C++ circular reference problem

Question

I have 2 classes: DataObject and DataElement. DataObject holds pointers to (only) DataElements, and a DataElement

Answer 1

Make the destructor for the first class defined outside of the class body and after the second class, e.g.

class DataElement;

class DataObject
{
    DataElement* elem;
public:
    ~DataObject();
};

class DataElement
{
    DataObject* obj;
public:
    ~DataElement() { delete obj; }
};

DataObject::~DataObject()
{
    delete elem;
}

Answer 2

Define the destructor in a .cpp file that includes both headers.