#define XXX do{XXX} while (0) Why this usage

#define XXX do{XXX} while (0) Why this usage

Often encounter a very "strange macro definition", Rt. (Osiba ... Don't think deep enough, whip, clap

Recently met this guy, Quora above love God answered this question, I also worship

Http://www.quora.com/What-is-the-purpose-of-using-do-while-0-in-macros

This is the C language inside the macro definition, no ambiguity or side effects of a common technique.

Consider the situation.

#define FOO (x) bar (x); Baz (x)

Foo (wolf);

in general, expand to the following code:

Bar (Wolf); Baz (Wolf);

It's all right, no problem. But if you encounter an if judgment statement, take a look at the following example.

F (!feral)    foo (Wolf);

This expands into the following form, (⊙o⊙) See, is there a situation where you do not want the thick line to pinch?

Here the IF statement can only be used for the first bar () function and cannot be used for the second Baz (). But this approach is probably not the programmer's intention.

The original idea is to make the Foo list as a whole.

if (!feral)    bar (Wolf); Baz (Wolf);

Equivalent to the following form

if (!feral)    bar (Wolf); Baz (Wolf);

left thedo/while (0) You don't want to play the macro definition like a function.

If you use this technique to define a macro definition,

#define FOO (x) do {bar (x), Baz (x);} while (0)

Or this macro definition, so use

F (!feral)    foo (Wolf);

It becomes the following form.

if (!feral) do    {bar (Wolf), Baz (Wolf);} while (0);

equivalent to

if (!feral) {    bar (wolf);    Baz (Wolf);

You might think, in this way, adding a {} doesn't solve the problem? Why do you want to do{} while (0)?
consider the following situation

#define FOO (x)  {bar (x); Baz (x);}

if (!feral)    foo (wolf); else    bin (Wolf);

this becomes the

if (!feral) {    bar (wolf);    Baz (Wolf);}; else    bin (Wolf);

Note that this makes the else the "infamous" dangling else.

#define XXX do{XXX} while (0) Why this usage