Is there a way to tell what class an object is?

**Loduwijk** · 03-23-2006

Is there a way to tell what type of class an object is?

I want to tag objects of a few types with an id integer, and I want the classes to have seperate id index counters. The problem I'm running into is with the base class, as it will increment the id index even though the object is not of the appropriate class.

Code:

#define OBJECT1 0
#define OBJECT2 1
#define OBJECT3 2

unsigned short int object1_count = 0, object2_count = 0, object3_count = 0;

class object1
{
  public:
  unsigned short int type, id;
  object1(void)
  {
    type = OBJECT1; id = object1_count++;
  }
};
class object2: public object1
{
  public:
  object2(void)
  {
    type = OBJECT2; id = object2_count++;
  }
};
class object3: public object1
{
  public:
  object3(void)
  {
    type = OBJECT3; id = object3_count++;
  }
};

As you can see, object1_count will be incremented even when it's not used. I want object1_count to be incremented only when the object is the base class, not the derived class.

I know I could use the constructor overloading thing I learned in my last thread to accomplish this, but since it's a test project and the whole purpose is to learn new things, I thought I'd look into this to see if I could do it another way.

If I have a way to say "if this object is of class object1", such as...

Code:

class object1
{
  public:
  object1(void)
  {
    if(this->class == object1){id = object1_count++;}
  }
};

Obviously, I don't think I can do it like that, but it was just an example to show what type of ability I'm after.

**hk_mp5kpdw** · 03-23-2006

Well, you can decrement the object1_counter variable in each of the derived class's constructors:

Code:

class object2: public object1
{
  public:
  object2(void)
  {
    type = OBJECT2; id = object2_count++;
    --object1_count;
  }
};

class object3: public object1
{
  public:
  object3(void)
  {
    type = OBJECT3; id = object3_count++;
    --object1_count;
  }
};

**ChaosEngine** · 03-23-2006

this is actually difficult enough to do when you take inheritance, copy constructors, etc into account. A simple way is to use a template like this

Code:

// A base class that provides counting
template<class T> class Counted {
  static int count;
public:
  Counted() {
    ++count;
  }
  Counted(const Counted<T>&) {
    ++count;
  }
  ~Counted() {
    --count;
  }
  static int getCount() {
    return count;
  }
};

template<class T> int Counted<T>::count = 0;

// Curious class definitions
class CountedClass : public Counted<CountedClass> {};
class CountedClass2 : public Counted<CountedClass2> {};

but to answer your original question

Code:

class object1
{
  public:
  object1(void)
  {
    if(this->class == object1){id = object1_count++;}
  }
};

you can actually do this (well not that syntax, but that effect)
look up RTTI, dynamic_cast and the typeid operator.

Personnally I dislike RTTI. It's usually a symptom of bad design and there's almost always a better solution.

**Loduwijk** · 03-23-2006

Took some time to find something that explained it well enough, but I did. I doubt I'll be able to remember that though, so I bookmarked it the explanation I found.

Thanks much.

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