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Re: ridiculous warning or not?

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Tim Rentsch <txr@alumni.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> writes:
[...]
>> As far as I can tell, the rule that `char f[];` is treated as `char
>> f[1];` isn't even stated explicitly.
>>
>> N1570 6.9.2p2, External definitions, says:
>>
>>     A declaration of an identifier for an object that has file scope
>>     without an initializer, and without a storage-class specifier or
>>     with the storage-class specifier static, constitutes a *tentative
>>     definition*. If a translation unit contains one or more tentative
>>     definitions for an identifier, and the translation unit contains
>>     no external definition for that identifier, then the behavior
>>     is exactly as if the translation unit contains a file scope
>>     declaration of that identifier, with the composite type as of
>>     the end of the translation unit, with an initializer equal to 0.
>>
>> An example (non-normative) says:
>>
>>     If at the end of the translation unit containing
>>
>>         int i[];
>>
>>     the array i still has incomplete type, the implicit initializer
>>     causes it to have one element, which is set to zero on program
>>     startup;
>>
>> So this:
>>
>>     char f[];
>>
>> is treated as:
>>
>>     char f[] = 0;
>
> No, it would be  'char f[] = {0};'  as the implementation is
> obliged to do by 6.7.9p16 -- implementations are obliged to
> follow "shall" directives.

Implementations are obliged to follow *all* normative requirements
in the standard.  6.9.2p2 requires an implicit "initializer equal
to 0".  I wouldn't say that `{0}` is "equal to 0".

>> which causes `f` to inherit its size from the initializer.
>>
>> Except that
>>
>>     char f[] = 0;
>>
>> is invalid.  6.7.9p16 says:
>>
>>     Otherwise, the initializer for an object that has aggregate or union
>>     type shall be a brace-enclosed list of initializers for the elements
>>     or named members.
>>
>> A twist: That "shall" is not in a constraint, so it's undefined
>> behavior, not something that requires a diagnostic.
>>
>> Apparently we have to interpret the "initializer equal to 0" to mean
>> `{ 0 }` rather than `0` in this case.  And we're supposed to infer
>> from that that `f` has exactly one element; apparently `{ 0 }` is
>> "equal to 0" but `{ 0, 0 }` isn't.
>
> { 0, 0 } is two initializers, not one.

{ 0, 0 } is one initializer, which contains two more initializers.

N1370 6.7.9p1:

    initializer:
        assignment-expression
        { initializer-list }
        { initializer-list , }

The idea that `char f[];` is implicitly `char f[1];` is sufficiently
non-obvious (and IMHO counterintuitive) that it should be stated
explicitly.  We shouldn't have to depend on a flimsy and IMHO incorrect
chain of reasoning that appears only in a non-normative example.

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

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Thread

ridiculous warning or not? kerravon <kerravon@w3.to> - 2012-08-20 02:41 -0700
  Re: ridiculous warning or not? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-20 07:55 -0400
  Re: ridiculous warning or not? army1987 <army1987@ask-for-it.invalid> - 2012-08-20 19:11 +0000
  Re: ridiculous warning or not? Kenneth Brody <kenbrody@spamcop.net> - 2012-08-20 16:09 -0400
    Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 07:29 -0700
  Re: ridiculous warning or not? Andrew Cooper <amc96@cam.ac.uk> - 2012-08-21 00:36 +0100
  Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-08-21 01:41 +0100
    Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-08-20 18:59 -0700
      Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 08:03 -0700
        Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-09-07 14:37 -0700
          Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 19:17 -0700
            Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-09-07 19:56 -0700
              Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-08 06:30 -0700
    Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 07:57 -0700
      Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-08 00:25 +0100
        Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-07 18:58 -0700
          Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-09-07 19:26 -0700
            Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-08 06:26 -0700
              Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-09-08 11:33 -0700
                Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 14:39 -0800
                Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-12-17 15:56 -0800
                Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-20 10:31 -0800
                Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-12-20 16:57 -0800
                Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-21 03:11 +0000
                Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-21 09:06 -0800
          Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-08 03:42 +0100
            Re: ridiculous warning or not? Tim Rentsch <txr@alumni.caltech.edu> - 2012-09-08 06:01 -0700
              Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-08 14:45 +0100
                Re: ridiculous warning or not? Keith Thompson <kst-u@mib.org> - 2012-09-08 11:28 -0700
                Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-08 22:53 +0100
  Re: ridiculous warning or not? kerravon <kerravon@w3.to> - 2012-08-21 03:25 -0700
    Re: ridiculous warning or not? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-21 11:20 -0700
      Re: ridiculous warning or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-08-21 21:07 +0100
        Re: ridiculous warning or not? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-21 14:39 -0700
          Re: ridiculous warning or not? Ike Naar <ike@iceland.freeshell.org> - 2012-08-21 22:09 +0000
            Re: ridiculous warning or not? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-08-21 15:13 -0700
              Re: ridiculous warning or not? Fred K <fred.l.kleinschmidt@gmail.com> - 2012-08-22 07:17 -0700

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