C++ class issue and identity

disruptive · 08-17-2010, 05:50 AM

Hi there

I am attempting to modify some code for my own purposes but am finding things tricky as the code is not so well documented with a lot a custom types. As I put things below you should hopefully build a better picture of what I am trying to do.

I have what looks like the following set up.

1. Create a existence of a class based on what appears in a text file. Call this class, class SpecialistType. There are a number of classes that can be instantiated here. Now class SpecialistType inherits class BType. i.e. we have the following sort of definition:

Code:

 class SpecialistType : public BType {

2. We also have a B class which is instantiated later on in the program. One of the variables passed to it on start-up what looks like a pointer of BType.

3. Later on the methods in B class are executed.

For example:

Code:

class B {
  public:
    B(.., .., ..., BType* bt)

Now, because each class does something but is related, I want to put some logic in the code that decides what to do depending on the earlier instantized SpecialistType class.

However I now want to figure out from bt, which type of SpecialistType we have already instantiated. I just don't know if there is anyway to do this, given that we cannot read from the text file anymore.

No I could prob do this by putting some extra fields in some of these classes - but I'd like to make minimum changes to the code. This might seem a silly question, but is there any way I can get a handle on what class was instantiated? Or is it an extra field in BType?

Thanks in advance.

dwhitney67 · 08-17-2010, 08:54 AM

Maybe RTTI can help you.

http://en.wikipedia.org/wiki/Run-time_type_information

P.S. Here's an example using typeid()

Code:

#include <iostream>
#include <typeinfo>

class Base
{
public:
   Base() {}
   virtual ~Base() {}
};

class DerivedA : public Base
{
public:
   DerivedA() {}
   virtual ~DerivedA() {}
};

class DerivedB : public Base
{
public:
   DerivedB() {}
   virtual ~DerivedB() {}
};


void whatIsIt(const Base* base)
{
   if (typeid(*base) == typeid(Base))
   {
      std::cout << "Base object." << std::endl;
   }
   else if (typeid(*base) == typeid(DerivedA))
   {
      std::cout << "DerivedA object." << std::endl;
   }
   else if (typeid(*base) == typeid(DerivedB))
   {
      std::cout << "DerivedB object." << std::endl;
   }
   else
   {
      std::cout << "Unknown class object." << std::endl;
   }
}

int main()
{
   Base* b  = new Base;
   Base* ba = new DerivedA;
   Base* bb = new DerivedB;

   whatIsIt(b);
   whatIsIt(ba);
   whatIsIt(bb);
}