Minimizing dependencies within generic classes for faster and smaller programs

Abstract

Generic classes can be used to improve performance by allowing compile-time polymorphism. But the applicability of compile-time polymorphism is narrower than that of runtime polymorphism, and it might bloat the object code. We advocate a programming principle whereby a generic class should be implemented in a way that minimizes the dependencies between its members (nested types, methods) and its generic type parameters. Conforming to this principle (1) reduces the bloat and (2) gives rise to a previously unconceived manner of using the language that expands the applicability of compile-time polymorphism to a wider range of problems. Our contribution is thus a programming technique that generates faster and smaller programs. We apply our ideas to GCC's STL containers and iterators, and we demonstrate notable speedups and reduction in object code size (real application runs 1.2x to 2.1x faster and STL code is 1x to 25x smaller). We conclude that standard generic APIs (like STL) should be amended to reflect the proposed principle in the interest of efficiency and compactness. Such modifications will not break old code, simply increase flexibility. Our findings apply to languages like C++, C#, and D, which realize generic programming through multiple instantiations. Copyright © 2009 ACM.