The initialization of the variables
b in your question are indeterminately sequenced in relation to each other. The initialization order is not guaranteed between them.
The initialization rules of C++ are quite complex, especially past C++11. The relevant part would be C++11 3.6.2, "Initialization of non-local variables" which speaks of zero initialization, then constant initialization. The formal terms are defined as
Together, zero-initialization and constant initialization are called static initialization; all other initialization
is dynamic initialization.
Your specific case sorts under dynamic initialization, even if the storage duration of the objects are static/thread storage. Static storage variables have the following rules, emphasis mine:
Dynamic initialization of a non-local variable with static storage duration is either ordered or
unordered. Definitions of explicitly specialized class template static data members have ordered initialization.
Other class template static data members (i.e., implicitly or explicitly instantiated specializations) have
unordered initialization. Other non-local variables with static storage duration have ordered initialization.
Variables with ordered initialization defined within a single translation unit shall be initialized in the order
of their definitions in the translation unit.
Regarding thread storage, the same section continues, emphasis mine:
If a program starts a thread (30.3), the subsequent initialization
of a variable is unsequenced with respect to the initialization of a variable defined in a different translation
unit. Otherwise, the initialization of a variable is indeterminately sequenced with respect to the initialization
of a variable defined in a different translation unit. If a program starts a thread, the subsequent unordered
initialization of a variable is unsequenced with respect to every other dynamic initialization. Otherwise,
the unordered initialization of a variable is indeterminately sequenced with respect to every other dynamic