The question refers to the following (partial) header file.

QueueElement *next; // pointer to next element or NULL if tail stda string data; struct Queue f *head; // first element to remove // last element after which you add struct left; *right; std:istring data; struct Tree *head; struct Node f stdistring name; struct Arc Node *start; Node *finish; double distance; class Graph public: double distance) private: stdiset nodes_; stdiset arcs_i