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Groups > comp.lang.c > #76177

Re: Forward declarations

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: Forward declarations
Date 2015-11-16 12:43 +0100
Organization A noiseless patient Spider
Message-ID <n2cfdd$2ui$1@dont-email.me> (permalink)
References (10 earlier) <3f4d20d4-503c-4fe2-8353-2f99a2620e63@googlegroups.com> <87twomsrc8.fsf@bsb.me.uk> <n2bbc2$1jm$1@dont-email.me> <87oaeuso40.fsf@bsb.me.uk> <n2cec9$uvf$1@dont-email.me>

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On 16/11/15 12:26, BartC wrote:
> On 16/11/2015 01:57, Ben Bacarisse wrote:
>> BartC <bc@freeuk.com> writes:
>>
>>> On 16/11/2015 00:47, Ben Bacarisse wrote:
> 
>>>> That's the good part.  I don't think many people reading this will come
>>>> away thinking that the pointer parameter declaration is a second
>>>> declaration for either "foo" or "bar".  After all there were two
>>>> functions and only two function declarations in the whole code example.
>>>
>>> Well I think this solution is unacceptable.
>>
>> That is your right.  What you accept, in this unqualified sense, is up
>> to you.  Had you said "I don't accept that both functions are declared
>> exactly once" you would have made a statement of fact (an incorrect
>> one).
> 
> Yes, there are only two conventional function definitions. I think I
> said that in my first post in the sub-thread.
> 
> But look at the subject line: it's about forward declarations. In my
> view, specifying a function pointer and telling the compiler how it
> ought to be called, constitutes most of a forward declaration.

Then your view is wrong.

A forward declaration for a function tells the compiler about a
/particular/ function.  The most critical piece of information here is
the name of the function.  Parameter and return types are important to
get correct, efficient and safe code - but it is the name that makes
this a forward declaration of a function.

Thus "int foo(int, char*);" declares a function just like "int x;"
declares a variable.


What Tim wrote was a function that took a function pointer as a
parameter.  The parameter simply gives the compiler a type (in this
case, a function pointer type) and a local name for that parameter.  It
is not a declaration of the parameter, or of the thing it points to -
that comes from the function's caller.

So when you write:

void qsort(void*, size_t, size_t,
	    int(*)(const void*,const void*));

you are declaring the function "qsort", but not making a function
declaration for the comparison function.

Even in its implementation:

void qsort(void* base, size_t nmemb, size_t size,
	    int(*compar)(const void*,const void*))
{
	...
}

you are only declaring and defining qsort.  "compar" is not a function
declaration or a forward declaration - there is no function called
"compar" in the code.  You would not call "void* base" a forward
declaration of the data that is going to be sorted by "qsort" - why
would you call the "compar" parameter a forward declaration of a function?


> 
> Because C still requires that. (My own language doesn't work the same
> way. The sole source of information as to how bas ought be called, comes
> from its definition. That is used to double-check the call to it in foo,
> when they are written without function pointers.)

I don't disagree that forward declarations could be made unnecessary in
C or a C-like language, at least when the same translation unit has the
definition.  But that is an independent matter from specifying the type
of a function pointer parameter.

> 
>>> The other unsatisfactory thing [...]
>>
>> I don't think anyone is claiming it's an example of a style to copy.
> 
> OK, so a more acceptable style would need forward declarations. The OP
> is saying that oughtn't be necessary. And I'm sure he meant being able
> to just leave them out without then having to rewrite the code in a
> convoluted and unsafe manner.
> 

In the case of two mutually recursive and dependent functions, a forward
declaration is probably the best solution in most cases.  Tim's point
was merely that it is not the /only/ solution.  A solution using
function pointers may be preferably if you want greater separation
between the functions, such as compiling them in different modules - you
certainly would not use function pointers merely to avoid forward
declarations, except as a teaching example.

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Thread

Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-10 14:20 -0800
  Re: Forward declarations jacobnavia <jacob@jacob.remcomp.fr> - 2015-11-10 23:38 +0100
    Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-10 15:00 -0800
      Re: Forward declarations Ian Collins <ian-news@hotmail.com> - 2015-11-11 12:24 +1300
        Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-10 15:53 -0800
          Re: Forward declarations gazelle@shell.xmission.com (Kenny McCormack) - 2015-11-11 00:01 +0000
          Re: Forward declarations Ian Collins <ian-news@hotmail.com> - 2015-11-11 13:46 +1300
            Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-10 21:05 -0800
    Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-11 00:21 +0000
  Re: Forward declarations Lőrinczy Zsigmond <zsiga@nospam.for.me> - 2015-11-11 10:16 +0100
    Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-11 07:19 -0800
  Re: Forward declarations David Brown <david.brown@hesbynett.no> - 2015-11-11 15:40 +0100
    Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-11 07:21 -0800
      Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-11 07:23 -0800
      Re: Forward declarations David Brown <david.brown@hesbynett.no> - 2015-11-11 16:29 +0100
        Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-11 07:31 -0800
        Re: Forward declarations Nick Bowler <nbowler@draconx.ca> - 2015-11-13 18:20 +0000
          Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-13 10:39 -0800
            Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-13 19:40 +0000
          Re: Forward declarations Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-14 11:42 -0800
            Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 12:32 -0800
              Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 00:15 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 16:34 -0800
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 01:54 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 18:00 -0800
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 02:13 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 18:26 -0800
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 02:34 +0000
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-15 11:11 +0000
              Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 02:36 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 19:25 -0800
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 19:37 -0800
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-14 20:11 -0800
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 10:36 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 06:16 -0800
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-15 14:44 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 09:41 -0800
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 18:26 +0000
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-15 18:42 +0000
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 18:53 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 12:58 -0800
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 12:49 -0800
                Re: Forward declarations Ian Collins <ian-news@hotmail.com> - 2015-11-16 10:01 +1300
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 14:54 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 09:53 -0800
                Re: Forward declarations Keith Thompson <kst-u@mib.org> - 2015-11-15 10:36 -0800
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 12:56 -0800
                Re: Forward declarations Ian Collins <ian-news@hotmail.com> - 2015-11-16 10:04 +1300
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 13:11 -0800
                Re: Forward declarations Ian Collins <ian-news@hotmail.com> - 2015-11-16 10:25 +1300
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 13:37 -0800
              Re: Forward declarations Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-15 13:13 -0800
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 13:16 -0800
                Re: Forward declarations David Brown <david.brown@hesbynett.no> - 2015-11-16 00:23 +0100
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 16:20 -0800
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 00:47 +0000
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-16 01:28 +0000
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 01:57 +0000
                Re: Forward declarations Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-15 21:15 -0500
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 11:05 +0000
                Re: Forward declarations Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-16 10:18 -0500
                Re: Forward declarations Richard Heathfield <rjh@cpax.org.uk> - 2015-11-16 06:44 +0000
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-16 11:26 +0000
                Re: Forward declarations David Brown <david.brown@hesbynett.no> - 2015-11-16 12:43 +0100
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-16 12:20 +0000
                Re: Forward declarations David Brown <david.brown@hesbynett.no> - 2015-11-16 14:38 +0100
                Re: Forward declarations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 13:26 +0000
                Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-16 13:53 +0000
                Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-15 18:12 -0800
      Re: Forward declarations pa@see.signature.invalid (Pierre Asselin) - 2015-11-12 01:54 +0000
        Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-12 02:08 -0800
    Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-12 09:58 +0000
  Re: Forward declarations John Bode <jfbode1029@gmail.com> - 2015-11-17 14:46 -0800
    Re: Forward declarations "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-17 14:59 -0800
    Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-17 23:35 +0000
      Re: Forward declarations BartC <bc@freeuk.com> - 2015-11-17 23:40 +0000
        Re: Forward declarations Keith Thompson <kst-u@mib.org> - 2015-11-17 19:48 -0800
      Re: Forward declarations glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-18 01:04 +0000

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