A forward service declaration guard.
Helps to resolve an issue with hybrid services, i.e. dynamic services, accompanied by static services in the same DLL. Only automatic instances of this class are supposed to exist. Those are created during (dynamic) service initialization and serve to:
(a) Ensure the service we are loading is ordered last in the repository, following any other services it may cause to register, as part of its own registration. This is a common case when loading dynamic services from DLLs - there are often static initializers, which register static services.
(b) The SDG instance destructor detects if the dynamic service initialized successfully and "fixes-up" all the newly registered static services to hold a reference to the DLL, from which they have originated.