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

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.