Thread: do {} while (0); ??

I just found this (browsing Python source code):

Code:

#define Py_CLEAR(op)				\
        do {                            	\
                if (op) {			\
                        PyObject *tmp = (PyObject *)(op);	\
                        (op) = NULL;		\
                        Py_DECREF(tmp);		\
                }				\
        } while (0)

I'm interested in the do..while block. Why did he / she / it do that ? I vaguely remember it being mentioned in one of my C++ books that I haven't read for ages, but can't remember for the life of me what the point of it is.

It's so you can put a semicolon after the macro when you use it, as if it were a real function.

esbo, seriously, stop posting.

But I can put a semicolon after a macro when I use it already. Or is this just the "goody" way to do it?

You don't want to leave an if statement on the outside of a macro, because it's essentially hidden to the coder. Consider if you did this:

Code:

if(strcmp(myVar,"Clear")==0)
   Py_CLEAR(toBeCleared)
else
   fprintf(stdout, "Not clearing.");

A superficial example with little to no difference either way, but regardless. Without the loop, there would be confusion as to what if that else belongs to. Of course, if you know the macro, you know what's happening. If always a good habit (as if macros were ever a good habit in the first place) to wrap if statements in a solid control structure in a macro, if you don't have an else.

Originally Posted by SlyMaelstrom

You don't want to leave an if statement on the outside of a macro, because it's essentially hidden to the coder. Consider if you did this:

Code:

if(strcmp(myVar,"Clear")==0)
   Py_CLEAR(toBeCleared)
else
   fprintf(stdout, "Not clearing.");

A superficial example with little to no difference either way, but regardless. Without the loop, there would be confusion as to what if that else belongs to. Of course, if you know the macro, you know what's happening. If always a good habit (as if macros were ever a good habit in the first place) to wrap if statements in a solid control structure in a macro, if you don't have an else.

Okies.

>If you don't like my posts don't read them!!
I don't like your posts because they're always wrong but you try to speak with authority. That's misinformation in my opinion, and it goes against everything we try to do here. Since my mission is to provide accurate information, I don't have the option of not reading your posts. Therefore, you would do well to learn what people try to teach you and apply that knowledge in the future. If not then we'll have no choice but to label you as a troll and deal with you accordingly.

>But I can put a semicolon after a macro when I use it already.
But does the compiler throw an error if you fail to do so? The while loop is a clever way to enforce common C syntax with macros.

Originally Posted by esbo

If you don't like my posts don't read them!!
I don't like your's so your on iggy.
Goodbybe!!

The problem with yours is that they're wrong! And how are beginners supposed to know that? Do you have a big warning in your signature saying, "Don't read my posts, they're most likely wrong!" ? You don't, and that's why you'd better refrain from posting.


On topic, another reason is that the do...while forces the semicolon. I think that's what zx-1 meant. It makes the macro "statement-like" instead of "expression-like".

Edit: Beaten on both points by Prelude ...

It's basically to make the macro syntactically a single statement
http://c-faq.com/cpp/multistmt.html

>so I can easily break out of the sequence without using goto statements
Oddly enough, I see that as a worse solution than goto. "Use judiciously" doesn't mean "avoid at all costs even when the result sucks".

I didn't mean that I religiously try to avoid using gotos, I do use them where I need to. To tell the truth, I don't really know what got me on to doing that, the general idea is the same, I guess I just sort of figured I might as well use a loop when I can...

Originally Posted by Prelude

>If you don't like my posts don't read them!!
I don't like your posts because they're always wrong but you try to speak with authority. That's misinformation in my opinion, and it goes against everything we try to do here. Since my mission is to provide accurate information, I don't have the option of not reading your posts. Therefore, you would do well to learn what people try to teach you and apply that knowledge in the future. If not then we'll have no choice but to label you as a troll and deal with you accordingly.

>But I can put a semicolon after a macro when I use it already.
But does the compiler throw an error if you fail to do so? The while loop is a clever way to enforce common C syntax with macros.

I have had the occasional complaint when using some macros inside if blocks. Complaints from my compiler that is.