why do you think the length member function is not pure?
> Pure functions have a lot of nice properties. Thread safety. A pure function with value parameters is completely thread safe. With reference or pointer parameters, even if they are const, you do need to be aware of the danger that another thread doing non-pure operations might mutate or free the data, but it is still one of the most powerful tools for writing safe multithreaded code.
Emphasis on 'value parameters'. The invisible 'this' argument is not passed by value, it is passed by reference. By Carmack's definition, C++ member functions are not pure.
I didn’t mean to sound patronizing, I apologize for that. Your question suggested that you aren’t that familiar with functional programming, so I was just trying to steer you in the right direction. A function that takes no parameters is not a pure function in general, unless all it does is return a constant value, which wouldn’t make much sense. length() does not make sense in a functional programming style.
and passing a sequence of characters? hello, C strlen( const char * s); ! with all the problems that has. and if you don't want to pass some horrible pointer, you have to pass some object containing the characters which must have a length function itself. hello, std::string!
what you are ignoring here is how functional types either ignore functionality under the hood, or sacrifice it for performance.
which is why functional programming will never catch on (look at lisps), imho. programmers really like to know wtf their code is actually doing on a mutable machine.
No, I mean, a standalone function with no parameters can only be functional if it always returns the same value, which is effectively no different than declaring a constant. The function always returns the same value all time (which wouldn't be so useful).
I think you are trying to find C++ examples to match the broader implications of functional programming (which is what I'm talking about), which will be difficult.
As far as whether "functional programming will catch on", I don't think it will for C++ so much, but it certainly has been very successful for many other languages. I earn my living writing only functional code, but most of the time it is not in C++.
parameter0.membfun()
It doesn't really matter whether the parameter name is separated from the function name by a dot, or a bracket. In a different syntax, it might have been written as "membfun(parameter0)" without any practical difference.
This means that Foo::length() can be pure no problem, because Foo is its parameter, and, as such, in its scope.