C++ template to cover const and non-const method

Question

I have a problem with duplication of identical code for const and non-const versions. I can illustrate the problem with some code. Here are two s

Answer 1

struct Aggregate
{
    int i;
    double d;

    template 
    void operator()(Visitor &v)
    {
        visit(this, v);
    }
    template 
    void operator()(Visitor &v) const
    {
        visit(this, v);
    }
  private:
    template
    static void visit(ThisType *self, Visitor &v) {
        v(self->i);
        v(self->d);
    }
};

OK, so there's still some boilerplate, but no duplication of the code that depends on the actual members of the Aggregate. And unlike the const_cast approach advocated by (e.g.) Scott Meyers to avoid duplication in getters, the compiler will ensure the const-correctness of both public functions.