Thread: Storing int and double type values under one variable name

Suppose you could store the value as a string and have different derived versions of Quantity return a double representation vs an int representation.

There are a number of ways of going about this.

Didn't he just mention a union?

Unions are bad. Review your requirements to make sure you really, really, really need to do what you're planning to do. Are you sure that a simple double doesn't suffice?

If you still think you need the construct, use a Boost.Variant.

http://www.boost.org/

When would "quantity" not be an integer? Usually, quantity would be a number of items, yes?

One solution would of course be to use a double at all times, and just make the value "int" when you need to for some mathematical reason.

Other alternatives involve using fixed point math.

But if you (Thanuja91) can explain in a bit more detail what you want the quantity to represent, and why it needs to be double and int.

--
Mats

Unions are the C solution. They work, but they do not provide type safety, so if you store a double, but try to get an int, then you won't get the desired value. This can be resolved by having another variable that stores the type of the value in the union. Then when your accessor tries to read an int it can check to make sure the union really does store an int.

Another solution is to have a base class pointer or reference. This requires a warper object, generally a template, that inherits from a generic object type. The disadvantage of this approach is that is is very verbose. You write a lot of code for something very simple.

The best solution may be to use a pre-written implementation such as Boost::Variant.

So, you would select (at some point) whether the data is a "number of items" or "volume" or "area", right?

This, to me, is a typical case where you would use two classes, something like this:

Code:

class Base 
{
private:
   double cost;    // Per item, volume or area.
public:
   virtual double CostOf();
   virtual const double getCost() const { return cost;};
   virtual void setCost(const double aCost) {cost = aCost; };
};

class Items : public Base
{
private:
   int nItems;
public:
   void SetItems(int aItems): nItems(aItems);
   virtual const double CostOf() const { return cost * nItems; };
};

class Items : public Base
{
private:
   int nItems;
public:
   void SetArea(const double aArea): area(aArea);
   virtual const double CostOf() const { return cost * area; };
};

I'll leave it to you to imlement volume [should you wish to have a different class].

--
Mats