Multi-Language Programming: The Challenge and Promise of Class-Level Interfacing
Many computer applications today involve modules written in different programming languages, and integrating these modules together is a delicate operation. This first requires the availability of formalisms to let programmers denote “foreign” entities like objects and subprograms as well as their associated types. Then, proper translation of what programmers express often calls for significant implementation effort, possibly down to the specification of very precise ABIs (Application Binary Interfaces). Meta-language based approaches a la CORBA/IDL are very powerful in this respect but typically aim at addressing distributed systems issues as well, hence entail support infrastructure that not every target environment needs or can afford. When component distribution over a network is not a concern, straight interfacing at the binary object level is much more efficient. It however relies on numerous low level details and in practice is most often only possible for a limited set of constructs.
Binary level interaction between foreign modules is traditionally achieved through subprogram calls, exchanging simple data types and relying on the target environment’s core ABI. Object Oriented features in modern languages motivate specific additional capabilities in this area, such as class-level interfacing to allow reuse and extension of class hierarchies across languages with minimal constraints. This paper describes work we have conducted in this context, allowing direct binding of Ada extensible tagged types with C++ classes. Motivated by extensions to the Ada typing system made as part of the very recent language standard revision, this work leverages the GCC multi-language infrastructure and implementation of the Itanium C++ ABI. We will first survey the issues and mechanisms related to basic inter-language operations, then present the interfacing challenges posed by modern object oriented features after a brief overview of the Ada, C++, and Java object models. We will continue with a description of our work on Ada/C++ class-level interfacing facilities, illustrated by an example.