Jaakko Järvi

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.

A comparative study of language support for generic programming

Many modern programming languages support basic generic programming, sufficient to implement type-safe polymorphic containers. Some languages have moved beyond this basic support to a broader, more powerful interpretation of generic programming, and their extensions have proven valuable in practice. This paper reports on a comprehensive comparison of generics in six programming languages: C++, Standard ML, Haskell, Eiffel, Java (with its proposed generics extension), and Generic C. By implementing a substantial example in each of these languages, we identify eight language features that support this broader view of generic programming. We find these features are necessary to avoid awkward designs, poor maintainability, unnecessary run-time checks, and painfully verbose code. As languages increasingly support generics, it is important that language designers understand the features necessary to provide powerful generics and that their absence causes serious difficulties for programmers.

An extended comparative study of language support for generic programming

Many modern programming languages support basic generics, sufficient to implement type-safe polymorphic containers. Some languages have moved beyond this basic support, and in doing so have enabled a broader, more powerful form of generic programming. This paper reports on a comprehensive comparison of facilities for generic programming in eight programming languages: C++, Standard ML, Objective Caml, Haskell, Eiffel, Java, Cn (with its proposed generics extension), and Cecil. By implementing a substantial example in each of these languages, we illustrate how the basic roles of generic programming can be represented in each language. We also identify eight language properties that support this broader view of generic programming: support for multi-type concepts, multiple constraints on type parameters, convenient associated type access, constraints on associated types, retroactive modeling, type aliases, separate compilation of algorithms and data structures, and implicit argument type deduction for generic algorithms. We find that these features are necessary to avoid awkward designs, poor maintainability, and painfully verbose code. As languages increasingly support generics, it is important that language designers understand the features necessary to enable the effective use of generics and that their absence can cause difficulties for programmers.

Algorithm specialization in generic programming: challenges of constrained generics in C++

Generic programming has recently emerged as a paradigm for developing highly reusable software libraries, most notably in C++. We have designed and implemented a constrained generics extension for C++ to support modular type checking of generic algorithms and to address other issues associated with unconstrained generics. To be as broadly applicable as possible, generic algorithms are defined with minimal requirements on their inputs. At the same time, to achieve a high degree of efficiency, generic algorithms may have multiple implementations that exploit features of specific classes of inputs. This process of algorithm specialization relies on non-local type information and conflicts directly with the local nature of modular type checking. In this paper, we review the design and implementation of our extensions for generic programming in C++, describe the issues of algorithm specialization and modular type checking in detail, and discuss the important design tradeoffs in trying to accomplish both.We present the particular design that we chose for our implementation, with the goal of hitting the sweet spot in this interesting design space.

Concept-controlled polymorphism

Concepts--sets of abstractions related by common requirements-- have a central role in generic programming. This paper proposes a general framework for using concepts to control polymorphism in different ways. First, concepts can be used to constrain parametric polymorphism, as exemplified by type classes in Haskell. Second, concepts can be used to provide fine-grained control of function and operator overloading. Finally, generic functions can be overloaded (specialized) based on concepts, rather than simply on types. We describe a C++ implementation of a new mechanism, which we call enable_if, and its role in concept-controlled polymorphism.

Associated types and constraint propagation for mainstream object-oriented generics

Support for object-oriented programming has become an integral part of mainstream languages, and more recently generic programming has gained widespread acceptance as well. A natural question is how these two paradigms, and their underlying language mechanisms, should interact. One particular design option, that of using subtyping to constrain the type parameters of generic functions, has been chosen in the generics of Java and those planned for a future revision of C#.Certain shortcomings have previously been identified in using subtyping for constraining parametric polymorphism in the context of generic programming.To address these, we propose extending object-oriented interfaces and subtyping to include associated types and constraint propagation.Associated types are type members of interfaces and classes. Constraint propagation allows certain constraints on type parameters to be inferred from other constraints on those parameters and their use in base class type expressions.The paper demonstrates these extensions in the context of C# (with generics), describes a translation of the extended features to C#, and presents a formalism proving their safety. The formalism is applicable to other mainstream object-oriented languages supporting F-bounded polymorphism, such as Java.

The Lambda library: unnamed functions in C++

The Lambda Library (LL) adds a form of lambda functions to C++, which are common in functional programming languages. The LL is implemented as a template library using standard C++; thus no language extensions or preprocessing is required. The LL consists of a rich set of tools for defining unnamed functions. In particular these unnamed functions work seamlessly with the generic algorithms in the C++ Standard Library. The LL offers significant improvements, in terms of generality and ease of use, compared to the current tools in the C++ Standard Library. Copyright

Efficient run-time dispatching in generic programming with minimal code bloat

Generic programming with C++ templates results in efficient but inflexible code: efficient, because the exact types of inputs to generic functions are known at compile time; inflexible because they must be known at compile time. We show how to achieve run-time polymorphism without compromising performance by instantiating the generic algorithm with a comprehensive set of possible parameter types, and choosing the appropriate instantiation at run time. Applying this approach naively can result in excessive template bloat: a large number of template instantiations, many of which are identical at the assembly level. We show practical examples of this approach quickly approaching the limits of the compiler. Consequently, we combine this method of run-time polymorphism for generic programming, with a strategy for reducing the number of necessary template instantiations. We report on using our approach in GIL, Adobes open source Generic Image Library. We observed a notable reduction, up to 70% at times, in executable sizes of our test programs. This was the case even with compilers that perform aggressive template hoisting at the compiler level, due to significantly smaller dispatching code. The framework draws from both the generic and generative programming paradigms, using static metaprogramming to fine tune the compilation of a generic library. Our test bed, GIL, is deployed in a real world industrial setting, where code size is often an important factor.

Library composition and adaptation using c++ concepts

Large scale software is composed of libraries produced by different entities. Non-intrusive and efficient mechanisms for adapting data structures from one library to conform to APIs of another are essential for the success of large software projects. Concepts and concept maps, planned features for the next version of C++, have been designed to support adaptation, promising generic, non-intrusive, efficient, and identity preserving adapters. This paper analyses the use of concept maps for library composition and adaptation, comparing and contrasting concept maps to other common adaptation mechanisms. We report on two cases of data structure adaptation between different libraries, indicating best practices and idioms along the way. First, we adapt GUI controls from several frameworks for use with a generic layout engine, extending the application of concepts to run-time polymorphism. Second, we develop a transparent adaptation layer between an image processing library and a graph algorithm library, enabling the efficient application of graph algorithms to the image processing domain.

Variadic templates for C

Generic functions and classes accepting a variable number of type arguments have proven to be a very useful, but missing, feature of C++. Numerous foundational libraries rely on clever template and preprocessor tricks to emulate such variadic templates. By several measures these emulations are inadequate. This paper describes how C++ can be extended with variadic templates, significantly improving existing implementations of widely used C++ libraries in terms of code size, quality of error diagnostics, compilation speed, and generality. Furthermore, variadic templates enable new applications, such as type-safe implementations of functions like printf, and improved support for generic mixin classes. We have implemented variadic templates as an extension of a production quality C++ compiler.