C++ primer中的一段代码,帮忙分析下为什么要定义两个front函数?
template <class Type> class Queue {
public:
// empty Queue
Queue(): head(0), tail(0) { }
// copy control to manage pointers to QueueItems in the Queue
Queue(const Queue &Q): head(0), tail(0)
{ copy_elems(Q); }
Queue& operator=(const Queue&);
~Queue() { destroy(); }
// return element from head of Queue
// unchecked operation: front on an empty Queue is undefined
Type& front() { return head->item; }
const Type &front() const { return head->item; } void push(const Type &); // add element to back of Queue
void pop (); // remove element from head of Queue
bool empty () const { // true if no elements in the Queue
return head == 0;
}
private:
QueueItem<Type> *head; // pointer to first element in Queue
QueueItem<Type> *tail; // pointer to last element in Queue
// utility functions used by copy constructor, assignment, and destructor
void destroy(); // delete all the elements
void copy_elems(const Queue&); // copy elements from parameter
};
为什么需要定义两个front函数?调用的时候如何区分呢?