G++ does not seem to search for classes for friend class declaration outside the current namespace.
The following code does not work with G++ 4.0.3/4.1.0. ---- class bar; namespace NS { class foo { private: friend class bar; static int var; }; int foo::var = 0; } class bar { public: bar() { NS::foo::var = 1; } }; int main() { bar i; } ------- Result is: nstest.cc: In constructor 'bar::bar()': nstest.cc:12: error: 'int NS::foo::var' is private nstest.cc:21: error: within this context The above code works with g++ 3.4.4. A work around for 4.x is defining friend class as "::bar". gcc -v: Using built-in specs. Target: i486-linux-gnu Configured with: ../src/configure -v --enable-languages=c,c++,java,fortran,objc,obj-c++,ada,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.1-1.4.2.0/jre --enable-mpfr --with-tune=i686 --enable-checking=release i486-linux-gnu Thread model: posix gcc version 4.1.0 (Debian 4.1.0-1) System: Linux maestro 2.6.16 #1 SMP PREEMPT Mon Mar 20 21:20:19 EET 2006 i686 GNU/Linux -- Summary: friend class declaration doesn't search out of the namespace Product: gcc Version: 4.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: pvuorela at iki dot fi http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27123