"Semi-const" methods?

[Magnus Lie Hetland]

I have a data structure that's generally static (const, or even 
immutable), but it has some utility storage, which caches certain 
results during use. This caching etc. doesn't really affect the 
semantics of the main object, and are reset between operations, so I 
think it still would be useful to declare (and statically check) that 
certain methods don't modify any of the *rest* of the structure (i.e., 
the "main parts").

I *could* declare the methods const, and pass in a second object (a 
non-const parameter) for the caching etc. Or I cast the relevant parts 
to const (assigning them to local variables) early on in the relevant 
methods (dropping the const modifier on the method itself -- sort of a 
bummer).

Any other ideas on how to handle this sort of "mostly const" or "const 
where it counts" stuff? Perhaps my design intentions here are off to 
begin with?-)

-- 
Magnus Lie Hetland
http://hetland.org