I understand what you've done; I'm asking that you not visually pollute the signature in the class definition with a non-obvious annotation linked to an implementation detail.

It is not important to the reader that this specific implementation of CDev::write doesn't use the argument. If it were, the place to note to the reader that the argument is unused is in the documentation block for the function. If it's important to note it to the compiler, the attribute should be placed near the implementation, so that when the implementation is changed the annotation can be removed.

An annotation of the kind you've used makes good sense when your readers are all familiar with it - as I noted, the fact that you felt it was necessary to explain the annotation to your reader (were you planning on doing that every time you did that, for every argument that isn't used in every method?) is a pretty good hint that it's not going to work.

The notation you've chosen is by no means unique to the meaning you've assigned, either; historically it was used to avoid namespace collisions between preprocessor macro definitions and function argument names, so it can easily be read more than one way...