Question

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

When one needs to execute multiple statements within preprocessor macro, it’s usually written like

#define X(a) do { f1(a); f2(a); } while(0)

so when this macro is used inside expressions like:

if (...)
    X(a);

it would not be messed up.

The question is: wherever I’ve seen such expression, it’s always do { ... } while(0);. Is there any reason to prefer such notion over (in my opinion more clear one) if (1) { ... }? Or am I wrong in my observations and they are equally popular?

Answer 1

Nope, you’re not wrong.

There’s actually a nice reason:

#define my_code if (1) { ... }

if (1)
    my_code;

The problem is with the ; ! It shouldn’t be there… and that would just look strange and not in the spirit of the language. You can either choose to have a code that expands in to two ; in a row, or a code that looks un-c-ish 🙂

On the other hand, the do-while construction does not have that problem.

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Also, as others mentioned, there’s an else problem:

if (1)
    my_code;
else { ... }

Ignoring the ; issuse, the else block now belongs to the wrong if.