From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH kvm-unit-tests 2/4] Introduce a C++ wrapper for the kvm
 APIs
Date: Wed, 24 Nov 2010 19:50:06 +0200
Message-ID: <4CED504E.60408@redhat.com>
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Cc: Anthony Liguori <anthony@codemonkey.ws>,
	Alexander Graf <agraf@suse.de>,
	Marcelo Tosatti <mtosatti@redhat.com>, kvm@vger.kernel.org
To: Gleb Natapov <gleb@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
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On 11/24/2010 07:41 PM, Gleb Natapov wrote:
> On Wed, Nov 24, 2010 at 07:39:39PM +0200, Avi Kivity wrote:
> >  On 11/24/2010 07:33 PM, Gleb Natapov wrote:
> >  >>
> >  >>   But I don't see the point.  If you look at my repository, there's
> >  >The point is that C++ is ugly language. The short code Avi sent remind
> >  >me perl (aka line noise). It is almost impossible to parse it into
> >  >what code it actually does. Most symbols are there for C++ syntax not
> >  >functionality.
> >
> >  No.  They're there for error handling.  A C++ wrapper, doesn't add
> >  any functionality, so you can say that all of the lines of codes do
> >  nothing and are just syntax.  But they do allow you to pair init and
> >  uninit (in the constructor and destructor).  When you use the
> >  wrapper (as opposed to the bare C interface) you get the value by
> >  not having to code long error handling sequences (with a high
> >  probability of getting them wrong and never finding out in testing).
> >
> So how errors are handled there? By throwing exceptions? Sorry this is
> not error "handling".

It's not in this code, but in it's callers.

   kvm::system sys;
   kvm::vm vm(sys);
   kvm::vcpu vcpu(vm, 0);
   mutex::guard guard(some_mutex);
   some_other_object blah;
   return do_something();

If an exception happens in any of these places, whatever's already 
constructed is rolled back and destroyed.  Contrast with

   struct kvm_system sys;
   struct kvm_vm vm;
   struct kvm_vcpu vcpu;
   struct some_other_object object;
   int r;

   r = kvm_system_init(&sys);
   if (r < 0)
       goto out_1;
   r = kvm_vm_init(&vm, &sys);
   if (r < 0)
       goto out_2;
   r = kvm_vcpu_init(&vcpu, &vm, 0);
   if (r < 0)
       goto out_3;
   mutex_aquire(&some_mutex);
   r = some_other_object_init(&object);
   if (r < 0)
       goto out_4;
   r = do_something();
   some_other_object_destroy(&object);
out_4:
   mutex_release(&mutex);
out_3:
   kvm_vcpu_destroy(&vcpu);
out_2:
   kvm_vm_destroy(&vm);
out_1:
   kvm_system_destroy(&sys);
   return r;

Which, in your opinion, is more readable?

-- 
error compiling committee.c: too many arguments to function