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

c loops
gilm picture gilm · Nov 2, 2008 · Viewed 153.3k times · Source

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

Greg Hewgill picture Greg Hewgill · Nov 2, 2008

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
    ....

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