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

Generic programming using C++ results in code that is efficient but inflexible. The inflexibility arises, because the exact types of inputs to generic functions must be known at compile time. We show how to achieve run-time polymorphism without compromising perfor-mance by instantiating the generic algorithm with a comprehensive set of possible parameter types, and choosing the appropriate in-stantiation at run time. The major drawback of this approach is ex-cessive template bloat, generating a large number of instantiations, many of which are identical at the assembly level. We show prac-tical examples in which this approach quickly reaches the limits of the compiler. Consequently, we combine the method of run-time polymorphism for generic programming with a strategy for reduc-ing the amount of necessary template instantiations. We report on using our approach in GIL, Adobe’s open source Generic Image Library. We observed notable reduction, up to 70 % at times, in ex-ecutable sizes of our test programs. Even with compilers that per-form aggressive template hoisting at the compiler level, we achieve notable code size reduction, due to significantly smaller dispatching code. The framework draws from both the generic programming and generative programming paradigm, using static metaprogram-ming 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