This methodology allows the mqtt driver to exist in non-global space (such as belonging to another wrapper class).
Other features here allow default constructor, with post constructed Initialization (but only a single time).
Also, the callback feature is backwards compatible with a global function callback OR the option of registering
other objects (by means of the ISubCallback interface) to a list of callback listeners.
This methodology allows the mqtt driver to exist in non-global space (such as belonging to another wrapper class). Other features here allow default constructor, with post constructed Initialization (but only a single time). Also, the callback feature is backwards compatible with a global function callback OR the option of registering other objects (by means of the ISubCallback interface) to a list of callback listeners.