Thread: deleting dynamic object problem

I have 3 different classes inherited from 1 base class.
Objects are instantiated dynamically.

My base class is called Sensor and my 3 inherited classes are called typeA, typeB and typeC

The user can pick which object they want to instntiate

Code:

   if (type == 1) {
      head();
      sensor_a[Sensor::GetSensorCount() + 1] = new typeA;
   } else if (type == 2) {
      head();
      sensor_b[Sensor::GetSensorCount() + 1] = new typeB;
   } else {
      head();
      sensor_c[Sensor::GetSensorCount() + 1] = new typeC;
   }

a running total of objects derrived from the 3 classes is kept in the base class and the objects are given an identity number from this.

If I want to remove an object how can I do this as I only know the object identity number?

e.g.

Code:

   cout << "Enter sensor number to remove from the network: ";
   cin >> num;
   delete sensor[num];
   sensor[num] = NULL;

obviously this won't work as my objects are not called sensor.


I have a function in my base class

Code:

type Sensor::GetSenType() {
   return sensor_type;
}

which refers to an enumarated type

Code:

enum type {type_a = 1, type_b, type_c};

and if I could get to this it would return what type (a, b or c) that particular object was and I could give the delete command the correct type.


Is there a way to do this?

Any comments appricieted.

Thanks

Maybe you ought to make a single sensor array with three elements instead of the three variables sensor_a, sensor_b and sensor_c.

I can't. This assignment is for an OO class, thus the whole reason for the assignment is to implement inheritence, aggregation and association.

No, polymorphism is the next topic. If you think this will help solve my problem I'll jump ahead and do some reading tonight.

Deffinitely meant to be 3 different classes.
Each derrived class has all the attributes of the base class but they each have additional functionality added.

e.g. sensor A measures everything in the base class plus oxygen content, sensor B has the base class plus sea temperature, etc...

I don't think anything prevents you from having only one array. Certainly not what you told us about the assignment.

Code:

Sensor *sensors[MAX_SENSORS];
sensors[0] = new typeA;
sensors[1] = new typeB;
sensors[2] = new typeC;

That's polymorphism btw, so read up on it. Make sure your destructors are marked as virtual.

Can I just ask someone to go a little deeper into what CornedBee has said.
It sounds like the answer I'm looking for, however polymorphism is a large topic and I haven't come accross my method yet.

Thanks.

typeA, typeB and typeC are all inherited from Sensor, therefore, a Sensor * pointer can also point to objects of typeA, typeB and typeC. When you delete everything you can just do:

Code:

delete [] sensors[0];
delete [] sensors[1];
delete [] sensors[2];

Errm, that would be without the [] for the deletes. The array elements are single objects.

This was the way I did it originally, however I came accross a problem when trying to refference data stored in the derrived objects so I assumed you couldn't do it that way.

Here is my code using the method you suggested:

Code:

   type = sensors[num]->GetSenType();
   
   switch (type) {
      case 1 : cout << "Sensor type: A\n"; break;
      case 2 : cout << "Sensor type: B\n"; break;
      case 3 : cout << "Sensor type: C\n"; break;
   }
   cout << "Air temperature: " << sensors[num]->GetAirTemp() << endl;
   cout << "Wind speed: " << sensors[num]->GetWindSpeed() << endl;
   cout << "Wind direction: " << sensors[num]->GetWindDir() << endl;
   if (type == 2)
      cout << "Sunlight intensity: " << sensors[num]->GetSunInt() << endl; //<---problem line
   if (type == 3) {
      cout << "Sea temperature: " << sensors[num]->GetSeaTemp() << endl;  //<---problem line
      cout << "Oxygen content: " << sensors[num]->GetOxCont()) << endl;   //<---problem line
   }
   cout << "Longitude " << sensors[num]->GetLong() << endl;
   cout << "Latitude " << sensors[num]->GetLat() << endl;

When I try to refference the data in the the derrived objects I get an error ' no matching function for call to `Sensor::GetSunInt()' '

How can I access this data?

Thanks

You have to cast it.

Code:

switch (type) {
  case 1: {
    // This doesn't seem to have any special handling
  } break;

  case 2: {
    SunSensor ........ = dynamic_cast<SunSensor *>(sensors[num]);
    cout << ss->GetSunInt();
  } break;

  case 3: {
    // Same here.
  } break;
}

You may have to enable RTTI in the compiler options to make dynamic_cast work.

However, it would probably make more sense to have one virtual function PrintInformation in the Sensor class which each derived class overrides to write their own data, because that's what polymorphism is about.

If you're not using Internet Explorer, take a look at what I've got to say on the topic.
http://stud3.tuwien.ac.at/~e0226430/...l#polymorphism
http://stud3.tuwien.ac.at/~e0226430/...eclasses.xhtml

If you are using IE, it will try to download the files instead of recognizing them for what they are, so get a better browser.

What would you consider to be a better browser?

Mozilla.
You'll never use IE again.

I use it on both Windows and Linux systems.

Just downloaded Mozilla, very nice! Thanks for the tip.

Originally Posted by CornedBee

However, it would probably make more sense to have one virtual function PrintInformation in the Sensor class which each derived class overrides to write their own data, because that's what polymorphism is about.

OK, that bit makes sense to me. I have now put a function in my classes

Code:

   virtual void PrintInformation();

and now in the derrived class implementation I will have the following

Code:

void PrintInformation() {
   cout << "Air temperature: " << sensors[num]->GetAirTemp() << endl;
   cout << "Wind speed: " << sensors[num]->GetWindSpeed() << endl;
   cout << "Wind direction: " << sensors[num]->GetWindDir() << endl;
   cout << "Sea temperature: " << sensors[num]->GetSunInt() << endl; 
   cout << "Longitude " << sensors[num]->GetLong() << endl;
   cout << "Latitude " << sensors[num]->GetLat() << endl;
}

obviously sensors[num] is not known here, so how do you get the function to display what you want?

Thanks