Re: GURU NEEDED : macro SQUARE(x) for any type x

[Keith Thompson]

PKM <phoenixstormcrow@gmail.com> writes:
> On Jan 13, 11:46 pm, bolega <gnuist...@gmail.com> wrote:
>> #define SQR(x) ({typedef xtype=x; xtype xval=x; xval*xval})  // NOTE,
>> closure or {} inside () is a valid idea in C, and thus no return is
>> needed.
>
> I don't see how this could possibly work:

It can't.

It depends on two gcc-specific extensions, and it gets both of them
wrong.

Here's a version that actually works with gcc:

#define SQR(x) ({typedef typeof(x) xtype; xtype xval=(x); xval * xval;})

The "typeof" keyword is non-standard, as is the use of a "statement
expression".

There is no general solution in standard C that avoids evaluating the
argument twice.

But in practice, this:

#define SQR(x) ((x) * (x))

works perfectly well.  The convention of using all-caps for macro names
warns the user that the argument might be evaluated more than once.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"