Problem understanding shared_ptr

Question

I have a:

template<class K,class V> struct Node { node_ptr parent_;//node_ptr is a shared_ptr<Node<K,V>> node_ptr& get_parent()const { return parent_; } void set_parent(node_ptr& p) { parent_ = p; } //the get set for left and right are analogical };

I cannot understand why this works:

auto zz = get_parent(get_parent(z)); rb_left_rotate(t,zz);

but this does not:

rb_left_rotate(t,get_parent(get_parent(z)));

by works I mean that inside rb_left_rotate I have:

template<class Tree_T, class Node_T> void rb_left_rotate(Tree_T& t,Node_T& x) { auto y = get_right(x); set_right(x,get_left(y)); if (get_left(y)) { set_parent(get_left(y),x); } auto tmp = get_parent(x); //y's current parrent is x set_parent(y,tmp);//by works I mean that this line WILL NOT set x to empty ...... }

I thought I had my crystal ball at hand, but I don't think it is able to see so far. You are presenting a class that has a member function get_parent that returns a reference to a shared pointer but you are then asking about a different code that uses a free function from which you do not state what is returned (my guess: the free function returns by value) — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Aug 5, 2011 at 18:16

Maxim Egorushkin · Accepted Answer · 2011-08-05 18:16:01Z

rb_left_rotate() accepts Node_T as a reference to non-const. Such a reference can only be bound to an l-value, that is, a non-temporary object. auto zz = get_parent(get_parent(z)); creates such an l-value named zz. In expression rb_left_rotate(t,get_parent(get_parent(z)));, on the other hand, the result of get_parent(z) is an r-value, i.e., a temporary value, which can not be bound to a reference to non-const.

This is not related to the fact that you are using a smart pointer.

The difference being (more or less) l-values are references and things with names, and r-values are things returned from functions (unless by reference).

Daniel Earwicker · Accepted Answer · 2011-08-05 18:15:04Z

What is the relationship between Node and Node_T?

How comes you declare get_parent as a no-parameter member function, but call it by passing a parameter?

The only thing that seems likely to modify x is the line:

set_right(x,get_left(y));

What does set_right do?

In general, you will probably get more predictable behaviour if you pass around shared_ptrs as true values, rather than references to shared_ptr.

Probably, what get_parent(y) does is just call y->get_parent(). It is an overload hook that can be overloaded for different types of y.
Node_T most likely can not be passed by value in that function since it is a node of the tree whose pointers must be reassigned.

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