How to allocate array in base constructor with size based on derived class?

Question

I have a hierarchy of classes. The base class uses some tuning parameters that are loadable from file (and reloadable during runtime). Each derived class may add some additional parameters. I am looking for a way to allocate a correctly sized parameters array in the base constructor, so that I don't have to deallocate and reallocate in the derived class. I was hoping for something like this, but it's not working (parameters always has 2 elements):

   class Base
   { static int nParms;
     virtual int getNParms() { return nParms;}
     float *parameters;
   public:
     Base() 
     { parameters= new float[this->getNParms()];
       parameters[0] = globalReloadableX;
       parameters[1] = globalReloadableY;
     }
   };
   int Base::nParams =2;

   class Derived : public Base
   { static int nParms;
     virtual int getNParms() { return nParms;}
   public:
     Derived() : Base()
     { parameters[2] = globalReloadableZ;
     }
   }
   int Derived::nParams =3;

I've seen this question, but the solution there doesn't quite work for me. I also tried making parameters a regular array in each class:

  class Base
  {  float parameters[2]
    ...
  class Derived : public Base
  {  float parameters[3]
    ...

but that makes Derived have 2 separate arrays.

Any ideas?

Answer 1

Why not pass the required array size as a parameter in the constructor of the base class?

(The reason the virtual function doesn't call the derived class is because that is how C++ virtual functions work; conceptually, until the derived class constructor completes, the object's type is still the base class.)