Using an attribute of an object as a default parameter in a function inside a class

Question

I'm trying to write the implement of BST in OOP style.I got some bugs while writting insertNode function like

error: invalid use of non-static data member ‘BST::root’

.How can i fix it?

struct node{
    int data;
    node* left;
    node* right;
    node(){}
    node(int data){
        this->data=data;
        this->left=NULL;
        this->right=NULL;
    }
};
struct BST{
    node* root;
public:
    BST(){root=NULL;}
    node*& getRoot(){return root;}
    void insert(int data,node*& t=this->root){
        if(!t){
            t=new node(data);
            return;
        }
        if(t->data<data) insert(data,t->left);
        else insert(data,t->right);
    }
};