Bjarne Stroustrup

Concepts: linguistic support for generic programming in C++

Generic programming has emerged as an important technique for the development of highly reusable and efficient software libraries. In C++, generic programming is enabled by the flexibility of templates, the C++ type parametrization mechanism. However, the power of templates comes with a price: generic (template) libraries can be more difficult to use and develop than non-template libraries and their misuse results in notoriously confusing error messages. As currently defined in C++98, templates are unconstrained, and type-checking of templates is performed late in the compilation process, i.e., after the use of a template has been combined with its definition. To improve the support for generic programming in C++, we introduce concepts to express the syntactic and semantic behavior of types and to constrain the type parameters in a C++ template. Using concepts, type-checking of template definitions is separated from their uses, thereby making templates easier to use and easier to compile. These improvements are achieved without limiting the flexibility of templates or decreasing their performance - in fact their expressive power is increased. This paper describes the language extensions supporting concepts, their use in the expression of the C++ Standard Template Library, and their implementation in the ConceptGCC compiler. Concepts are candidates for inclusion in the upcoming revision of the ISO C++ standard, C++0x.

Specifying C++ concepts

C++ templates are key to the design of current successful mainstream libraries and systems. They are the basis of programming techniques in diverse areas ranging from conventional general-purpose programming to software for safety-critical embedded systems. Current work on improving templates focuses on the notion of concepts (a type system for templates), which promises significantly improved error diagnostics and increased expressive power such as concept-based overloading and function template partial specialization. This paper presents C++ templates with an emphasis on problems related to separate compilation. We consider the problem of how to express concepts in a precise way that is simple enough to be usable by ordinary programmers. In doing so, we expose a few weakness of the current specification of the C++ standard library and suggest a far more precise and complete specification. We also present a systematic way of translating our proposed concept definitions, based on use-patterns rather than function signatures, into constraint sets that can serve as convenient basis for concept checking in a compiler.

A history of C++: 1979–1991

This paper outlines the history of the C++ programming language. The emphasis is on the ideas, constraints, and people that shaped the language, rather than the minuitiae of language features. Key design decisions relating to language features are discussed, but the focus is on the overall design goals and practical constraints. The evolution of C++ is traced from C with Classes to the current ANSI and ISO standards work and the explosion of use, interest, commercial activity, compilers, tools, environments, and libraries.

Evolving a language in and for the real world: C++ 1991-2006

This paper outlines the history of the C++ programming language from the early days of its ISO standardization (1991), through the 1998 ISO standard, to the later stages of the C++0x revision of that standard (2006). The emphasis is on the ideals, constraints, programming techniques, and people that shaped the language, rather than the minutiae of language features. Among the major themes are the emergence of generic programming and the STL (the C++ standard librarys algorithms and containers). Specific topics include separate compilation of templates, exception handling, and support for embedded systems programming. During most of the period covered here, C++ was a mature language with millions of users. Consequently, this paper discusses various uses of C++ and the technical and commercial pressures that provided the background for its continuing evolution.

Lock-free dynamically resizable arrays

We present a first lock-free design and implementation of a dynamically resizable array (vector). The most extensively used container in the C++ Standard Template Library (STL) is vector, offering a combination of dynamic memory management and constant-time random access. Our approach is based on a single 32-bit word atomic compare-and-swap (CAS) instruction. It provides a linearizable and highly parallelizable STL-like interface, lock-free memory allocation and management, and fast execution. Our current implementation is designed to be most efficient on multi-core architectures. Experiments on a dual-core Intel processor with shared L2 cache indicate that our lock-free vector outperforms its lock-based STL counterpart and the latest concurrent vector implementation provided by Intel by a large factor. The performance evaluation on a quad dual-core AMD system with non-shared L2 cache demonstrated timing results comparable to the best available lock-based techniques. The presented design implements the most common STL vectors interfaces, namely random access read and write, tail insertion and deletion, pre-allocation of memory, and query of the containers size. Using the current implementation, a user has to avoid one particular ABA problem.

Classes: an abstract data type facility for the C language

Language constructs for definition and use of abstract data types ease the design and maintenance of large programs. This paper describes the C class concept, an extension to the C language providing such constructs. A class is defined using standard C data types and functions, and it can itself be used as a building block for new classes. A class provides a way of restricting access to a data structure to a specific set of functions associated with it, without incurring significant overheads at compile time or at run time.The C class concept is introduced by small examples of its use, and familiarity with the C language [2] is assumed. Appendix A is a complete small C program using classes.Classes have been in use for more than a year on a dozen PDP11 and VAX UNIX systems [1], and they are currently used for a diverse set of projects on more than 30 systems. Classes are currently implemented by an intermediate pass of the cc compiler, called the class pre-processor, which is invoked when the directive #class is found in a C source file. The class pre-processor is easily ported to a system with a version of the portable C compiler. A Motorola68000 version is in use.

Open multi-methods for c++

Multiple dispatch - the selection of a function to be invoked based on the dynamic type of two or more arguments - is a solution to several classical problems in object-oriented programming. Open multi-methods generalize multiple dispatch towards open-class extensions, which improve separation of concerns and provisions for retroactive design. We present the rationale, design, implementation, and performance of a language feature, called open multi-methods, for C++. Our open multi-methods support both repeated and virtual inheritance. Our call resolution rules generalize both virtual function dispatch and overload resolution semantics. After using all information from argument types, these rules can resolve further ambiguities by using covariant return types. Great care was taken to integrate open multi-methods with existing C++ language features and rules. We describe a model implementation and compare its performance and space requirements to existing open multi-method extensions and workaround techniques for C++. Compared to these techniques, our approach is simpler to use, catches more user mistakes, and resolves more ambiguities through link-time analysis, runs significantly faster, and requires less memory. In particular, the runtime cost of calling an open multi method is constant and less than the cost of a double dispatch (two virtual function calls). Finally, we provide a sketch of a design for open multi-methods in the presence of dynamic loading and linking of libraries.

Why C++ is not just an object-oriented programming language

C++ directly supports a variety of programming styles. In this, C ++ deliberately differs from languages designed to support a single way of writing programs. This paper briefly presents key programming styles directly supported by C++ and argues that the support for multiple styles is one of its major strengths. The styles presented include: traditional C-style, concrete classes, abstract classes, traditional class hierarchies, abstract classes and class hierarchies, and generic programming. To provide a context for this overview, I discuss criteria for a reasonable and useful definition of ‘‘object-oriented programming.’’

Software Development for Infrastructure

Infrastructure software needs more stringent correctness, reliability, efficiency, and maintainability requirements than non- essential applications. This implies greater emphasis on up-front design, static structure enforced by a type system, compact data structures, simplified code structure, and improved tool support. Education for infrastructure and application developers should differ to reflect that emphasis. This Web extra video features Bjarne Stroustrup of Texas A&M University discussing how C++ can help improve the reliability, maintainability, and performance of infrastructure software. He also describes features that are part of the latest versions of the C++ language.

Rejuvenating C++ programs through demacrofication

We describe how legacy C++ programs can be rejuvenated using C++11 features such as generalized constant expressions, perfect forwarding, and lambda expressions. In general, this work develops a correspondence between different kinds of macros and the C++ declarations to which they should be transformed. We have created a set of demacrofication tools to assist a developer in the rejuvenation of C++ programs. To evaluate the work, we have applied the rejuvenation tools to a number of C++ libraries to assess the extent to which these libraries might be improved by demacrofication. Results indicate that between 68 and 98% of potentially refactorable macros could be transformed into C++11 declarations. Additional experiments demonstrate why these numbers are not readily achieved using fully automated rejuvenation tools. We also discuss some techniques to further assist in automating rejuvenation process.