"When I use -Wall (or -Wunused), g++ warns about
unused parameters. But the parameters have to be there, for use
in derived class functions. How do I get g++ to stop complaining?"
The answer is to simply omit the names of the unused parameters when defining the function. This makes clear, both to g++ and to readers of your code, that the parameter is unused. For example:
int Foo::bar(int arg) { return 0; }
will give a warning for the unused parameter arg. To suppress
the warning write
int Foo::bar(int) { return 0; }