C ++指针引用混乱

Question

    struct leaf
    {
        int data;
        leaf *l;
        leaf *r;
    };
    struct leaf *p;


void tree::findparent(int n,int &found,leaf *&parent)

This is piece of code of BST. I want to ask. why

 leaf *&parent

Why we need "reference mark" here?

parent is also a leaf, why can't I just use leaf* parent ?

code below for your reference. Thank you!

void tree::findparent(int n,int &found,leaf *&parent)
{
    leaf *q;
    found=NO;
    parent=NULL;

    if(p==NULL)
        return;

    q=p;
    while(q!=NULL)
    {
        if(q->data==n)
        {
            found=YES;
            return;
        }
        if(q->data>n)
        {
            parent=q;
            q=q->l;
        }
        else
        {
            parent=q;
            q=q->r;
        }
    }
}

Answer 1

You are passing the pointer parent in by reference , so that you can modify that pointer:

parent=q;

If you passed the pointer in by value , the modifications would be to a copy of the pointer that expires at the end of the function.

Answer 2

When you use REFERENCE TO POINTER, you can change the value of the pointer. You may need to use this schema in link list implementation to change the head of the list.

void passPointer(int *variable)
{
    *variable = (*variable)*2;
    variable = NULL; // THIS CHANGES THE LOCAL COPY NOT THE ACTUAL POINTER
}
void passPointerReference(int* &variable)
{
    *variable = (*variable)*3;
    variable = NULL; // THIS CHANGES THE ACTUAL POINTER!!!!
}
int main()
{    
    int *pointer;
    pointer = new int;
    *pointer = 5;
    passPointer(pointer);
    cout << *pointer; // PRINTS 10
    passPointerReference(pointer);
    cout << *pointer; // GIVES ERROR BECAUSE VALUE OF pointer IS NOW 0.
    // The constant NULL is actually the number 0.
}