memory allocation and inherited classes in C++

Question

Say I have these structs:

struct Base{
 ...
}

struct Derived:public Base{
 //everything Base contains and some more
}

I have a function in which I want to duplicate an array of these and then alter it.

void doStuff(Base *data, unsigned int numItems){
 Base *newdata = new Base[numItems];
 memcpy(newdata, data, numItems*sizeof(Base));
 ...
 delete [] newdata;
}

But if I used this function like so:

Base *data = new Derived[100];
doStuff(data, 100);

It wouldn't work, would it? Because Derived1 is larger than Base, so allocating for Base is not enough memory?

Answer 1

You could do this easily with a template:

template< class T >void doStuff(T *data, unsigned int numItems)
{
    T *newdata = new T[numItems];
    memcpy( newdata, data, sizeof( T ) * numItems );
    ...
    delete [] newdata;
}

Edit as per the comments: If you wanted to do this for a mixed collection things will get more complicated quickly ... one possible solution is this:

struct Base{
    virtual Base* CopyTo()      { return new Base( *this ); }
};

struct Derived:public Base{
    virtual Derived* CopyTo()   { return new Derived( *this ); }

};

void doStuff( Base** ppArray, int numItems )
{
    Base** ppNewArray   = new Base*[numItems];
    int count = 0;
    while( count < numItems )
    {
        ppNewArray[count] = ppArray[count]->CopyTo();
        count++;
    }

    // do stuff

    count = 0;
    while( count < numItems )
    {
        delete ppNewArray[count];
        count++;
    }
    delete[] ppNewArray;
}