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Message-ID: <20010328044032.15122.qmail@web6405.mail.yahoo.com>
Date: Wed, 28 Mar 2001 16:40:32 +1200 (NZST)
From: =?iso-8859-1?q?Danny=20Smith?= <danny_r_smith_2001 AT yahoo DOT co DOT nz>
Subject: static constant initialisation fails for dllimported classes
To: Cygwin <cygwin AT sources DOT redhat DOT com>,
        Mingw <mingw-users AT lists DOT sourceforge DOT net>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: 8bit

Static constant initialisation of data in C++ classes works when linking
statically, but not with dllimported classes.  	This is a bug with older
versions of gcc as well as gcc-2.95.3.

The following code used to build dll:

dllclass.h
======================================
#if BUILDING_DLL
# define DLLIMPORT __declspec (dllexport)
#else /* Not BUILDING_DLL */
# define DLLIMPORT __declspec (dllimport)
#endif /* Not BUILDING_DLL */

class DLLIMPORT 
DllClass {
public:
  DllClass(); 
  unsigned int a_method () const;
  static int non_const_int;	/* initialised in dllclass.cc */
  static const unsigned int const_int=256;
  char buffer[const_int];  
};
==========================================

dllclass.cc
===========================================
#include "dllclass.h"
#include <string.h>
DllClass::DllClass(){
  memset(buffer,0,const_int); 
}

unsigned int 
DllClass::a_method () const { 
  return const_int;
}
int
DllClass::non_const_int;

============================================

Dll build correctly.
non_const_int is exported as DATA.
const_int is not exported. That's fine.

This is client code:
usedll.cc
=========================================
#include <stdio.h>
#include "dllclass.h"

int main () {
  DllClass A;
  printf("a_method = %d\n", A.a_method());
}
==========================================

This fails to compile with error:
dllclass.h:13: initialized variable `const int DllClass::const_int' is marked
dllimport


In this case, (integral const), one workaround is the enum hack.
-  static const unsigned int const_int=256;
+  enum {const_int=256};

The problem occurs because class members get the dllimport/export status
of their class. 
IMO static const data members of classes initialised in class definition should
not "inherit" the dllimport attribute of their class.
The following w32-specific patch to gcc/config/i386/winnt.c fixes the problem.
I have tested with STLport, which uses static const initialisation of
fmtflags (in ios_base) and locale categories, and  no problems yet. 

However, I am very, very new to the insides of gcc and this patch
probably breaks soemthing else. Can somebody who "knows what they are
doing" check.


--- gcc/config/i386/winnt.c.orig	Wed Jan 19 19:30:10 2000
+++ gcc/config/i386/winnt.c	Tue Mar 27 22:03:47 2001
@@ -250,6 +250,11 @@ i386_pe_dllimport_p (decl)
   context = associated_type (decl);
   if (context)
     {
+    /* Don't use context to mark initialised variables as dllimport */
+      if (TREE_CODE (decl) == VAR_DECL
+        && (DECL_INITIAL (decl)
+          && ! TYPE_NEEDS_CONSTRUCTING (TREE_TYPE (decl))))
+        return 0;
       imp = lookup_attribute ("dllimport",
 			      TYPE_ATTRIBUTES (context));
       if (imp)

Danny

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