http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58605
Bug ID: 58605 Summary: atomic<T>::atomic() disobeys [atomics.types.operations.req]p4 for types with user-defined default constructors Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: jyasskin at gcc dot gnu.org [atomics.types.operations.req]p4 says: A::A() noexcept = default; Effects: leaves the atomic object in an uninitialized state. [ Note: These semantics ensure compatibility with C. — end note ] http://gcc.gnu.org/viewcvs/gcc/trunk/libstdc%2B%2B-v3/include/std/atomic?revision=201315&view=markup#l160 includes: template<typename _Tp> struct atomic { private: _Tp _M_i; If _Tp has a non-trivial default constructor, this will cause atomic<_Tp>'s default constructor to initialize the member. Using a bare member also adds requirements on the signature of _Tp::_Tp(), in particular that it must now be noexcept. libc++ handles this by defining a "mutable _Atomic(_Tp)" member instead: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/atomic?revision=180945&view=markup#l558