Q: Iphone auto clear cache of apps?

No, you have to clear them manually. Double-click the Home button then tap and hold the app in the menu that appears until it starts to wiggle. Tap the red minus sign that appears

Read here > http://support.apple.com/kb/HT4211

I guess chengchow88 was referring to memory size for "Documents & Data" in the apps.

Settings -> General -> Usage

Similar issue : https://discussions.apple.com/message/18920427#18920427

Firstly I'd like to make it clear that I am not recommending the following, as improper use could corrupt an app and/or iPhone.

However, using iExplorer from a desktop computer allows a user to browse to an app's cache and delete its contents if required

At this point, for months I have been contacting app developers and Apple requesting the ability to clear an app's cache. I have only 1 app that I use that supports this feature - WallpapersHD.

All apps should support cache clearing. Especially apps that bloat like: Netflix, YouTube, Amazon Instant Video, TUAW, iMore, Twitter, TWC Max, Weather [Yahoo], Trailers [Apple], Bible [YouVersion]...  Practically any and every app I have ever used that communicates with a remote server out there in the world somewhere.

iOS needs a cache clearing option to assist in managing space because deleting and re-installing apps is a terrible way to get that space back.