Question

Is global memory initialized in C++? And if so, how?

(Second) clarification:

When a program starts up, what is in the memory space which will become global memory, prior to primitives being initialized? I'm trying to understand if it is zeroed out, or garbage for example.

The situation is: can a singleton reference be set - via an instance() call, prior to its initialization:

MySingleton* MySingleton::_instance = NULL;

and get two singleton instances as a result?

See my C++ quiz on on multiple instances of a singleton...

Answer 1

From the standard:

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) 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 [plain old data] types (3.9) with static storage duration initialized with constant expressions (5.19) 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. [Note:8.5.1 describes the order in which aggregate members are initialized. The initial- ization of local static objects is described in 6.7.]

So yes, globals which have static storage duration will be initialized. Globals allocated, e.g., on the heap will of course not be initialized automatically.