I don't use pure functions all the time, but I appreciate them for the help they give to reduce bugs. A problem is that pure functions are mostly for functional programming, not for OOP.

For classes the pure functions are less useful, they can be virtual and be overridden, but they can't change the instance, so they are a subset of the const methods.

So it can be invented an attribute for class/struct methods that like 'pure' helps reducing side effects, but more useful (and less restrictive) for OOP. This attribute can be a superset of pure, it allows to call pure functions/methods, but pure functions can't call them.

A possible way to design this idea: an attribute that allows a method to modify only its instance attribute and nothing else (no the input arguments, variable in outer scopes, no modify other object, etc.).

I don't know if this can be useful in practice. It doesn't look good. Maybe you can invent something else.

Bye,
bearophile