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Comment by trealira | Hacker News Reader

It could help in generic macros, like this:

  #define SWAP(var1, var2) do { \
      auto tmp = var1; \
      var1 = var2; \
      var2 = tmp; \
  } while (0)

Previously, you would have needed a third macro for the type, or you would have needed to do a byteswap to be generic.

True, and now we also get typeof() so there is more than one way!

  const typeof(var1) tmp = var1;

Why’s that inside do/while?

To scope the temporaries?

No, a block would be enough for that. It's to make ; handling natural when the macro is used.

Forces the use of a semicolon:

SWAP(…);

If you simply put braces around it, you'd generate syntactically invalid code when you don't use braces for if-else statements.

Consider this:

  if (e)
      SWAP(a, b);
  else
     something_else();

Currently, that expands to this, which is still valid code:

  if (e)
      do {
          // ...macro...
      } while(0);
  else
      something_else();

If it were just wrapped in braces, the code would be parsed like this:

  // One-armed if-statement
  if (e) {
      // ...macro...
  }

  // Empty statement
  ;

  // Another statement
  else something_else();

It's incorrect syntax to start a statement with else, so the compiler will say something like "Unexpected 'else' at line N."

I would not have come up with that on my own. Thanks for the explanation!

No problem! I didn't come up with it on my own, either. I simply read it somewhere, and now I'm repeating it.

"What's the best way to write a multi-statement macro?"