The array you are initialising in your constructor Doctor::Doctor()
is a local variable called 'patientArray', not the class variable that you are then deleting in the destructor.
To fix the problem, change the constructor to this:
Doctor::Doctor() : doctorName("need a name."), patientArraySize(100), numOfPatient(0) {
// Create a dynamic array for patients below:
// stringPtr_t* pArray;
// Delete local variable declaration that was here: stringPtr_t* patientArray;
//
patientArray = new string[patientArraySize];