Cling - The LLVM-based C++ Interpreter

<!--HTML-->Cling (http://cern.ch/cling) is a C++ interpreter, built on top of clang (http://clang.llvm.org) and LLVM (http://llvm.org). Like its predecessor CINT, cling offers an interactive, terminal-like prompt. It enables exploratory programming with rapid edit / run cycles. The ROOT team has more than 15 years of experience with C++ interpreters, and this has been fully exploited in the design of cling. However, matching the concepts of an interpreter to a compiler library is a non-trivial task; we will explain how this is done for cling, and how we managed to implement cling as a small (10,000 lines of code) extension to the clang and llvm libraries. The resulting features clearly show the advantages of basing an interpreter on a compiler. Cling uses clang's praised concise and easy to understand diagnostics. Building an interpreter on top of a compiler library makes the transition between interpreted and compiled code much easier and smoother. We will present the design, e.g. how cling treats the C++ extensions that used to be available in CINT. We will also present the new features, e.g. how C++11 will come to cling, and how dictionaries will be simplified due to cling. We describe the state of cling's integration in the ROOT Framework.