In C++03 mode template class atomic<T> deliberately has no declared constructors, because examples like GetUniqueInteger, shown in the Atomic Operations section, are commonly required to work correctly even before all file-scope constructors have been called. If atomic<T> declared a constructor, a file-scope instance might be initialized after it had been referenced.
As for any C++ class with no declared constructors, an object X of type atomic<T> is automatically initialized to zero in the following contexts:
X is declared as a file-scope variable or as a static data member of a class.
x is a member of a class and explicitly listed in the constructor's initializer list.
The code below illustrates these points.
atomic<int> x; // zero-initialized because it is at file scope class Foo { atomic<int> y; atomic<int> notzeroed; static atomic<int> z; public: Foo() : y() // zero-initializes y. { // notzeroed has unspecified value here. } }; atomic<int> Foo::z; // zero-initialized because it is a static member
In C++11 mode, template class atomic<T> has two constructors :
atomic() = default; default constructor generated by compiler. This constructor behaves same as if there were no user defined constrcutors declared at all. This constructor keeps backward compatibilty with C++03 mode, e.g. allow zero-initilization of global objects.
constexpr atomic(T arg); this constructor allows initialization of atomic variable during translation time, only if the argument is itself a translation time constant, otherwise initialization is performed at run time.