WG21/N0238;X3J16/93-0051

Thomas Plum        Plum Hall Inc, #261 PO Box 111333
plum@plumhall.com                 Kamuela HI 96743 USA
                                  808-882-1255   FAX 808-882-1556 [new]
Re: Array of unknown bound

1.  The type ``array of unknown bound of T'' must be part of the type
system.  For example, it is required for  extern int a[]; .  Refer to
such an array as an "incomplete array type".

2.  However, the full set of "compatible type" rules from C will create
a problem in C++.  Shopiro suggested the following example which is
valid in C but dangerous (under any set of rules) in C++:

file 1:
extern int a[];
typedef int UNKA[];
void f(UNKA*);
 ...
f(&a);

file 2:
int a[N] = {0};
typedef int AN[N];
void f(AN*);
 ...
f(&a);

In C, the two invocations of  f  must resolve to the same function  f ,
but this is impossible in C++.

3.  Functions with a pointer-to-incomplete-array parameter pose a
problem for "name-mangling" schemes in some systems, as reported by
Gibbons.

4.  Given that the C/C++ behaviors must differ in some way, where should
we draw the line?  If functions with pointer-to-incomplete-array
parameter are prohibited (with diagnostic required), a C programmer
would be guaranteed of an explicit diagnostic for a program whose
behavior might change in C++.  If such functions are allowed in C++,
there will be incompatibility situations whose diagnosis relies upon
the (currently unresolved) diagosis requirements for the one-definition
rule.  Therefore I propose the following additional requirement upon
function declarations, which is to be in the "diagnostic required"
category:

If the type of a function parameter includes a type of the form
"pointer to array of unknown bound of T", the program is ill-formed.
[Or use "Required Diagnostics", as determined editorially.]

5.  The current rules for standard conversions prohibit some conversions
which are valid in ISO C.  This should be noted in Chapter 19.  Example:

extern int a[];
typedef int AN[N];
AN *p = &a;	/* valid in ISO C, invalid in C++ */

6.  Some members may want to impose additional requirements.  These can
be proposed and added later.