Re: Detecting Incomplete Array Initialization at Compile Time?

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From	Keith Thompson <kst-u@mib.org>
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Subject	Re: Detecting Incomplete Array Initialization at Compile Time?
Date	Thu, 21 Jul 2016 12:42:24 -0700
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Ken Brody <kenbrody@spamcop.net> writes:
> On 7/21/2016 2:39 PM, David T. Ashley wrote:
>> Found a bug related to incomplete array initialization yesterday.  The form of the bug was:
>>
>> enum
>>   {
>>   Apples,
>>   Oranges,
>>   Grapefruits,
>>   MaxElements
>>   };
>>
>> int myarray[MaxElements] =
>>    {
>>    429,
>>    258
>>    //Oops, missing initializer--not intended.
>>    //C-language defaults to 0.
>>    };
>>
>> Question:  Is there any way to detect incomplete initialization at compile-time?  (Any obscure feature of the language I'm not aware of?)
>>
>> Caveat:  I've oversimplified the problem for the purposes of example.  Also, this occurred in an embedded system, and doing anything at runtime is off the table.
>>
>
> This might do what you want:
>
> enum
>    {
>    Apples,
>    Oranges,
>    Grapefruits,
>    MaxElements
>    };
>
> int myarray[] = /* Leave off the array size here. */
>     {
>     429,
>     258
>     //Oops, missing initializer--not intended.
>     //C-language defaults to 0.
>     };
>
> static char MyArraySizeCheck[ ( sizeof(myarray)/sizeof(myarray[0]) )
>                                == MaxEmements ? 1 : -1 ];
>
> If the size is "wrong", then the compiler will complain about the "-1" array 
> size on "MyArraySizeCheck".

That's a hack to implement a static assert.  (I'm not using the word
"hack" negatively.)

If you have C11 support, or a compiler that implements it as an
extension, you can just use static assert:

    _Static_assert(sizeof myarray / sizeof myarray[0] == MaxElements,
                   "Extra or missing elements");

(I'd suggest using #if and #error, but sizeof can't be evaluated in
preprocessor expressions.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Detecting Incomplete Array Initialization at Compile Time? "David T. Ashley" <dashley@gmail.com> - 2016-07-21 11:39 -0700
  Re: Detecting Incomplete Array Initialization at Compile Time? Ken Brody <kenbrody@spamcop.net> - 2016-07-21 14:47 -0400
    Re: Detecting Incomplete Array Initialization at Compile Time? Keith Thompson <kst-u@mib.org> - 2016-07-21 12:42 -0700
  Re: Detecting Incomplete Array Initialization at Compile Time? Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-21 20:07 -0700

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