The compiler issues spurious warnings such as "warning: '__cur' might be used uninitialized in this function", where on the corresponding line, __cur is the lvalue in an assignment. The warnings are not issued at -O0. I noticed this problem when dealing with nested STL containers. This is not a duplicate of bug 5035. Here is the test code: #include <vector> #include <set> int main(int, char**) { std::vector<std::vector<std::set<int> > > basis; basis.clear(); return 0; } I'll attach the preprocessed output. Command line and compiler output follows: % g++ -v -save-temps -Wall -o uninitialized -O2 uninitialized.cc Reading specs from /usr/lib/gcc/i686-pc-cygwin/3.4.4/specs Configured with: /gcc/gcc-3.4.4/gcc-3.4.4-1/configure --verbose --prefix=/usr --exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --enable-languages=c,ada,c++,d,f77,java,objc --enable-nls --without-included-gettext --enable-version-specific-runtime-libs --without-x --enable-libgcj --disable-java-awt --with-system-zlib --enable-interpreter --disable-libgcj-debug --enable-threads=posix --enable-java-gc=boehm --disable-win32-registry --enable-sjlj-exceptions --enable-hash-synchronization --enable-libstdcxx-debug : (reconfigured) Thread model: posix gcc version 3.4.4 (cygming special) (gdc 0.12, using dmd 0.125) /usr/lib/gcc/i686-pc-cygwin/3.4.4/cc1plus.exe -E -quiet -v -D__CYGWIN32__ -D__CYGWIN__ -Dunix -D__unix__ -D__unix -idirafter /usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../include/w32api -idirafter /usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../i686-pc-cygwin/lib/../../include/w32api uninitialized.cc -mtune=pentiumpro -Wall -O2 -o uninitialized.ii ignoring nonexistent directory "/usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../i686-pc-cygwin/include" ignoring duplicate directory "/usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../i686-pc-cygwin/lib/../../include/w32api" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++ /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/i686-pc-cygwin /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/backward /usr/local/include /usr/lib/gcc/i686-pc-cygwin/3.4.4/include /usr/include /usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../include/w32api End of search list. /usr/lib/gcc/i686-pc-cygwin/3.4.4/cc1plus.exe -fpreprocessed uninitialized.ii -quiet -dumpbase uninitialized.cc -mtune=pentiumpro -auxbase uninitialized -O2 -Wall -version -o uninitialized.s GNU C++ version 3.4.4 (cygming special) (gdc 0.12, using dmd 0.125) (i686-pc-cygwin) compiled by GNU C version 3.4.4 (cygming special) (gdc 0.12, using dmd 0.125). GGC heuristics: --param ggc-min-expand=47 --param ggc-min-heapsize=32702 /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_vector.h: In member function `std::vector<_Tp, _Alloc>& std::vector<_Tp, _Alloc>::operator=(const std::vector<_Tp, _Alloc>&) [with _Tp = std::set<int, std::less<int>, std::allocator<int> >, _Alloc = std::allocator<std::set<int, std::less<int>, std::allocator<int> > >]': /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_vector.h:715: warning: '__result' might be used uninitialized in this function /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_uninitialized.h:82: warning: '__cur' might be used uninitialized in this function /usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_uninitialized.h:82: warning: '__cur' might be used uninitialized in this function /usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../i686-pc-cygwin/bin/as.exe -o uninitialized.o uninitialized.s /usr/lib/gcc/i686-pc-cygwin/3.4.4/collect2.exe -Bdynamic --dll-search-prefix=cyg -o uninitialized.exe /usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../crt0.o -L/usr/lib/gcc/i686-pc-cygwin/3.4.4 -L/usr/lib/gcc/i686-pc-cygwin/3.4.4 -L/usr/lib/gcc/i686-pc-cygwin/3.4.4/../../.. uninitialized.o -lstdc++ -lgcc -lcygwin -luser32 -lkernel32 -ladvapi32 -lshell32 -lgcc
Created attachment 9629 [details] Preprocessed test case
With mailine compiler (GCC-4.2.x), I cannot reproduce the reported behaviour. -- Gaby