Serverless functions have lately been getting traction in the world of high-performance applications where the dynamic scheduling features that serverless cloud environments exhibit can be used to offload CPU-intensive work to the cloud. This is especially advantageous for workloads where dynamic parallelism is required. However, using serverless platforms for this purpose remains difficult in languages like C++ which is traditionally used for high-performance application. To solve this problem we introduce cppless, a single-source programming model for high-performance serverless applications. Cppless allows users to write serverless functions together with the code that uses them to a low transparent offloading. This allows us to provide a common abstraction layer for serverless platforms and enables composable, modular architectures that make use of serverless functions. We evaluate Cppless using several high-performance problems. The results show that Cppless provides a low-overhead interface for serverless applications.