Why do{}while(0) in #MACROS?

[Steven T. Hatton]

I've noticed in different places where people who write #MACROS use a null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT (EXCEPTION); } \
} while (0)

What does that accomplish?

--
If our hypothesis is about anything and not about some one or more
particular things, then our deductions constitute mathematics. Thus
mathematics may be defined as the subject in which we never know what we
are talking about, nor whether what we are saying is true.-Bertrand Russell

[Artie Gold]

Steven T. Hatton wrote:

I've noticed in different places where people who write #MACROS use a null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT (EXCEPTION); } \
} while (0)

What does that accomplish?

It enables you to have a macro define a local scope, confine an `if'
statement or both -- and still end it with a semicolon.

HTH,
--ag

--
Artie Gold -- Austin, Texas
http://it-matters.blogspot.com (new post 12/5)
http://www.cafepress.com/goldsays

[Steven T. Hatton]

Artie Gold wrote:

Steven T. Hatton wrote:

I've noticed in different places where people who write #MACROS use a
null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT (EXCEPTION); } \
} while (0)

What does that accomplish?

It enables you to have a macro define a local scope, confine an `if'
statement or both -- and still end it with a semicolon.

HTH,
--ag

Why not simpley use '{' and '}'?
#include <iostream>

int main () {
{
if(true) {
std::cout << "I don't get it." << std::endl;
}
};
}

--
If our hypothesis is about anything and not about some one or more
particular things, then our deductions constitute mathematics. Thus
mathematics may be defined as the subject in which we never know what we
are talking about, nor whether what we are saying is true.-Bertrand Russell

[Victor Bazarov]

Steven T. Hatton wrote:

Artie Gold wrote:

Steven T. Hatton wrote:

I've noticed in different places where people who write #MACROS use
a null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT
(EXCEPTION); } \ } while (0)

What does that accomplish?

It enables you to have a macro define a local scope, confine an `if'
statement or both -- and still end it with a semicolon.

HTH,
--ag

Why not simpley use '{' and '}'?
#include <iostream>

int main () {
{
if(true) {
std::cout << "I don't get it." << std::endl;
}
};
}

Try it. Compare the results.

#define LIMITSCOPE(what ) { int what = 42; printf("LIMITIN G " ## #what); }
#include <stdio.h>
int main(int argc, char *argv[]) {
if (argc > 1)
LIMITSCOPE(abc) ; // pay attention to this line
else
printf("not more");
}
---------------------------------
#define LIMITSCOPE(what ) do { \
int what = 42; printf("LIMITIN G " ## #what); } while(0)
#include <stdio.h>
int main(int argc, char *argv[]) {
if (argc > 1)
LIMITSCOPE(abc) ; // pay attention to this line
else
printf("not more");
}
V

[Artie Gold]

Steven T. Hatton wrote:

Artie Gold wrote:

Steven T. Hatton wrote:

I've noticed in different places where people who write #MACROS use a
null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT (EXCEPTION); } \
} while (0)

What does that accomplish?

It enables you to have a macro define a local scope, confine an `if'
statement or both -- and still end it with a semicolon.

HTH,
--ag

Why not simpley use '{' and '}'?
#include <iostream>

int main () {
{
if(true) {
std::cout << "I don't get it." << std::endl;
}
};
}

Think what happens if such a macro (without the do/while(0) wrapper) is
used as the `then' clause of an if. [Hint, the extra `;' would then
introduce a null statement.]

--ag

--
Artie Gold -- Austin, Texas
http://it-matters.blogspot.com (new post 12/5)
http://www.cafepress.com/goldsays

[Steven T. Hatton]

Artie Gold wrote:

Steven T. Hatton wrote:

Artie Gold wrote:

Steven T. Hatton wrote:
I've noticed in different places where people who write #MACROS use a
null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT (EXCEPTION); }
\
} while (0)

What does that accomplish?

It enables you to have a macro define a local scope, confine an `if'
statement or both -- and still end it with a semicolon.

HTH,
--ag

Why not simpley use '{' and '}'?
#include <iostream>

int main () {
{
if(true) {
std::cout << "I don't get it." << std::endl;
}
};
}

Think what happens if such a macro (without the do/while(0) wrapper) is
used as the `then' clause of an if. [Hint, the extra `;' would then
introduce a null statement.]

--ag

I see from Victor's example what happens. I figured it was something like
that, but until recently, I have systematically avoided omitting the
brackets in if(), for() and while() statements even when the are not
necessary. As you can see, that was the form I chose without considering
whether it was needed or not.

Furthermore, I am of the opinion that Cpp must be destroyed.
- Cato the Elder (Marcus Porcius Cato)[*]
[*]D&E, Chapter 18
--
If our hypothesis is about anything and not about some one or more
particular things, then our deductions constitute mathematics. Thus
mathematics may be defined as the subject in which we never know what we
are talking about, nor whether what we are saying is true.-Bertrand Russell

[Paul Groke]

Furthermore, I am of the opinion that Cpp must be destroyed.
- Cato the Elder (Marcus Porcius Cato)[*]

Shouldn't that be "ceterum censeo C++ esse delendam" ?
And the correct translation would be
"Furthermor e I think that C++ is to be destroyed" AFAIK.

[leonardo77]

Paul Groke wrote:

"Furthermor e I think that C++ is to be destroyed" AFAIK.

I don't think Cpp and C++ are the same thing.
;-)

[Paul Groke]

leonardo77 wrote:

Paul Groke wrote:

"Furthermor e I think that C++ is to be destroyed" AFAIK.

I don't think Cpp and C++ are the same thing.
;-)

Then just search and replace. ;-)

[Mike Smith]

Steven T. Hatton wrote:

I've noticed in different places where people who write #MACROS use a null
do/while. For example:

do { POINTER = new CONSTRUCTOR; \
if (POINTER == 0) { errno = ENOMEM; ACE_THROW_INT (EXCEPTION); } \
} while (0)

What does that accomplish?

You can break out of it if you want, which you cannot do with a simple
local scope.

--
Mike Smith