Question

I am having difficulty understanding why I am getting this error. I am referring to both Josuttis’ STL book and other resources and it seems that the way I have declared my iterator below should work:

#ifndef LRU_H
#define LRU_H

#include <queue>
#include <iterator>


class LRU
{
public:

   LRU();                           // default constructor
   LRU(int);                        // constructor with argument
   ~LRU();                          // destructor 

   // Methods
   //
   void enqueue(int);               // add datum to the queue
   void dequeue();                  // remove datum from the queue
   void replace();                  // replacement algorithm
   void displayQueue() const;       // display contents of queue

private:

   // Member Data
   //
   const int MAX_SIZE;
   int m_currentCount;

   std::queue<int> m_buffer;
   std::queue<int>::const_iterator iter;

};

#endif

But the line where I declare my const_iterator generates the following compiler error:

In file included from main.cpp:10:
lru.h:41: error: 'const_iterator' in class 'std::queue<int, std::deque<int, std::allocator<int> > >' does not name a type
In file included from lru.cpp:10:
lru.h:41: error: 'const_iterator' in class 'std::queue<int, std::deque<int, std::allocator<int> > >' does not name a type
lru.cpp: In constructor 'LRU::LRU()':
lru.cpp:17: error: class 'LRU' does not have any field named 'm_pos'
lru.cpp: In constructor 'LRU::LRU(int)':
lru.cpp:23: error: class 'LRU' does not have any field named 'm_pos'

Compilation exited abnormally with code 1 at Thu Nov 15 10:47:31

Is there something special about declaring an iterator in a class that results in the error?

Answer 1

The container adapter std::queue has no publicly accessible iterators. Since the std::queue<int> is hidden in the LRU implementation, you could consider using an std::deque<int> instead. std::deque is the default container used by std::queue under the hood, so you would incur no performance penalty by using it. I think it is safe to use it as long as you do not make the non-queue operations public in your LRU interface.