Attempting to declare a previously-declared extern "C" function as a friend
within a class definition fails with an error that the friend declaration
ambiguates the original extern "C" declaration. It appears that the friend
declaration is taken as an overload with "C++" linkage, since G++ then
complains that the class members are private within the context of the extern
"C" function definition.
$ cat friend.cxx
extern "C"
{
int fork (void);
};
class frok
{
int this_errno;
friend int fork (void);
};
extern "C" int
fork (void)
{
frok grouped;
return grouped.this_errno;
}
$ g++-4 -c friend.cxx -o friend.o
friend.cxx:10:24: error: new declaration 'int fork()'
friend.cxx:4:7: error: ambiguates old declaration 'int fork()'
friend.cxx: In function 'int fork()':
friend.cxx:9:7: error: 'int frok::this_errno' is private
friend.cxx:17:18: error: within this context
$
According to, for example, "C++ in a nutshell"
(http://books.google.co.uk/books?id=91JTA9B_m44C&pg=PA170&lpg=PA170&dq=friend+%22storage+class%22&source=bl&ots=HrN4X1Y5wu&sig=N9rnB8r_YnxbH2hiWGqtbAWbWyk&hl=en&ei=oDt_SqmvGJKNjAe9k93wAQ&sa=X&oi=book_result&ct=result&resnum=5#v=onepage&q=friend%20%22storage%20class%22&f=false),
a function in a friend declaration should "retain its original linkage" when it
has already been declared. That is referring to the case of applying a storage
class specifier (static) to the prior declaration that isn't syntactically
valid within a friend declaration, but it seems like it should apply here too;
the linkage should still be retained, even though I'm not changing the
storage-class specifier.
There may be some reason why this isn't valid C++, but I think it's probably
a weakness in the parsing of friend declarations.
$ g++-4 -v
Using built-in specs.
Target: i686-pc-cygwin
Configured with: /gnu/gcc/gcc-patched/configure --prefix=/opt/gcc-tools -v
--wit
h-gmp=/usr --with-mpfr=/usr --enable-bootstrap
--enable-version-specific-runtime
-libs --enable-static --enable-shared --enable-shared-libgcc
--disable-__cxa_ate
xit --with-gnu-ld --with-gnu-as --with-dwarf2 --disable-sjlj-exceptions
--disabl
e-symvers --disable-libjava --disable-interpreter --program-suffix=-4
--disable-
libgomp --enable-libssp --enable-libada --enable-threads=posix --with-arch=i686
--with-tune=generic CC=gcc-4 CXX=g++-4 CC_FOR_TARGET=gcc-4 CXX_FOR_TARGET=g++-4
--with-ecj-jar=/usr/share/java/ecj.jar LD=/opt/gcc-tools/bin/ld.exe
LD_FOR_TARGE
T=/opt/gcc-tools/bin/ld.exe AS=/opt/gcc-tools/bin/as.exe
AS_FOR_TARGET=/opt/gcc-
tools/bin/as.exe --disable-win32-registry --disable-libgcj-debug
--enable-langua
ges=c,c++,ada
Thread model: posix
gcc version 4.5.0 20090730 (experimental) (GCC)
--
Summary: Can't declare an extern "C" friend.
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: davek at gcc dot gnu dot org
GCC build triplet: i686-pc-cygwin
GCC host triplet: i686-pc-cygwin
GCC target triplet: i686-pc-cygwin
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41020
