How can I improve this design that forces me to declare a member function const and declare variables mutable?

Question

For some reason I am iterating over elements of a class in an std::set and would like to slightly modify the keys, knowing that the order will be unchanged.

Answer 1

Another option is to const_cast to a reference type :

class Foo
{
public:
    void incrementB() const { ++ const_cast< int& >( b_ ); }
private:
    int b_;
};

But as sehe already said, you shouldn't modify set's elements.