Re: C++17 and "Alice in Wonderland"

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2021-03-24 13:49:51

 Â I also do not even begin to understand why 'bool (*)(char const)' 
becomes 'bool (*)(char)' in any logical world. Clearly a char const is 
not the same as a char. Who in their right minds would make up a rule 
that says they are the same, whether in a function declaration or not ? 
They clearly are not, unless you are out to destroy the notion that 
top-level consts in function declarations mean anything.

That is pretty easy: The function type(!) does NOT include the const of 
value parameters. My goto for this is godbolt and an easy template test: 
https://godbolt.org/z/7MvbWb3EW

This is why in the function declaration you can omit the const even when 
you use it in the definition and if you think about it it makes sense: 
If the function does or does not modify a copied (i.e. by-value) 
parameter doesn't matter to the caller at all.

>
> Honestly if this is now C++, something seriously has gone wrong 
> AFAICS. No doubt I am missing something basic, but this is truly 
> "Alice in Wonderland" stuff to me. OK, I have had my rant. In what I 
> was working on I will just specify that T as a top-level const is 
> disallowed. My design can get by on that, rather than come up with 
> some tortured code that allows T as a top-level const in the sort of 
> signature above which works in both C++14 and C++17 on up.
In short the takeaway here is: Function value parameters in function 
signatures are never const.