Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: "span"
Date: Fri, 22 Mar 2024 08:51:18 -0300
Organization: A noiseless patient Spider
Lines: 84
Message-ID: <utjrbm$2st57$1@dont-email.me>
References: <span-20240322122631@ram.dialup.fu-berlin.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Fri, 22 Mar 2024 11:51:19 -0000 (UTC)
Injection-Info: dont-email.me; posting-host="b69c71425e7b8f370ab82e463a5eae2f";
	logging-data="3044519"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/TYOCD+n0d5pcfSucotKlaMobe8lO5Rek="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:fyhWOf4RbvvJrdHU8EuQWE83wi4=
In-Reply-To: <span-20240322122631@ram.dialup.fu-berlin.de>
Content-Language: en-US
Bytes: 3377

On 22/03/2024 08:30, Stefan Ram wrote:
>    Some people suggested to introduce a feature "span" (probably a
>    type) into C++, so that when one has an int array "a" and calls
>    "f( a )", the length of a is being transferred to "f", which "f"
>    is defined using "void f(span<int> a)". The "span" in "f" then
>    knows the length of "a". This is supposed to be less error prone
>    than passing a pointer with a separate length argument.
> 
>    Of course, I immediately wondered whether one could implement
>    such a "span" for C, and here's a draft:
> 
> #include <stdio.h>
> 
> #define SPAN_PARAM(x,a,n) x*a,size_t const n
> #define SPAN(a) a,(sizeof a/sizeof 0[a])
> 
> void print( SPAN_PARAM( int, a, n ))
> { for( size_t i = 0; i < n; ++i )
>    printf( "%lld: %d\n", i, a[ i ]); }
> 
> int main( void )
> { int a[ 3 ]={ 4, 6, 8 };
>    print( SPAN(a) ); }
> 
>    . But since C is another language, there are other forces at work,
>    which means that the overall usability of such macros in C might not
>    make their definition and use worthwhile. Of course, the "smart" span
>    type in C++ surely can do more than my simple macros can do in C!


(
C++ proposals use new type<T> instead of qualifiers. This creates a big 
   mess on the type system and incompatibility with C.
I don't have idea how/why they do this constantly.
)

C does not need this for parameters since the current array syntax 
already cover that.

void f(int n, int a[n])

The problem is the syntax for pointers, this may be necessary in other 
contexts other than in parameters.


C already have a syntax for pointer to array

int (*)[2] p;

The problem of this syntax is C code normally use pointer directly
and a pointer to array requires *p everywhere. Having an specific syntax 
for pointer length makes the code transition from  non-annotated to 
annotated much easier. It also avoid lots of (*p)[index] and have just 
p[index].


Does anyone have a suggestion for syntax to give pointers the length 
information?

One try is

int *[2] p;

but the problem of this syntax is it is ambiguous with (cast)

(int *[2])  //pointer to 2 ints, or pointer to array of 2 ints

This alternative

(int [2]*)
is harder to parse since first it thinks it is array then it need fo 
"fix" to be a pointer.