do { } while(0) vs. if (1) { } in macros [duplicate]

问题

Possible Duplicate:
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?

回答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.

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.

回答2:

if can be as safe as do/while only if there is else branch. E.g.:

#define X(a) if(1) { f1(a); f2(a); } else{}

Is as safe as:

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

So that the user can't do:

X(...) else ...;

One difference is that when using do/while it requires a ; at the end, whereas if/else doesn't.

回答3:

Consider this:

if(foo)
    X(a);
else
    whatever();

This would expand to:

if(foo)
    if(1) { ... }
else
    whatever();

Which is bad because now the else belongs to the wrong if.

回答4:

Using do... while allows you to break out if necessary.

回答5:

When you use #define X(a) do { ... } while(0) form, it forces you to put ; at the end of the statement X(1).

来源：https://stackoverflow.com/questions/10720465/do-while0-vs-if-1-in-macros

标签

c-preprocessor