问2-3树和如何构建构造函数

EN

template<class E>
class BTree
{
public:
    BTree();
    ~BTree();

    class BNode
    {
    public:
        // I'm not sure if this is quite enough for the constructor
        BNode() {_right == nullptr;};

        typedef struct Entry
        {
            E value;
            BNode<E>* _left;
            // Constructor for the entry
            Entry();
        };

        E fValue() {return _first.value;}
        E sValue() {return _second.value;}
        BNode<E>* fLeft() {return _first._left;}
        BNode<E>* sLeft() {return _second._left;}

        //check if the node is 2 or 3 pointers
        bool IsThree() 
        {
            if(_first._left != nullptr && _second._left != nullptr
                && _second._right != nullptr)
                _three = true;
            else
                _three = false;
            return _three;
        }
        Entry fEntry() {return _first;}
        Entry sEntry() {return _second;}
        BNode<E>* right() {return _right;}

    private:
        bool _three;
        Entry _first, _second;
        BNode<E>* _right;
    };
private:
    BNode* root_;
};
template<class E>
BTree<E>::BNode<E>::Entry::Entry()
{
    _left = nullptr;
}