Relaxing the one definition rule in interpreted C++

Abstract

Most implementations of the C++ programming language generate binary executable code. However, interpreted execution of C++ sources has its own use cases as the Cling interpreter from CERN's ROOT project has shown. Some limitations are derived from the ODR (One Definition Rule) that rules out multiple definitions of entities within a single translation unit (TU). ODR is there to ensure uniform view of a given C++ entity across translation units. Ensuring uniform view of C++ entities helps when producing ABI compatible binaries. Interpreting C++ presumes a single evergrowing translation unit that define away some of the ODR use-cases. Therefore, it may well be desirable to relax the ODR and, consequently, to support the ability of developers to override any existing definition for a given declaration. This approach is especially well-suited for iterative prototyping. In this paper, we extend Cling, a Clang/LLVM-based C++ interpreter, to enable redefinitions of C++ entities at the prompt. To achieve this, top-level declarations are nested into inline namespaces and the translation unit lookup table is adjusted to invalidate previous definitions that would otherwise result in ambiguities. Formally, this technique refactors the code to an equivalent that does not violate the ODR, as each definition is nested in a different namespace. Furthermore, any previous definition that has been shadowed is still accessible by means of its fully-qualified name. A prototype implementation of the presented technique has been integrated into the Cling C++ interpreter, showing that our technique is feasible and usable.