Re: c++/8041: Array to pointer conversion in cast expression

Re: c++/8041: Array to pointer conversion in cast expression

[René Møller Fonseca]

The following reply was made to PR c++/8041; it has been noted by GNATS.

From: =?ISO-8859-1?Q?Ren=E9_M=F8ller_Fonseca?= <fonseca@mip.sdu.dk>
To: gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org,
	gcc-prs@gcc.gnu.org, austern@apple.com
Cc:  
Subject: Re: c++/8041: Array to pointer conversion in cast expression
Date: Fri, 14 Feb 2003 19:58:47 +0100

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8041
 
 This "implicit casting" problem seems to be related (checked with GCC 3.2 under GNU/Linux and Solaris).
 
 #include <iostream>
 
 using namespace std;
 
 class MyClass {
 public:
 
   template<size_t SIZE>
   MyClass(const int (&x)[SIZE]) throw() {
     cout << "int[" << SIZE << "]" << endl;
   }  
 
   template<size_t SIZE>
   MyClass(long (&x)[SIZE]) throw() {
     cout << "long[" << SIZE << "]" << endl;
   }
 };
 
 MyClass myIntFunction() throw() {
   static const int INT_ARRAY[999] = {}; // const
   return INT_ARRAY; // this fails to compile
   // GCC ERROR: conversion from `const int*' to non-scalar type `MyClass'
 }
 
 MyClass myLongFunction() throw() {
   static long LONG_ARRAY[1234] = {}; // mutable
   return LONG_ARRAY; // this fails to compile
   // GCC ERROR: conversion from `long int*' to non-scalar type `MyClass'
 }
 
 int main() {
   static const int INT_ARRAY[123] = {};
 
   MyClass a(INT_ARRAY); // this works
   MyClass b = INT_ARRAY; // this works
   
   myIntFunction();
   myLongFunction();
   return 0;
 }
 

Re: c++/8041: Array to pointer conversion in cast expression

[bangerth]

Synopsis: Array to pointer conversion in cast expression

State-Changed-From-To: open->analyzed
State-Changed-By: bangerth
State-Changed-When: Thu Dec 19 17:27:01 2002
State-Changed-Why:
    Confirmed.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8041

c++/8041: Array to pointer conversion in cast expression

[austern]

>Number:         8041
>Category:       c++
>Synopsis:       Array to pointer conversion in cast expression
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Wed Sep 25 11:26:02 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     austern@apple.com
>Release:        2.96 (Redhat), 3.1, 3.2, 3.3 TOT
>Organization:
>Environment:
Linux
>Description:
Consider the following file:
void foo() {
  typedef int (&AP)[2][2];
  int a[2][2];

#ifdef NEW_STYLE_CAST
  AP ap = static_cast<AP>(a);
#else
  AP ap = (AP) a;
#endif
}

With all of the versions I listed above: this code compiles without error if -DNEW_STYLE_CAST is on the command line, but omitting it yields this error message:
  ap.cc:8: cannot convert `int (*)[2]' to `int[2][2]' in converting

The reason for the error message is that the compiler is applying the array-to-pointer standard conversion, so the argument to the cast expression is of type int (*)[2], which can't be cast to type AP.

I believe the compiler is incorrect in applying that standard conversion for the old-style cast, and that the static_cast behavior is right.  (Obviously one of the two must be wrong.)

Here's an analysis from William Miller, posted on the committee reflector:
  5.2.9p2 says that "static_cast<T>(e)" is well-formed if the
  declaration "T t(e);" is, and the effect is the same as using
  "t".  In this case, if "int (&t)[2](a)" is well-formed, the
  static_cast is as well.  In other words, it depends on how
  "a" is declared, but it's not automatically an error.
>How-To-Repeat:

>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: