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We cannot use a C++ static initializer to initialize certain globals because libc calls us before our C++ initializers run.

There is some deep dyld and Mach-O voodoo that I don't understand when it comes to the Objective-C runtime! This was a fun question to try dig into.

The function _objc_init() (in objc-os.mm) performs:

Bootstrap initialization. Registers our image notifier with dyld. Called by libSystem BEFORE library initialization time

_objc_init() calls runtime_init(), a portion of which is objc::allocatedClasses.init();. allocatedClasses is declared as static ExplicitInitDenseSet allocatedClasses; in objc-runtime-new.mm.

So it appears libSystem calls _objc_init() which in turns calls runtime_init() which in turn uses a C++ static variable that hasn't yet been constructed because the library itself hasn't been initialized, hence the need for ExplicitInit.

We also don't want a global pointer to some globals because of the extra indirection.

I don't know why this design wasn't chosen. Perhaps for cache locality?

ExplicitInit / LazyInit wrap doing it the hard way.

The init() methods in these classes use placement new to explicitly initialize the underlying type into the space reserved for it in _storage. This would normally happen automatically during static initialization if the type was instantiated directly. In this case space is reserved for the underlying object so it may be explicitly initialized by calling init() which constructs the object.