220 8395 <52C7F46C.3060702@wanadoo.fr> article Path: news.gmane.org!not-for-mail From: "Vicente J. Botet Escriba" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: [exception_ptr] exception_ptr_cast: cast to the stored exception a la any_cast Date: Sat, 04 Jan 2014 12:45:48 +0100 Lines: 178 Approved: news@gmane.org Message-ID: <52C7F46C.3060702@wanadoo.fr> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------090606000102070200000607" X-Trace: ger.gmane.org 1388835945 31948 80.91.229.3 (4 Jan 2014 11:45:45 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 4 Jan 2014 11:45:45 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDH67CONY4PBB27IT6LAKGQE6TFVVII@isocpp.org Sat Jan 04 12:45:51 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-la0-f70.google.com ([209.85.215.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VzPfn-00017N-SY for gclcip-std-proposals@m.gmane.org; Sat, 04 Jan 2014 12:45:47 +0100 Original-Received: by mail-la0-f70.google.com with SMTP id e16sf1881274lan.5 for ; Sat, 04 Jan 2014 03:45:47 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:message-id:date:from:user-agent:mime-version:to :subject:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe:content-type; bh=k6MDUIF+anVhABiPTBdJ+JmmnELyZUbA/kwM0gMK6TQ=; b=T8T8LAKWmnnQgRu8IYJZJjuZthFEk7bsUhC3JlzVBEMf0fXzNhtAaob6Dc3GFw3nEz DKAf8AB6eJtEDit0P+VpoXGMwsrqqlcfJcoA/ZEzMg343vzCVS8/7hLX+BwxwN3AftHU K4/egseWo53k4cSeLsMa6vn0UcL6eIrC8z9sU/eJsEKFO+Ksuy9XIUBKwyd09vpGUfvB pfXKc8ogtDLEiYDoR9uv4jjEJt3Otwaonx5+ppO5mjuFEFZBieVv2Gxwa2VWJ2dZSbdS 5wZX04MamJAsTZFkQ/yiuXc4x3WBIsrsjgWlPTv7GXYbbpx4pxAPIsVY6wJvzMXV11/M G1Bg== X-Gm-Message-State: ALoCoQkVoKWaYULOGcsQDDAb6qqrjs24iHwaHWu7qrpUN/+kBnp9Kkc30K+sH6fvbomBIkd/6hZH X-Received: by 10.112.130.196 with SMTP id og4mr25784956lbb.13.1388835947456; Sat, 04 Jan 2014 03:45:47 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.94.34 with SMTP id cz2ls200952wib.41.gmail; Sat, 04 Jan 2014 03:45:46 -0800 (PST) X-Received: by 10.194.2.70 with SMTP id 6mr47018085wjs.25.1388835946627; Sat, 04 Jan 2014 03:45:46 -0800 (PST) Original-Received: from smtp.smtpout.orange.fr (smtp03.smtpout.orange.fr. [80.12.242.125]) by mx.google.com with ESMTP id m13si4889419wjr.119.2014.01.04.03.45.46 for ; Sat, 04 Jan 2014 03:45:46 -0800 (PST) Received-SPF: neutral (google.com: 80.12.242.125 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) client-ip=80.12.242.125; Original-Received: from new-host.home ([92.139.143.25]) by mwinf5d50 with ME id 9nll1n00D0Z5MbQ03nllbh; Sat, 04 Jan 2014 12:45:46 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 X-Original-Sender: vicente.botet@wanadoo.fr X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 80.12.242.125 is neither permitted nor denied by best guess record for domain of vicente.botet@wanadoo.fr) smtp.mail=vicente.botet@wanadoo.fr Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:8395 Archived-At: This is a multi-part message in MIME format. --------------090606000102070200000607 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Hi, I'm working on a proposal of an expected class based on the Expected from Alexandrescu. Expected provides a function hasException defined as follows: template bool hasException() const { try { if (!gotHam) std::rethrow_exception(spam); } catch (const E& object) { return true; } catch (...) { } return false; } But this function doesn't allow to get the stored exception so that we cannot access specifics members in exception E. if (y.hasException()) { .... // no way to get the values that are not dividible } Of course, the user could always do try { if (! y.valid()) std::rethrow_exception(y.exception_ptr()); } catch (const NotDivisible& ex) { std::cout < const E* expected_cast(const expected*); if (auto ex = expected_cast(&y)) { std::cout <num <<"," <den< Hi,

I'm working on a proposal of an expected<T> class based on the Expected<T> from Alexandrescu.
Expected<T> provides a function hasException defined as follows:

template <class E>
bool hasException() const {
  try {
     if (!gotHam) std::rethrow_exception(spam);
  } catch (const E& object) {
     return true;
  } catch (...) {
  }
  return false;
}

But this function doesn't allow to get the stored exception so that we cannot access specifics members in exception E.

if (y.hasException<NotDivisible>())
{
... // no way to get the values that are not dividible
}

Of course, the user could always do

  try {
     if (! y.valid()) std::rethrow_exception(y.exception_ptr());
  } catch (const NotDivisible& ex) {
     std::cout <<ex.num <<"," <<ex.den<<std::endl;
  } catch (...) {
  }

But, this is not optimal, as we need to use a try-catch block and rethrow the exception.

I would like to define a dynamic cast that returns a pointer to the stored exception it the type match

template <class E, class T>
const E* expected_cast(const expected<T>*);

if (auto ex = expected_cast<NotDivisible*>(&y))
{
   std::cout <<ex->num <<"," <<ex->den<<std::endl;
}

But I don't see how to implement it without having the possibility to cast the exception_ptr instance to the stored exception. Do you know how this can be implemented with the current exception_ptr interface?

If instead of storing the exception on a exception_ptr, we store it on the std::experimental::any we can use std::experimental::any_cast to get the stored exception. However, while std::experimental::any could be used to store any explicit exception, it cannot be used to store the current exception.

Is there a way to define a exception_ptr_cast (similar to the one of std::experimental::any_cast) for std::exception_ptr?
Are there some restrictions to the stored exceptions?
It is worth writing a standard proposal?

Best,
Vicente


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