IOSToolChain.cs does not include settings needed to enable full C++11 under iOS; so standard features like std::chrono, std::unique_ptr<>, etc raise compilation errors.
See for a proposed fix: Does iOS building support new features of C++ 11? - Mobile - Unreal Engine Forums
Ya, we just updated the iOS Tool Chain this week to enable C++11 for iOS exactly the same as specified in that thread. It will be in the next release.
-Pete
Great! Thanks for the update.