where is different???

[wende]

1.
#define MA \
....
{
i = j;
}
....
2
#define MA \
....
do{
i = j;
}while(0);

thanks for your answer...

[Kevin Easton]

wende <sj**********@yahoo.co.uk> wrote:

1.
#define MA \
...
{
i = j;
}
...
2
#define MA \
...
do{
i = j;
}while(0);

The usual idiom is to leave off the trailing semicolon - and it's only
useful with multiple-statement macros, so it is this:

#define MA \
{ \
i = j; \
j = k; \
}

versus this:

#define MA \
do { \
i = j; \
j = k; \
} while (0)

With the former definition, this code:

if (test)
MA;
else
puts("no");

is an error, but with the latter definition it is not. This is because
MA; is two statements with the former definition and one statement with
the latter.

- Kevin.

[Alex]

Kevin Easton <[e-mail]> wrote:

wende <sj**********@yahoo.co.uk> wrote:
<snip original>
The usual idiom is to leave off the trailing semicolon - and it's only
useful with multiple-statement macros, so it is this: #define MA \
{ \
i = j; \
j = k; \
} versus this: #define MA \
do { \
i = j; \
j = k; \
} while (0) With the former definition, this code: if (test)
MA;
else
puts("no"); is an error, but with the latter definition it is not. This is because
MA; is two statements with the former definition and one statement with
the latter.

With the first definition, this will work:

if(test)
MA
else
puts("no");

However, the requirement of the trailing semicolon in the
second definition is much more idiomatic.

Alex

[Kevin Easton]

Peter "Shaggy" Haywood <ph******@alphalink.com.au.stop.spam> wrote:

Groovy hepcat Kevin Easton was jivin' on Thu, 14 Aug 2003 23:40:06 GMT
in comp.lang.c.
Re: where is different???'s a cool scene! Dig it!

#define MA \
{ \
i = j; \
j = k; \
}

versus this:

#define MA \
do { \
i = j; \
j = k; \
} while (0)

With the former definition, this code:

if (test)
MA;
else
puts("no");

is an error, but with the latter definition it is not. This is because
MA; is two statements with the former definition and one statement with
the latter.

Piffle! They're both perfectly legal. You've never heard of a null
statement? Look that up in your C manual.

How about *you* look up the syntax of the if statement in *your* C
manual?

The expansion of the given if statement using the former definition of
MA is:

if (test)
{
i = j;
j = k;
};
else
puts("no");

Which isn't legal. There can only be *one* statement between the test
and else part of an if statement.

- Kevin.