Dealing with lazy computation in C++ classes

Question

Let's say I have a class:

class NumberCollection
{
public:
    typedef std::set<int> SetType;
    typedef SetType::iterator iterator;
    void insert(int n);

    iterator begin();
    iterator end();
    size_t size() const;

    iterator difficultBegin();
    iterator difficultEnd();
    size_t difficultSize() const;    

private:
    SetType easySet_, difficultSet_;
}

Where insert() adds an element to easySet_. difficultSet_'s members change depending on the members of easySet_.

The problem I am having is that, multiple insertions means that difficultSet_ is constantly recalculated. So I want difficultSet_ to be calculated lazily (i.e., only when difficultBegin(), difficultEnd(), or difficultSize() are called). The problem is, then I actually have to make difficultSet_ into a mutable because otherwise difficultSize() cannot operate on it.

So now my class declaration looks like

class NumberCollection
{
public:
    typedef std::set<int> SetType;
    typedef SetType::iterator iterator;
    void insert(int n);

    iterator begin();
    iterator end();
    size_t size() const;

    iterator difficultBegin();
    iterator difficultEnd();
    size_t difficultSize() const;    

private:
    SetType easySet_; 
    mutable SetType difficultSet_;
    mutable bool upToDate_;
}

I feel like this is bad design though. Is there a better way?

Answer 1

That's totally the way to do it. Const can mean binary const, or it can mean conceptually const. Using mutable means you're doing the later, which is fine.

Answer 2

To help understand why to use mutable, we can explore other options.

You can solve the same problem using const_cast:

size_t NumberCollection::difficultSize() const
{
     if(!upToDate_)
     {
          NumberCollection& nonConst = const_cast<NumberCollection&>(*this);
          nonConst.difficultSet_ = PerformExpensiveCalculationFunction();
          nonConst.upToDate_ = true;
     }
     // etc....
}

Having offered this solution, I'll say that it's inferior to using mutable. If a member is marked as mutable, then simply by looking at the header I can gather how you are treating it. I don't get this information if you use const_cast.

But then somebody might take the other side of the debate, and say that it's better not to expose implementation details in the header.