On 2014-04-30, at 11:21 PM, Matthew Woehlke <mw_triad@users.sourceforge.net=
> wrote:

> void foo(int bar)
> {
> #if EXPR
> report(bar)
> #endif
> ...do some other stuff that doesn't use 'bar'...
> }
>=20
> Yes, this happens in real code. (Often the PP conditional is related to i=
f debugging is enabled or not.)

Then [[unused]] would only apply in the other PP branch. Perhaps it the sta=
ndard could recommend not warning about using an [[unused]] entity, but it =
still looks like a bug unless you do

void foo(
 #if ! (EXPR)
   [[unused]]
 #endif
   int bar)

The current solution to this issue is to artificially use (but not ODR-use)=
 the variable:

#if EXPR
report(bar)
#else
(void) bar;
#endif

This "contains the damage" to the use site.

Of course you could wrap [[unused]] in a conditionally-defined macro, but y=
ou can do the same with the void cast (and that's already common practice).

> There's also the issue that not naming the argument makes the code harder=
 to read ("what's this argument that you don't care about?").

I think that's solved by having a function prototype naming the variable, a=
nd leaving it anonymous only in the definition.

I do think [[unused]] might be a reasonable thing to have, but I wouldn't w=
ant it to be hobbled by a common use-case still involving later use. It mig=
ht be nice to tag globals and class members, and get a whole-program warnin=
g which could be specifically asked-for. But I don't know whether that's pr=
actical.

--=20

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And there is a simple standard solution to that:


On Wed, Apr 30, 2014 at 11:21 AM, Matthew Woehlke <
mw_triad@users.sourceforge.net> wrote:

>
> void foo(int bar)
> {
> #if EXPR
>   report(bar)
>

#else
(void)bar;


> #endif

  ...do some other stuff that doesn't use 'bar'...
> }
>
> Yes, this happens in real code. (Often the PP conditional is related to if
> debugging is enabled or not.)
>

I agree that this is a common pattern, say for example:

int rc = doSomething();
assert(!rc);                    // no error

the approach I have been following is the one shown inlined in your code:

assert(!rc); (void)rc;

Not pretty, but functional.

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<div dir=3D"ltr">And there is a simple standard solution to that:<div class=
=3D"gmail_extra"><br><br><div class=3D"gmail_quote">On Wed, Apr 30, 2014 at=
 11:21 AM, Matthew Woehlke <span dir=3D"ltr">&lt;<a href=3D"mailto:mw_triad=
@users.sourceforge.net" target=3D"_blank">mw_triad@users.sourceforge.net</a=
>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><br>
void foo(int bar)<br>
{<br>
#if EXPR<br>
=C2=A0 report(bar)<br></blockquote><div>=C2=A0</div><div>#else</div><div>(v=
oid)bar;</div><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"m=
argin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
#endif=C2=A0</blockquote><blockquote class=3D"gmail_quote" style=3D"margin:=
0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
=C2=A0 ...do some other stuff that doesn&#39;t use &#39;bar&#39;...<br>
}<br>
<br>
Yes, this happens in real code. (Often the PP conditional is related to if =
debugging is enabled or not.)<br></blockquote><div><br>I agree that this is=
 a common pattern, say for example:<br><br>int rc =3D doSomething();<br>
assert(!rc); =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0// no error=C2=A0<br><br>the approach I have been following is the o=
ne shown inlined in your code:=C2=A0<br><br>assert(!rc); (void)rc;<br><br>N=
ot pretty, but functional.</div></div></div></div>

<p></p>

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.