Error in deleting treap

Question

i have a procudere deletes for my treap, and on the line p=merge(l, merge(m, rs)); i have error error: non-const lvalue reference to type 'nodeptr' (aka 'node *') cannot bind to a temporary of type 'nodeptr' So here is implementation of deletes and merge

nodeptr treap::merge(nodeptr &l, nodeptr &r){
    nodeptr result;
    if (!l){
        result=r;
    }
    else if(!r){
        result=l;
    }
    else if(l->cnt > r->cnt){
        l->right=merge(l->right, r);
        result=l;
    }
    else{
        r->left=merge(l, r->left);
        result=r;
    }
    return result;
}

void treap::deletes(nodeptr &p, int x){
    nodeptr l, r, m, rs;
    split(p, x-1, l, r);
    split(r, x, m, rs);
    if (m){
        if (!m->left){
            m=m->right;
        }
        else{
            m=m->left;
        }
    }
    p=merge(l, merge(m, rs));
}

And how i implement treap as a data structure.

typedef struct node *nodeptr;
struct node{
    int x;
    long y;
    node* left ;
    node* right ;
    int cnt;
    node(int key=0, long prior=0):  x(key), y(prior), left(NULL), right(NULL), cnt(0)  {}
};

class treap{
public:
    int cnt( nodeptr &p);
    bool find(nodeptr &p, int x);
    void update_cnt(nodeptr &p);
    void split(nodeptr &p, int x, nodeptr &l, nodeptr &r);
    void insert(nodeptr &p, nodeptr &q);
    nodeptr merge(nodeptr &l, nodeptr &r);
    void deletes(nodeptr &p, int x);
};

Can you say me, why is there this error? I think that all is fine. Sorry, if the question is nooby. Thanks in advance.

Answer 1

You have hit the annoyance of references over pointers. When you say p=merge(...) the compiler is trying to assign the temporary nodeptr returned by merge to the reference. But then the temp is out of scope and p no longer holds a valid reference. Sadly, sometimes you need to use pointers.