Re: Variable-sized types vs. templates

[Jason Merrill <jason at redhat dot com>]

On Tue, 8 Oct 2002 13:35:41 +0100, Neil Booth <neil@daikokuya.co.uk> wrote:

> Jason Merrill wrote:-
>
>> Similarly, I think it's appropriate to say that neither VLA nor VM types
>> are valid template type arguments.  I'd also say that a VLA is not a valid
>> type for a struct member; I don't understand why C99 seems to allow that.
>> Perhaps it wants to allow it so that uses in different functions can have
>> different sizes, but that seems incompatible with the stronger C++ model of
>> struct types.

> I think they wanted to formalize hacks like:
>
> /* Chained list of answers to an assertion.  */
> struct answer
> {
>   struct answer *next;
>   unsigned int count;
>   cpp_token first[1];
> };
>
> Where first is an array of size determined at run time.

But first isn't a VLA.  To actually use a VLA in a struct would mean an
example like

  int foo_length;
  struct foo
  {
    int arr[foo_length];
  };

  void f ()
  {
    foo_length = 42;
    {
      foo bar;
      bar.arr[12] = 24;
    }
  }

which seems rather more ugly than the 'struct hack' you refer to.  And,
since it can only be used for local variables, rather useless.  Now that I
think of it, this is probably more what they had in mind:

  void f (int n)
  {
    struct foo { ...; int arr[n]; } bar;
  }

So, to refine my earlier suggestion, in C++ I'd prohibit VLAs in types with
linkage.  A local class such as this is already an invalid template type
parm.

Jason