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Re: Do you like that ?

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Kaz Kylheku <864-117-4973@kylheku.com> writes:

> On 2023-11-05, Bonita Montero <Bonita.Montero@gmail.com> wrote:
>> using XHANDLE = std::unique_ptr<void, decltype([]( void *h ) { h != 
>> INVALID_HANDLE_VALUE && h && CloseHandle( h ); })>;
>>
>> A Win32 HANDLE-wrapper in just one line in C++20.
>> C++20 allows lambdas in an unevaluated context.
>
> So what?
>
> 1960 Lisp allows lambdas in unevaluated context.
>
>   (QUOTE (LAMBDA (H)
>            (AND (NOT (INVALIDHANDLE H))
>                 (CLOSEHANDLE H))))
>
> This is just C++ catching up to something that existed when
> Stroustrup was in elementary school.

I was going to say the usage here is not a good look for C++.  As a
language with compile-time type checking why is decltype needed?  It's
usually because C++ can't express the type involved any other way and
that's not something to shout about.

But then it turns out I don't know what's going on.  This program:

  #include <iostream>
  #include <memory>

  using uptr_int = std::unique_ptr<int, decltype([](int *ip) {
     std::cout << "delete (" << *ip << ")\n";
  })>;

  int main()
  {
     int x = 42;
     uptr_int ip(&x);
  }

prints "delete (42)".  Why?  I thought decltype was just being used to
give a type for the std::unique_ptr template instantiation, but it seems
to supply the function too.  What's going on?

Having used C++ from the early CFront days, and watched balloon in size
and complexity, I knew the day would come when even a short C++ program
would be mystery to me.  That day has come!

-- 
Ben.

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Thread

Do you like that ? Bonita Montero <Bonita.Montero@gmail.com> - 2023-11-05 20:06 +0100
  Re: Do you like that ? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-11-05 20:02 +0000
    Re: Do you like that ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-05 21:42 +0000
      Re: Do you like that ? David Brown <david.brown@hesbynett.no> - 2023-11-05 23:09 +0100
    Re: Do you like that ? Bonita Montero <Bonita.Montero@gmail.com> - 2023-11-06 09:33 +0100
      Re: Do you like that ? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-11-06 17:50 +0000
        Re: Do you like that ? Bonita Montero <Bonita.Montero@gmail.com> - 2023-11-06 19:01 +0100
          Re: Do you like that ? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-11-06 18:59 +0000
            Re: Do you like that ? Bonita Montero <Bonita.Montero@gmail.com> - 2023-11-06 20:07 +0100
              Re: Do you like that ? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-11-06 19:10 +0000
                Re: Do you like that ? David Brown <david.brown@hesbynett.no> - 2023-11-07 08:57 +0100
        Re: Do you like that ? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-06 12:17 -0800

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