When are static C++ class members initialized?

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The standard guarantees two things – that objects defined in the same translation unit (usually it means .cpp file) are initialized in order of their definitions (not declarations):

3.6.2

The storage for objects with static storage duration (basic.stc.static) shall be zero-initialized (dcl.init) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization. Objects of POD types (basic.types) with static storage duration initialized with constant expressions (expr.const) shall be initialized before any dynamic initialization takes place. Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.

The other guaranteed thing is that initialization of static objects from a translation unit will be done before use of any object or function from this translation unit:

It is implementation-defined whether or not the dynamic initialization (dcl.init, class.static, class.ctor, class.expl.init) of an object of namespace scope is done before the first statement of main. If the initialization is deferred to some point in time after the first statement of main, it shall occur before the first use of any function or object defined in the same translation unit as the object to be initialized.

Nothing else i guaranteed (especially order of initialization of objects defined in different translation units is implementation defined).

EDIT
As pointed in Suma’s comment, it is also guaranteed that they are initialized before main is entered.

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