My job is to refactor an old library for GIS vector data processing. The main class encapsulates a collection of building outlines, and offers different methods for checking data consistency. Those checking functions have an optional parameter that allows to perform some process. For instance: std::vector<Point> checkIntersections(int process_mode = 0); This method tests if some building outlines are intersecting, and return the intersection points. But if you pass a non null argument, the method will modify the outlines to remove the intersection. I think it's pretty bad (at call site, a reader not familiar with the code base will assume that a method called checkSomething only performs a check and doesn't modifiy data) and I want to change this. I also want to avoid code duplication as check and process methods are mostly similar. So I was thinking to something like this: // a private worker std::vector<Point> workerIntersections(int process_mode = 0) { // it's the equivalent of the current checkIntersections, it may perform // a process depending on process_mode } // public interfaces for check and process std::vector<Point> checkIntersections() /* const */ { workerIntersections(0); } std::vector<Point> processIntersections(int process_mode /*I have different process modes*/) { workerIntersections(process_mode); } But that forces me to break const correctness as workerIntersections is a non-const method. How can I separate check and process, avoiding code duplication and keeping const-correctness?