Re: how to structure a class that may hold two kind of values

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 08 Feb 2008 07:49:04 -0500

Message-ID:

<daniel_t-1B1B09.07490408022008@earthlink.vsrv-sjc.supernews.net>

"fabricio.olivetti@gmail.com" <fabricio.olivetti@gmail.com> wrote:

I am designing a class to read a data file and provide access to
another class, but as this dataset may contain either double or int
values, and some of them may be very large I'd like to create a class
that can decide upon allocating a vector of "char" (for the integral
values may be enough) or a vector of double.
But I can't see how can I do that...using templates I still have to
determine, prior the class declaration, which type this class will
hold.

Of course I could declare something like this:

class foo{

     private:
          vector< char > cData;
          vector< double > dData;
          bool type;
     public:
          double getData(unsigned i);
};

and always return a double (casting the char when required) and using
just the required data type using a flag to determine which type is
used by the class.
How would be the most elegant and optimized way of doing that?

If all the elements of the dataset can be the same type, (once you
determine which type it can be) then:

class foo {
public:
   virtual ~foo() { }
   virtual double getData(unsigned i) = 0;
};

class CharFoo {
   vector< char > data;
public:
   virtual double getData(unsigned i);
};

class DoubleFoo {
   vector< char > data;
public:
   virtual double getData(unsigned i);
};

If all of the elements can't be the same type (for example if you can
only represent some of the doubles as chars in a particular dataset,)
then store them all as doubles because the amount of memory you would
need to keep track of which ones were chars and which were doubles would
overwhelm the memory you would be saving by compressing the storage to
begin with.

Also, if your datasets are so big, maybe you should use deque instead of
vector.