Nullable values in C++

Question

I'm creating a database access layer in native C++, and I'm looking at ways to support NULL values. Here is what I have so far:

class CNullValue
{
public:
    static CNullValue Null()
    {
        static CNullValue nv;

        return nv;
    }
};

template<class T>
class CNullableT
{
public:
    CNullableT(CNullValue &v) : m_Value(T()), m_IsNull(true)
    {
    }

    CNullableT(T value) : m_Value(value), m_IsNull(false)
    {
    }

    bool IsNull()
    {
        return m_IsNull;
    }

    T GetValue()
    {
        return m_Value;
    }

private:
    T m_Value;
    bool m_IsNull;
};

This is how I'll have to define functions:

void StoredProc(int i, CNullableT<int> j)
{
    ...connect to database
    ...if j.IsNull pass null to database etc
}

And I call it like this:

sp.StoredProc(1, 2);

or

sp.StoredProc(3, CNullValue::Null());

I was just wondering if there was a better way than this. In particular I don't like the singleton-like object of CNullValue with the statics. I'd prefer to just do

sp.StoredProc(3, CNullValue);

or something similar. How do others solve this problem?

Answer 1

Replace IsNull with HasValue and you've got the .NET Nullable type.

Of course.. this is C++. Why not just use a pointer to a "primitive" type?

Answer 2

Boost.Optional probably does what you need.

boost::none takes the place of your CNullValue::Null(). Since it's a value rather than a member function call, you can do using boost::none; if you like, for brevity. It has a conversion to bool instead of IsNull, and operator* instead of GetValue, so you'd do:

void writeToDB(boost::optional<int> optional_int) {
    if (optional_int) {
        pass *optional_int to database;
    } else {
        pass null to database;
    }
}

But what you've come up with is essentially the same design, I think.