How can I initialize objects from base class array, containing different derived classes, using a for loop?

Question

I'm truly amazed by the idea behind the OOP, since I find it so much easier to think of the classes and the objects, as if they are real ones. I have a base class Tooth and derived classes for the different groups of teeth (they have their specifics). Their constructors are taking as argument the texture, which is drawn; For upper teeth it goes something like this:

class Tooth 
{ 
   public:
   Tooth(Texture &texture);
};

class Molar : public Tooth { };
class Premolar : public Tooth { };
class Frontal : public Tooth { };

main()
{
   Molar *molar[6];
   Premolar *premolar[4];
   Frontal *frontal[6];

   Texture uppertextures[8];   //only 8 textures, since the other half of the teeth are mirrored

   Tooth* upperTeeth[16] = { molar[0], molar[1], molar[2], premolar[0], premolar[1], frontal[0], frontal[1], frontal[2], frontal[3], frontal[4], frontal[5], premolar[2], premolar[3], molar[3], molar[4], molar[5] }; // < the order in which the teeth have to be added to the graphic scene;

   int x = 0     //for the position of the tooth on the screen;
   int texture = 0;    //we will need it to assign the textures in order;

   for(int i = 0; i<16; i++)
   {
       upperTeeth[16] = new !?!?!?(uppertextures[i])
       upperTeeth[16]->setPositionX(i+x) // some function for setting the position on the screen from left to right;
       
       if(i<8) texture++;     //setting the textures for the teeth on the left;
       if(i>8) texture--;    //setting the textures for the teeth on the right(the same, but in reverse order);
       
       upperTeeth[16]->setPositionY(y)
}


}

As you can see the problem is with the allocation. I want to create new[] the same way I create array of the Base class, and fill in the order of the derived classes, in which the objects appear. Of course if you have completely different solution, you can share it, but that's the most clever way I could think of.

Answer 1

The reason a lot of people are going to tell you to "get a textbook" is because there's a lot of machinery going on in C++. But don't worry, there's plenty of learning you can do without one. Practice is probably the best way to learn.

... There's a few things to consider finding a solution to your problem.

First, there's the constructors you've set up. You've got child classes that inherit from a class that has a constructor with arguments. That's not good, it basically means you can't create your child classes. You'll just get errors if you try. So I've changed all the constructors to "default" so that it actually runs, but you should make child constructors that build the base class too, like

Molar(Texture& texture) 
    : Tooth(texture) 
{
    // ... do whatever here
}

Then there's ownership. Which class "owns" the tooth data? You've got your arrays, but they are arrays of pointers so they don't actually own the data. So you can own the data somewhere else, to make sure it doesn't get trash-collected outside of your function context. Or just make it so the arrays actually contain the teeth classes. I've done that in my solution because it's simpler. But you can see upperTeeth is still a pointer array because you likely don't want to own two copies of the objects.

Molar molar[6];
Premolar premolar[4];
Frontal frontal[12];

Texture uppertextures[8];   // Only 8 textures, since the other half of the teeth are mirrored

// The order in which the teeth have to be added to the graphic scene
Tooth *upperTeeth[16] = { &molar[0], &molar[1], &molar[2], &premolar[0], &premolar[1], &frontal[0], &frontal[1]
                          , &frontal[2], &frontal[3], &frontal[4], &frontal[5], &premolar[2], &premolar[3], &molar[3]
                          , &molar[4], &molar[5] };

In this example the arrays would be destroyed upon leaving the context. With your use of a pointer array and the new constructor you can keep the array in use after the method finishes, but keep track of it and delete it when you no longer need it, aka in the destructor. The simpler way is to define the same "owner arrays" as in my example, in the Header of the main class.

And finally for the logic bit at the bottom, this is what I replaced your code with:

   int x = 0;     // For the position of the tooth on the screen;
   int y = 0;     // Why not a y variable as well?
   int texture = 0;    // We will need it to assign the textures in order;

   for(int i = 0; i<16; i++) {
       upperTeeth[i]->setPositionX(i + x); // Some function for setting the position on the screen from left to right;

       if (i < 8) texture++;     // Setting the textures for the teeth on the left;
       if (i > 8) texture--;    // Setting the textures for the teeth on the right(the same, but in reverse order);

       upperTeeth[i]->setTexture(texture); // alternative to your constructor

       upperTeeth[i]->setPositionY(y);
   }

Watch that you missed using the [i] instead of [16]. Alternatively, you can directly loop through your array using

 int i = 0;
 for (auto tooth : upperTeeth) {
      tooth->setPositionX(i); // you still need i for your case.
      i++;
 }

Hope I managed to clear up some things!

Answer 2

I finally did what I wanted to. The problem was purely logical. And to all of you, who say I shouldn't subclass - I already made a version of the program, in which there are no derived classes of tooth, and at some point I felt it way too limiting. Now I'm posting the code that worked:

    class Tooth 
    { 
       public:
       Tooth(Texture &texture);
    };
    
    class Molar : public Tooth { };
    class Premolar : public Tooth { };
    class Frontal : public Tooth { };
    
    main()
    {
       Molar *molar[6];
       Premolar *premolar[4];
       Frontal *frontal[6];
    
           Texture texture[8];   //only 8 textures, since the other half of the teeth are mirrored
         
           int toothindex[16] = { 0, 1, 2, 0, 1, 0, 1, 2, 3, 4, 5, 2, 3, 3, 4, 5 };
        
           int textureindex = 0;    //we will need it to assign the textures in order;
        
           for(int i = 0; i<16; i++)
           {
               if(i < 3 || i>12) 
               molar[toothindex[i]] = new Molar(texture[index]);
            
               if(i==3; i==4; i==11; i==12)
               premolar[toothindex[i]] = new Premolar(texture[textureindex]);
    
              if(i>4 && i<12) frontal[toothindex[i]] = new Frontal(texture[textureindex])
    
           if(i<7) texture++; 
           if(i>7) texture--;  
          }
   }  
       

Thanks for telling me about the so called undefined behavior and proposing the if statement. All I had to do was to create an array of ints, and use it to initialize the specific classes in the for loop. Now it looks something like this: https://i.stack.imgur.com/0yZv5.jpg

Oh... and since I'm using array of POINTERS, for anyone who seeks the answer, don't forget to make some kind of for loop to delete the objects, when you dont need them:

for (int i = 0; i<6; i++)
{
    if(i<4) delete premolar[i];
    delete molar[i];
    delete frontal[i];
}