Is C++ static member variable initialization thread-safe?

Question

Hi!

According to following resources, in C++(Specially Visual C++) scoped static variable initialization isn't thread safe. But, global static variables are safe.

http://stackoverflow.com/questions/1052168/thread-safe-static-variables-without-mutexing

http://blogs.msdn.com/oldnewthing/archive/2004/03/08/85901.aspx

So, is following code with static member variable thread-safe?

class TestClass
{
public:
   static MyClass m_instance;
}

Myclass TestClass::m_instance;

Thanks in advance!

Answer 1

Yes(*). When global statics are initialized, there is only one thread around and all constructors are called on it. This is not true for function's statics, though.

(*) One can possibly make global statics not thread-safe by creating threads in some of the constructors and scheduling some initialization stages on these threads. In this case usual thread safety rules apply.

Answer 2

It's more a question of function-scoped static variables vs. every other kind of static variable, rather than scoped vs. globals.

All non-function-scope static variables are constructed before main(), while there is only one active thread. Function-scope static variables are constructed the first time their containing function is called. The standard is silent on the question of how function-level statics are constructed when the function is called on multiple threads. However, every implementation I've worked with uses a lock around the constructor (with a twice-checked flag) to guarantee thread-safety.