Re: Regarding assignment to struct

Andrey Tarasevich <noone@noone.net> writes:

On Mon 5/5/2025 1:43 PM, Keith Thompson wrote:

What C90 constraint does it violate?  Both gcc and clang reject it
with "-std=c90 -pedantic-errors", with an error message "ISO C90
forbids subscripting non-lvalue array", but I don't see a relevant
constraint in the C90 standard.

>
The "constraint" in C89/90 is simply the fact that C89/90 _requires_
an lvalue (of array type) in order to apply array to pointer
conversion. Here's is the original wording:
>
  Except when it is the operand of the sizeof operator or the unary &
  operator, or is a character string literal used to initialize an
  array of character type, or is a wide string literal used to
  initialize an array with element type compatible with wchar-t, an
  *lvalue* that has type “array of type” is converted to an expression
  that has type “pointer to rype” that points to the initial element
  of the array object and is not an lvalue.
>
The presence of that "*lvalue*" requirement is what prevented up from
using `[]` operator on non-lvalue arrays in C89/90, because `[]`
critically relies on that conversion.
>
In C11 the wording has changed:
>
  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. If the array object has register storage class, the behavior
 is undefined.
>
Note that the "lvalue" requirement has disappeared from this
wording. That is exactly why since C99 we can apply `[]` to non-lvalue
arrays.

Yes, this was discussed in this thread several weeks ago.

A quibble: There are no implicit constraints.  All constraints
are in sections clearly marked "Constraints".  I believe that the
C90 constraint that's violated by `(a = b).a[5]` is the one that
requires the operands of [] to have pointer and integer types.
Since `(a = b).a` does not undergo array-to-pointer conversion
*in C90*, it's of array type.

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */