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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Function factories |
| Date | 2012-08-23 16:31 -0700 |
| Organization | None to speak of |
| Message-ID | <ln4nntgp7e.fsf@nuthaus.mib.org> (permalink) |
| References | (8 earlier) <k162la$vqr$7@solani.org> <503690A7.6030404@verizon.net> <k1642l$1rd$2@news.albasani.net> <ln8vd5gtts.fsf@nuthaus.mib.org> <k169mp$1rd$4@news.albasani.net> |
Melzzzzz <mel@zzzzz.com> writes:
> On Thu, 23 Aug 2012 14:51:59 -0700
> Keith Thompson <kst-u@mib.org> wrote:
>> Melzzzzz <mel@zzzzz.com> writes:
>> [...]
>> > Well compiler warnings are enough aren't they? Cast just masks
>> > warning and makes compiler happy which is not good thing.
>> > Well if you have to search trough code to find that one error is
>> > much more difficult rather then just check compiler warning.
>> > I don;t like to shut up compiler warnings with casts and that's
>> > why C's approach is better.
>>
>> The code in question was:
>>
>> double d;
>> int *pi = &d;
>>
>> and the question is whether a compiler should reject it with an error
>> message or accept it with a mere warning.
>>
>> Personally, I would never deliberately write that. As far as the C
>> standard is concerned, it's a constraint violation, requiring a
>> diagnostic from any conforming compiler. If a compiler issues a
>> warning and continues to compile the code, it might seem reasonable
>> to assume that it will generate a conversion from `double*` to `int*`
>> -- but in fact the language standard does not say or imply that
>> that's what will happen. The behavior is undefined; a compiler could
>> legally generate code that sets `pi` to a null pointer, for example.
>
> I didn't knew that this is undefined behavior.
>
>>
>> Whereas if I *want* to do that conversion, there's a perfectly valid
>> way to do it that likely won't result in a warning:
>>
>> double d;
>> int *pi = (int*)&d;
>>
>
> Problem is that we wont get warning in this case IMO.
A compiler *could* issue a warning (because the conversion is
potentially unsafe), but most don't. A cast, particularly a pointer
cast, is effectively a way of telling the compiler "Shut up, I know what
I'm doing".
Are you saying you'd *want* a warning for the cast?
>> A problem I have with gcc is that it doesn't clearly distinguish
>> between warnings about constraint violations, like the above, and
>> warnings about legal but questionable code, such as writing `if (x =
>> 42)` rather than `if (x == 42)`. Again, this doesn't violate the C
>> standard; I just personally find it annoying.
>>
>> That's just my opinion (that all syntax errors and constraint
>> violations should be treated as fatal errors, not as warnings -- or
>> at least that there should be an option that works that way). What's
>> yours? Which errors do you think should be fatal, and which should
>> merely trigger a warning?
>
> I don't think that converting one pointer type to other is error.
Converting from one pointer type to another using a cast is not an
error, though the result is implementation-defined, and using the
converted pointer can lead to undefined behavior.
*Assigning* a pointer value of one type to an object of another
incompatible pointer type is a constraint violation (apart from
special cases involving void*). Likewise for initialization and
argument passing.
> It seems to me that in spirit of C it should be allowed as it is.
> It should issue error if types are incompatible, say pointer to int
> has less bytes than pointer to double.
The word "incompatible" has a specific meaning in C, and it's not
related to the sizes of the types. For example, `int*` and `long*` are
incompatible, even if `int` and `long` happen to have exactly the same
size and representation.
> Problem is that if code that uses such conversions will work at all.
> Say copying data as int / reading as double.
> It might work in one case and might not in other.
> It would be very difficult to find out bug (wrong result etc) without
> compiler warning IMO.
I'm still not sure what you're saying. A compiler *must* issue a
diagnostic for an assignment between incompatible pointer types.
In my opinion, that diagnostic *should* be a fatal error.
Are you saying that a mere warning is preferable? Why?
--
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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Function factories Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-08-21 20:36 +0000
Re: Function factories Nobody <nobody@nowhere.com> - 2012-08-21 21:55 +0100
Re: Function factories Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-08-22 16:36 +0000
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-22 13:01 -0400
Re: Function factories Edward Rutherford <edward.p.rutherford79@REMOVETHIS.gmail.com> - 2012-08-22 18:33 +0000
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-22 22:26 -0400
Re: Function factories David Brown <david@westcontrol.removethisbit.com> - 2012-08-22 08:34 +0200
Re: Function factories Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-08-22 01:52 -0700
Re: Function factories David Brown <david@westcontrol.removethisbit.com> - 2012-08-22 11:34 +0200
Re: Function factories Gareth Owen <gwowen@gmail.com> - 2012-08-24 17:30 +0100
Re: Function factories Gareth Owen <gwowen@gmail.com> - 2012-08-24 17:29 +0100
Re: Function factories jacob navia <jacob@spamsink.net> - 2012-08-22 12:32 +0200
Re: Function factories tom st denis <tom@iahu.ca> - 2012-08-22 07:40 -0700
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-22 11:05 -0400
Re: Function factories jacob navia <jacob@spamsink.net> - 2012-08-23 11:08 +0200
Re: Function factories Chicken McNuggets <chicken@mcnuggets.com> - 2012-08-23 13:27 +0100
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 11:13 -0400
Re: Function factories "Heinrich Wolf" <invalid@invalid.invalid> - 2012-08-23 18:18 +0200
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 12:40 -0400
Re: Function factories Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2012-08-23 09:29 -0700
Re: Function factories jacob navia <jacob@spamsink.net> - 2012-08-23 18:44 +0200
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 12:46 -0400
Re: Function factories Chicken McNuggets <chicken@mcnuggets.com> - 2012-08-23 18:02 +0100
Re: Function factories jacob navia <jacob@spamsink.net> - 2012-08-23 19:24 +0200
Re: Function factories eq mail <eq.mail.id@gmail.com> - 2012-08-23 18:07 -0700
Re: Function factories Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-08-23 21:30 -0400
Re: Function factories eq mail <eq.mail.id@gmail.com> - 2012-08-23 21:46 -0700
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 13:35 -0400
Re: Function factories jacob navia <jacob@spamsink.net> - 2012-08-23 20:13 +0200
Re: Function factories eq mail <eq.mail.id@gmail.com> - 2012-08-23 17:57 -0700
Re: Function factories Andrew Cooper <amc96@cam.ac.uk> - 2012-08-25 02:35 +0100
Re: Function factories Melzzzzz <mel@zzzzz.invalid> - 2012-08-23 21:21 +0200
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 15:36 -0400
Re: Function factories Melzzzzz <mel@zzzzz.invalid> - 2012-08-23 22:09 +0200
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 16:20 -0400
Re: Function factories Melzzzzz <mel@zzzzz.com> - 2012-08-23 22:33 +0200
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 17:07 -0400
Re: Function factories Keith Thompson <kst-u@mib.org> - 2012-08-23 14:51 -0700
Re: Function factories Melzzzzz <mel@zzzzz.com> - 2012-08-24 00:09 +0200
Re: Function factories James Kuyper <jameskuyper@verizon.net> - 2012-08-23 19:01 -0400
Re: Function factories Keith Thompson <kst-u@mib.org> - 2012-08-23 16:31 -0700
Re: Function factories Melzzzzz <mel@zzzzz.com> - 2012-08-24 18:31 +0200
Re: Function factories Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-08-24 02:37 -0700
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