Re: Throw from a destructor

From:

Joe Van Dyk <joe.vandyk@boeing.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 7 Jul 2006 22:14:31 GMT

Message-ID:

<J220G5.8nD@news.boeing.com>

Phlip wrote:

C++ers:

I have this friend who thinks C++ cannot throw from a destructor.

I think throwing from a destructor is bad Karma, and should be designed
around, but that C++ cannot strictly prevent the actual event.

(One compiler-oriented reason is a compiler might not be able to detect
that the call-tree from a destructor leads to a throw.)

Put another way, suppose I have a big object, aBigSimCity, and it has a
.shutdown() method. It must call before ~BigSimCity() calls, and it might
throw. This implies my Wrapper class, containing aBigSimCity, should never
declare a destructor like this: ~Wrapper() { aBigSimCity.shutdown(); }.

.shutdown() might throw, and I don't feel like wrapping it in
try{}catch(). ~Wrapper should not throw, hence I need a better system to
break things down.

So which of us is right?

(Scott Meyer's Effective C++ 3rd edition, Item 8 addresses exactly this.)

How about something like (almost verbatim from the book):

class BigSimCity
{
    void shutdown()
    {
       // do stuff that may throw
       closed_ = true;
    }
    ~BigSimCity() // would this have an empty throw() specification?
    {
       if (!closed)
       {
         try { shutdown(); }
         catch (...) { // log or something }
       }
    }
   private:
     bool closed;
};

You give the client the chance to shutdown() on their own if they want
catch it on their own. And if not, then shutdown()'s called by the
destructor.

Joe