do { ... } while (0) — what is it good for? [duplicate]

Question

Possible Duplicate:
Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?

I've been seeing that expression for over 10 years now. I've been trying to think what it's good for. Since I see it mostly in #defines, I assume it's good for inner scope variable declaration and for using breaks (instead of gotos.)

Is it good for anything else? Do you use it?

Answer 1

It's the only construct in C that you can use to #define a multistatement operation, put a semicolon after, and still use within an if statement. An example might help:

#define FOO(x) foo(x); bar(x)  if (condition)     FOO(x); else // syntax error here     ...; 

Even using braces doesn't help:

#define FOO(x) { foo(x); bar(x); } 

Using this in an if statement would require that you omit the semicolon, which is counterintuitive:

if (condition)     FOO(x) else     ... 

If you define FOO like this:

#define FOO(x) do { foo(x); bar(x); } while (0) 

then the following is syntactically correct:

if (condition)     FOO(x); else     .... 

Answer 2

It is a way to simplify error checking and avoid deep nested if's. For example:

do {   // do something   if (error) {     break;   }   // do something else   if (error) {     break;   }   // etc.. } while (0);