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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: absence of max
Date: Sat, 03 Dec 2016 11:08:41 -0800
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jameskuyper@verizon.net writes:
> On Saturday, December 3, 2016 at 11:15:35 AM UTC-5, David Brown wrote:
>> On 03/12/16 16:32, James R. Kuyper wrote:
> ...
>> > C2011 adds a feature that allows us to check this directly (which won't
>> > necessarily convince Jerry, either):
>> >
>> > #include <stdio.h>
>> > #define TYPE(a) _Generic(a, \
>> >     int*:"int *", int[4]:"int[4]", default:"other")
>> >
>> > typedef int thing[4];
>> > void foo(thing bar)
>> > {
>> >    thing boz;
>> >    printf("%s:%s\n", "bar", TYPE(bar));
>> >    printf("%s:%s\n", "boz", TYPE(boz));
>> > }
>> >
>> > int main(int argc, char *argv[])
>> > {
>> >     foo(&argc);
>> >
>> >     return 0;
>> > }
>> >
>> > Unfortunately, the compiler I currently have access to doesn't support
>> > C2011. Therefore, I can't confirm whether I've used _Generic correctly,
>> > nor can I confirm the results. Could someone who has a compiler that
>> > does support _Generic() check this out?
>> 
>> I had a look using <https://godbolt.org>, but it looks like both get 
>> printed as "int *".  Maybe _Generic also turns arrays into int* like 
>> parameter passing?  I don't know the details of _Generic well enough to 
>> tell.
>
> "Except when it is the operand of the sizeof operator, the _Alignof
> operator, or the unary & operator, or is a string literal used to
> initialize an array, an expression that has type ‘‘array of type’’ is
> converted to an expression with type ‘‘pointer to type’’ that points
> to the initial element of the array object and is not an lvalue."
> (6.3.2.1p3)
>
> I think there should also have been an exception for _Generic, but
> that's not what was actually done. Therefore, my suggestion is not as
> useful as I thought it would be.

But it can be made useful by taking the address of the operand:

#include <stdio.h>
#define TYPE(a) _Generic(&a, \
    int**:"int *", int(*)[4]:"int[4]", default:"other")

typedef int thing[4];
void foo(thing bar)
{
   thing boz;
   printf("%s:%s\n", "bar", TYPE(bar));
   printf("%s:%s\n", "boz", TYPE(boz));
}

int main(int argc, char *argv[])
{
    foo(&argc);

    return 0;
}

Output:

bar:int *
boz:int[4]

-- 
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"