Question

I have some C++ code where I need to implement cache replacement using LRU technique.
So far I know two methods to implement LRU cache replacement:

1. Using timeStamp for each time the cached data is accessed and finally comparing the timeStamps at time of replacement.
2. Using a stack of cached items and moving them to the top if they are accessed recently, so finally the bottom will contain the LRU Candidate.

So, which of these is better to be used in production code?
Are their any other better methods?

Answer 1

Recently I implemented a LRU cache using a linked list spread over a hash map.

    /// Typedef for URL/Entry pair
    typedef std::pair< std::string, Entry > EntryPair;

    /// Typedef for Cache list
    typedef std::list< EntryPair > CacheList;

    /// Typedef for URL-indexed map into the CacheList
    typedef boost::unordered_map< std::string, CacheList::iterator > CacheMap;

    /// Cache LRU list
    CacheList mCacheList;

    /// Cache map into the list
    CacheMap mCacheMap;

It has the advantage of being O(1) for all important operations.

The insertion algorithm:

// create new entry
Entry iEntry( ... );

// push it to the front;
mCacheList.push_front( std::make_pair( aURL, iEntry ) );

// add it to the cache map
mCacheMap[ aURL ] = mCacheList.begin();

// increase count of entries
mEntries++;

// check if it's time to remove the last element
if ( mEntries > mMaxEntries )
{
    // erease from the map the last cache list element
    mCacheMap.erase( mCacheList.back().first );

    // erase it from the list
    mCacheList.pop_back();

    // decrease count
    mEntries--;
}