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Re: Q: how to make an object that throws

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Ross Finlayson <ross.a.finlayson@gmail.com> writes:

> Hello I hope I might consult your expertise, I'm wondering about
> a use-case where for a given library object, to provide an instance
> of same type, that on any access, throws an exception.
>
> The idea is that there's an idea in a different language where
> the runtime throws a null_pointer_dereference exception
> when dereferencing a null pointer, yet in C++ it's signal segfault.
>
> So, what I wonder about is how to make an object in C++,
> of a given standard library type, and any arbitrary type,
> that on any access, throws.
>
> I suppose I might static_cast an object yet I wonder how
> to make it so then that any access to it throws.
>
> As you might imagine it's a use-case for exceptions
> as part of flow-of-control, that in the other language
> just is null yet in C++ would be for "an object of all types",
> that throws on any access.
>

I believe there is no portable way to implement this.

However, if you are running GCC on supported platforms, you can try
the `-fnon-call-exceptions` option.

--8<---------------cut here---------------start------------->8---
// main.cpp
#include <sys/types.h>
#include <signal.h>
#include <iostream>
#include <exception>

extern "C" void handle_signal(int signum) {
  throw std::runtime_error("Invalid access");
}

int main()
{
  struct sigaction act;
  act.sa_handler = &handle_signal;
  sigemptyset(&act.sa_mask);
  act.sa_flags = SA_NODEFER;
  sigaction(SIGSEGV, &act, nullptr);

  int *object = NULL;

  try {
    // write
    *object = 100;
  } catch (std::exception& exc) {
    std::cerr << "Caught exception: " << exc.what() << '\n';
  }

  try {
    // read
    int value = *object;
    std::cout << "value = "  << value << '\n';
  } catch (std::exception& exc) {
    std::cerr << "Caught exception: " << exc.what() << '\n';
  }
}
--8<---------------cut here---------------end--------------->8---

Compile the source file with

  g++ -fnon-call-exceptions  -o test main.cpp

Run ./test, it outputs

  Caught exception: Invalid access
  Caught exception: Invalid access

-- 
Zhengyi Fu

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Thread

Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-22 19:11 -0700
  Re: Q: how to make an object that throws Sam <sam@email-scan.com> - 2024-04-22 22:32 -0400
    Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-22 21:51 -0700
      Re: Q: how to make an object that throws Sam <sam@email-scan.com> - 2024-04-23 07:26 -0400
        Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-23 08:39 -0700
          Re: Q: how to make an object that throws Sam <sam@email-scan.com> - 2024-04-23 22:08 -0400
            Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-23 20:07 -0700
              Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-23 22:10 -0700
  Re: Q: how to make an object that throws Zhengyi Fu <i@fuzy.me> - 2024-04-23 11:08 +0800
    Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-22 21:52 -0700
    Re: Q: how to make an object that throws wij <wyniijj5@gmail.com> - 2024-04-25 02:56 +0800
      Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-24 12:02 -0700
    Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-24 13:56 -0700
      Re: Q: how to make an object that throws Ross Finlayson <ross.a.finlayson@gmail.com> - 2024-04-27 08:09 -0700
    Re: Q: how to make an object that throws Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-04-25 14:08 -0700

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