Re: "span"

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.