std::atomic<T>::atomic (3) - Linux Man Pages
atomic() noexcept = default; (1) (since C++11)
constexpr atomic( T desired ) noexcept; (2) (since C++11)
atomic( const atomic& ) = delete; (3) (since C++11)
Constructs new atomic variable.
1) The default constructor is trivial: no initialization takes place other than zero_initialization of static and thread-local objects. std::atomic_init may be used to complete initialization.
2) Initializes the underlying value with desired. The initialization is not atomic.
3) Atomic variables are not CopyConstructible.
desired - value to initialize with
The default-initialized std::atomic<T> does not contain a T object, and its only valid uses are destruction and initialization by std::atomic_init, see LWG_2334