Memory Allocation

[Toot]

First, I'd use initialisations instead of code body, e.g.

ArrayD::ArrayD()
: size(0),
  data(NULL)
{
}
// etc

Although you won't notice it on consturctors of this size, it's good practice and for complex classes you might notice a performance gain.

Also, I think you need to check that the array has been allocated in the destuctor before delete'ing it:

ArrayD::~ArrayD()
{
    if (data)
        delete [] data;
}

Lastly, you don't need to specifiy the scope of your variables inside the class's body - so constructor 3 could become

ArrayD::ArrayD(int nSize, const double* pData)
:   size(nSize),
    data(new double[nSize])
{
    // constructor 3:
    if (data)
    {
        for (int i = 0; i < nSize; i++)	
            data[i] = pData[i];
    }
}

Toot