do while(false) pattern [duplicate]

Question

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

Why is the do while(false) necessary in the macros below?

#define LOG(message, ...) \
do { \
Lock<MutualExclusion> lock (logMutex); \

 .... a lot of code ...
} while (false)

I dont think it serves any functional purpose. Am I overlooking something?

Answer 1

It turns a block into a single statement. If you just use a block (i.e. code enclosed in {}) strange things can happen, for example

#define STUFF() \   { do_something(); do_something_else(); }  if (cond)     STUFF(); else     //... 

the extra semi-colon breaks the syntax. The do {} while(false) instead is a single statement.

You can find more about this and other macro tricks here.

Answer 2

So you are forced to add semicolon at the end of the macro, when you use it. This is a common idiom and only way to enforce it.

Answer 3

If somebody has code that does this:

if (something)
    LOG("My log message");

That would expand to:

if (something)
    Lock<MutualExclusion> lock (logMutex);
    // A bunch of other code

Which is incorrect (only the first line would be under the if statement).

The macro makes sure that the macro call is inside of a block of code.