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)

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."