Re: C++ PATCH to reject initializating flexible array members in constructors (PR c++/72775)

[Martin Sebor <msebor at gmail dot com>]

On 12/09/2016 08:09 AM, Jakub Jelinek wrote:
> On Fri, Dec 09, 2016 at 08:04:11AM -0700, Martin Sebor wrote:
> > I'm not sure I understand what you mean.  Since the following is
> > valid and meaningful (i.e., initializes the array with  the elements
> > of the string):
> >
> >   struct Foo {
> >    int a;
> >    char ary[4];
> >    Foo () : ary ("bob") {}
> >   };
>
> Sure, that has a clear meaning.
>
> > then (IMO) so is this:
> >
> >   struct Foo {
> >    int a;
> >    char ary[];
> >    Foo () : ary ("bob") {}
> >   };
>
> So what does this mean?  Assume that the Foo object lives on the heap
> and has been allocated with extra space after it?  Or shall global/automatic
> objects of type Foo be allocated with the extra space (like
> struct Foo x = { 5, "bob" }; in C would do if the Foo () ... line is
> removed?
> Something different?

It means the same thing as the first case, down to sizeof (Foo).
All of Foo's ctors would have to agree on the size of the initializer
(with an error if they didn't and could be seen in the same translation
unit).  In most cases the notation from the bug report would then be
the preferred one to use.

  struct Foo {
    int a;
    char ary[] = "bob";
    Foo () { }
  };

Martin