Re: What do you think of this caching optimization strategy?

"Victor Bazarov" <>
25 Jul 2006 06:30:57 -0400
francis_r wrote:

I would like to know if the caching strategy used in code below is a
good idea. The code explains my question... (It is a simplified
version of what I'm currently working on.)

class X
// Datastructure Action
struct Action {
enum Type {
Type_C // etc...
int mID;
Type mType;
std::string mDescription;

// Collection of actions
std::vector< Action > mActions; // contains many (say 500+) actions

// Extract all ActionIDs for a given ActionType
const std::vector< int >& GetActionIDs( Action::Type inActionType )
// Use caching
static std::map< Action::Type, std::vector< int > > fCache;

// First search cache
std::map< Action::Type, std::vector< int > >::iterator theIt =
fCache.find( inActionType );

// If ActionType not found in cache then iterate all Actions
if( theIt == fCache.end() )
std::vector< int > theActionIDs;
for( int theIndex = 0; theIndex != mActions.size(); ++theIndex )
if( mActions[ theIndex ].mType == inActionType )
theActionIDs.push_back( mActions[ theIndex ].mID );
// Store result in cache
fCache[ inActionType ] = theActionIDs;

This statement doesn't check if 'theActionIDs' is empty and will
create an empty entry in the cache. Is that acceptable?

// Return result from cache ( so we can return a reference)
return fCache[ inActionType ];
// Else immediately return the result from the cache
return theIt->second;

I have recently been writing code like this. Do you think it's a good

It seems incomplete. How do you clear the cache and force it to be
re-collected if the contents of 'mActions' change?

If for some reason 'mActions' is a "grow only" container, your scheme
can work, but it still seems (a) underdesigned and (b) wasteful because
it stores copies of vectors in the cache instead of, say, pointers to
the vectors.

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