C++ is not a purely functional language, but the type-system is (caveat WIP things like concepts). Even if all you're doing is "vector-in, vector-out", the type

C++ is not a purely functional language, but the type-system is (caveat WIP things like concepts). Even if all you're doing is "vector-in, vector-out", the types of those vectors matters immensely. This is especially true in languages like C++ where the type-system is very deep but also very hard to debug or change at varying levels of abstraction.