std::flip: A Fictional Yet Insightful C++ Utility Author: The Sparkelling Bedangler Date: September 25, 2025 --- Overview std::flip is presented as a hypothetical utility in C++ (from <functional>) that reverses the order of parameters of a callable (function or function object). Although it does not exist in the C++ standard library, the article explores its conceptual usefulness, origins, and practical applications by borrowing ideas known in functional programming. --- Key Concept Functionality: Accepts a Callable f(a, b) and returns g(b, a). Helps compose functions, especially predicates, and reconcile inconsistent APIs. --- Motivating Example: Tree Nodes Function isancestorof checks if maybeparent is an ancestor of maybechild by walking up the parent chain. Using std::flip: Flips argument order rather than writing a separate function. --- Origins in Functional Programming Many functional languages and libraries provide a flip function: Haskell, PureScript, OCaml, Idris: Prelude or standard libraries have flip. Elm: Used to have flip. Boost.Hana (C++), FunctionalPlus (C++), Toolz (Python), Ramda (JS), fp-ts (TS), LanguageExt (C#): Provide implementations of flip. Most flip only the first two parameters, inspired by Haskell's model, where arbitrary arity flipping is complicated. D language provides std.functional.reverseArgs that reverses all parameters. --- Common Use Cases Flipping Predicates Using std::flip to invert a binary predicate's argument order allows building relational operators based on <: | Comparison | Generic Implementation | Function Object | Predicate Call | Higher-Order Function | |-----------------|------------------------|--------------------|-------------------------|----------------------------| | a < b | a < b | std::less | pred(a, b) | pred | | a < b reversed| b < a | std::greater | pred(b, a) | std::flip(pred) | | a <= b | !(b < a) | std::lessequal | !pred(b, a) | std::notfn(std::flip(pred)) | | a >= b | !(a < b) | std::greaterequal| !pred(a, b) | std::notfn(pred) | Note: std::notfn is another higher-order function from <functional> complementing this pattern. --- Simple Generic Algorithms Checking reverse sorted: Implementing std::upperbound via std::lowerbound: Note: Earlier C++ standards restricted this due to strict weak ordering requirements but modern standards have relaxed these constraints. --- Longest Subsequence Problems Using Boost.Algorithm's (experimental) longestincreasing_subsequence to build variants: