How does a virtual proxy implement lazy loading, and what determines when the real object actually gets created?

A virtual proxy implements the same interface as the real, expensive object, but its constructor deliberately does none of the expensive work -- it just stores enough information, like a file name or an identifier, to construct the real object later. It holds a field for the real object that starts out null, which represents 'not created yet.' Every method the client calls on the proxy first checks that field: if it's still null, the proxy constructs the real object right there, triggering the expensive work for the first time, and stores the result; if it's already set, the proxy skips straight to delegating the call. This means the real object gets created exactly once, on the first genuine use, and never at all if the client never actually needs it.