What is this weird function parameter?

Question

today I came across this weird code:

auto rovoid_iterator
    (
        Construct ROII* const at,
        auto(ROII&)(auto(*)(Str&&)noexcept->void) ->void //WTF??
    ) -> void;

What the hell is this weird second parameter??

Thanks in advance!

Answer 1

Okay, let's deconstruct this abomination.

First, there's a inner type:

auto(*)(Str&&) noexcept -> void

This is a pointer to function taking a Str rvalue-reference as parameter. It's also a noexcept function.

Let's call that S

using S = auto(*)(Str&&) noexcept -> void;

Then the outer part of the parameter can be subtitued like that:

auto(ROII&)(S) -> void

As you stated in the comments, ROII is an empty macro. So in the end it reads like that:

auto(&)(S) -> void

That code appear to be a parameter which would be a reference to function that take a S which is and return void.

Answer 2

Okay I asked the dev who wrote this:

auto(ROII&)(auto(*)(Str&&)noexcept->void) ->void 

Is a reference to a function which takes a function pointer as argument. This function pointer is a pointer because its okay to pass nullptr if you dont need it, but the first function must be passed thats why it is a reference. The second pointer is a pointer to a function wich is noexcept and takes a rvalue reference to a string as paremeter. ROII marks game ready functions.