Cling: The new interactive interpreter for ROOT 6

Cling is an interactive C++ interpreter, built on top of Clang and LLVM compiler infrastructure. Like its predecessor Cint, Cling realizes the read-print-evaluate-loop concept, in order to leverage rapid application development. Implemented as a small extension to LLVM and Clang, the interpreter reuses their strengths such as the praised concise and expressive compiler diagnostics. We show how to match the interpreter concept to the compiler library and generalize common set of requirements for building up an interactive interpreter. We reason the design and implementation decisions as solution to the challenge of implementing interpreter behaviour as an extension of the compiler library. We present the new features, e.g. how C++11 will come to Cling and how Cint-specific extensions are being adopted. We clarify the state of integration in the ROOT framework and the induced change set. We explain how ROOT dictionaries are simplified due to the new interpreter.