ means C OR C++ , since it can also be used in C++.
-sandeep

[Ian Collins]

ma************@gmail.com wrote:

>
I think ## (token pasting operator ) can be used in parsing
configuration files

How can it? Pasting tokens is a compile time operation, not a run time one.

The are mag case where you may want to post tokens, one example form
some code I'm working on:

void check(char const* fn, int result, char const* file, int line);

#define CheckOk(fn, args) check(#fn, fn##args, __FILE__, __LINE__)

Which passes the function name and the result of its call to the
function check().

Note: C/C++ means C OR C++ , since it can also be used in C++.

Then write it that way!

--
Ian Collins.

[CBFalconer]

Ian Collins wrote:

ma************@gmail.com wrote:

.... snip ...

>

>Note: C/C++ means C OR C++ , since it can also be used in C++.

Then write it that way!

That complicated solution actually solves the perennial problem :-)

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[Justin Spahr-Summers]

On Aug 22, 12:41 am, Ian Collins <[e-mail]:

The are mag case where you may want to post tokens, one example form
some code I'm working on:

void check(char const* fn, int result, char const* file, int line);

#define CheckOk(fn, args) check(#fn, fn##args, __FILE__, __LINE__)

Which passes the function name and the result of its call to the
function check().

Of course, if you're not afraid to use C99, you could just do

#define CheckOk(fn, ...) check(#fn, fn(__VA_ARGS__), __FILE__,
__LINE__)

which, to me, is a lot easier to understand and use at first glance.
But to each their own, and you may have reservations against C99.

[Ian Collins]

Justin Spahr-Summers wrote:

On Aug 22, 12:41 am, Ian Collins <[e-mail]:

>The are mag case where you may want to post tokens, one example form
some code I'm working on:

void check(char const* fn, int result, char const* file, int line);

#define CheckOk(fn, args) check(#fn, fn##args, __FILE__, __LINE__)

Which passes the function name and the result of its call to the
function check().

Of course, if you're not afraid to use C99, you could just do

#define CheckOk(fn, ...) check(#fn, fn(__VA_ARGS__), __FILE__,
__LINE__)

which, to me, is a lot easier to understand and use at first glance.
But to each their own, and you may have reservations against C99.

We have to retain "backwards" compatibility with C++!

--
Ian Collins.

[Chris Torek]

In article <5j*************@mid.individual.net>,
Ian Collins <ia******@hotmail.comwrote:

>The are mag case where you may want to [paste] tokens, one example form
some code I'm working on:

void check(char const* fn, int result, char const* file, int line);

#define CheckOk(fn, args) check(#fn, fn##args, __FILE__, __LINE__)

Which passes the function name and the result of its call to the
function check().

If you mean to use this as, e.g.:

CheckOk(foo, (1, 2.5))

which should pass to check() these four arguments:

- the string "foo" (or more precisely, the address of the 'f' in
this string),
- the result of calling foo(1, 2.5),
- the source file, and
- the source file line number

then the macro should be defined *without* using the token-pasting
operator. Pasting together the two tokens <fooand <(results
in the invalid pp-token <foo(>, which gives undefined behavior.
(Thus, it may or may not work, depending on the implementation.)

"Real" examples of "proper" use of token pasting are relatively
rare. One can construct artificial examples easily enough though --
for instance, your CheckOk macro might be used instead to call
two variants of every function name:

#define Check2(fn, args) \
(check(#fn "1", fn ## 1 args, __FILE__, __LINE__), \
check(#fn "2", fn ## 2 args, __FILE__, __LINE__))

The expansion of Check2(foo, (1, 2.5)) is then:

check("foo" "1", foo1 (1, 2.5), "sourcefile.c", 42),
check("foo" "2", foo2 (1, 2.5), "sourcefile.c", 42)

(with the expansion occupying a single line, broken into two
lines only for newsgroup posting purposes).
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Ian Collins]

Chris Torek wrote:

In article <5j*************@mid.individual.net>,
Ian Collins <ia******@hotmail.comwrote:

>The are mag case where you may want to [paste] tokens, one example form
some code I'm working on:

void check(char const* fn, int result, char const* file, int line);

#define CheckOk(fn, args) check(#fn, fn##args, __FILE__, __LINE__)

Which passes the function name and the result of its call to the
function check().

If you mean to use this as, e.g.:

CheckOk(foo, (1, 2.5))

which should pass to check() these four arguments:

- the string "foo" (or more precisely, the address of the 'f' in
this string),
- the result of calling foo(1, 2.5),
- the source file, and
- the source file line number

then the macro should be defined *without* using the token-pasting
operator. Pasting together the two tokens <fooand <(results
in the invalid pp-token <foo(>, which gives undefined behavior.
(Thus, it may or may not work, depending on the implementation.)

Thanks for pointing that out Chris, I'll update the code.

--
Ian Collins.