Design question involving casting away constness

Hello Group,

There is some legacy code that i have to make changes to and for which
i need your suggestions.
I am told some technical architect has written this code for some sort
of an UMTS class (a fat interface IMHO) with 250+ methods and 50+
members; and i am not supposed to touch it (nor do i fully understand
it in the first place!).

The simlplifed version with the crux of the problem looks like
this ...
// start snip
class A{
    public:
       void func() { // does some useful calculation }
       void func() const { // left blank, notoriously?? }
};

// here is one of the methods with following semantics
void doCalc(const A& umtsObjIN) {
    // here i need to do some useful calculation via the class A's
members
}
// end snip

Since, the const version of the member-functions are undefined, i have
to call the non-const version; somehow from doCalc(...). Three
approaches that i can envision right now ...

1. const_cast<A*>(&umtsObjIN)->func();
2. ((A)umtsObjIn).func(); // 'C' style
3. Make a copy of umtsObjIn into a non-const temporary object and then
invoke the func.
    A copyofA = untsObjIN; copyofA.func();

Options 1 and 2, i think are UB** in C++ because the the object that
is passed as argument is instantiated as 'const A aObj;' during
application initialization. (** casting away constness of an object
which was originally defined as const) or is it O.K.?
Option-3 works. There are many such methods called many times! Clearly
in this situation, using option-3 everytime doCalc is called is
costly; but is it the only way?

I tried something like this ..
// start-snip
// i touched class A, breaking the rules and added a virtual method
but,
// i do not wish to touch the original author's methods here.
class A{
    public:
    // rest of the stuff
    virtual void adaptFunc(); // override this
};

class B : public A {
    public:
       void adaptFunc() {
          A::func();
       }
};
// end-snip

But dosen't work as it breaks the 'const' contract of 'this' pointer.

Any suggestions from u folks here? May be some sort of an adapter
class?
Or should i start looking for that technical architect :-)

Thanks in advance
Abhay

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