C++: Using typeid before heap allocation

Question

class A
{
protected:
    int a;
public:
    A();
    A(int);
    virtual void print()=0;
    virtual ~A();
};

class B: public A
{
    int b;
public:
    B();
    B(int,int); //initialize attributes a and b 
    void print(); //print a and b
};

class C: public A
{
    float c;
public:
    C();
    C(float,int); //initialize attributes a and c
    void print(); //print a and c
};

class D
{
    int size; //number of objects in v
    A **v; /* an array(a vector) of A pointers that allows me to have both B and C type objects */
public:
    D();
    D(int);
    D(D&);
    ~D();
    D operator=(const D&);
    void PrintAll();
};

---

All methods for D:

D::D()
{
    v=NULL;
}
D::D(int x)
{
    size=x;
    v=new A*[x];
    for(int i=0;i<x;i++)
    {
        if(i%2==0)
            v[i]=new B(4,7);
        else
            v[i]=new C(3,5);
    }
}

D::D(D& other)
{
    size=other.size;
    v=new A*[size];
    for(int i=0;i<size;i++)
    {
        if(i%2==0)
        {
            v[i]=new B();
            *v[i]=other.v[i][0];
        }
        else
        {
            v[i]=new C();
            *v[i]=other.v[i][0];
        }
    }
}

D::~D()
{
    if(v!=NULL)
    {
        for(int i=0;i<size;i++)
        {
            delete v[i];
        }
        delete[] v;
    }
}
D D::operator=(const D& other)
{
    if(v!=NULL)
    {
        for(int i=0;i<size;i++)
        {
            delete v[i];
        }
        delete[] v;
    }
    size=other.size;
    v=new A*[size];
    for(int i=0;i<size;i++)
    {
        if(i%2==0)
        {
            v[i]=new B();
            *v[i]=other.v[i][0];
        }
        else
        {
            v[i]=new C();
            *v[i]=other.v[i][0];
        }
    }
    return *this;
}
void D::PrintAll()
{
    cout<<"Printall():"<<endl;
    for(int i=0;i<size;i++)
        v[i]->print();
}

As you can see, the D class constructor makes the objects of type B or C as i is odd or even. If I know this, then I know how to write the operator= and the copy constructor for D. But If the class D constructor would make the objects of type B or C randomly, then how can I write the copy constructor (and operator=) for class D? My guess is I have to use typeid operator to solve this problem.

Answer 1

Define pure virtual method clone as part of your interface A definition - clone shall return a copy of the object. Override and implement it in each of your B and C classes. In D class copy constructor and assignment operator implementations use A's interface to create the required class instance instead of explicit call to new: v[i] = other.v[i]->clone();. No need for RTTI, normal polymorphism will do.