Re: [Boost-users] [scoped_ptr] Transferring pointers to scoped_ptr member

From: Christian Larsen (contact_at_[hidden])
Date: 2008-08-17 17:28:14

David Abrahams skrev:
> on Sun Aug 17 2008, Christian Larsen <contact-AT-dword.dk> wrote:
> 
>> Hi,
>>
>> This is just make sure I'm not being too careful in this situation. So
>> please confirm whether this is correct.
>>
>> I have a class with two scoped_ptr members, and I want to pass two
>> pointers to the constructor, which will then transfer the ownership of
>> both to the two member scoped_ptrs. I assume the following is not
>> safe:
>>
>> (I didn't try compiling; using namespace boost and std.)
>>
>>     struct B {};
>>
>>     class A
>>     {
>>     public:
>>         A(B* b1, B* b2)
>>             : b1_(b1), b2_(b2)
>>         {}
>>     private:
>>         scoped_ptr<B> b1_;
>>         scoped_ptr<B> b2_;
>>     };
>>
>>     // Create an A this way:
>>     A a(new B, new B);
>>
>> Is it correct that this could fail if one of the 'new B's throw, thus
>> leaking before the constructor ever started?
> 
> Correct.  Please see the guideline at
> http://www.boost.org/doc/libs/1_36_0/libs/smart_ptr/shared_ptr.htm#BestPractices

Ah yes, that clarifies it. :) Somehow it didn't show up in my search
results. But it baffles me, that the order isn't even specified to be
"unordered evaluation of the arguments to one function at a time", i.e.
as in the example

      f(shared_ptr<int>(new int(2)), g());

it surprises me that "new int(2)", and then "g()" can be evaluated
_before_ the shared_ptr constructor.

>> If that is so, is this then the correct way to achieve the same in a
>> safe manner?
>>
>>     struct B {};
>>
>>     class A
>>     {
>>     public:
>>         A(auto_ptr<B>& b1, auto_ptr<B>& b2)
>>             : b1_(b1.release()), b2_(b2.release())
>>         {}
>>     private:
>>         scoped_ptr<B> b1_;
>>         scoped_ptr<B> b2_;
>>     };
>>
>>     // Create an A this way:
>>     A a(auto_ptr<B>(new B), auto_ptr<B>(new B));
> 
> No, that has the same problem (your auto_ptr's aren't named).  the 2nd
> new could throw before any auto_ptrs are constructed, because the order
> of evaluation is unspecified.

Ok, so the correct way to do it in the last example is this?

      auto_ptr<B> b1(new B);
      auto_ptr<B> b2(new B);
      A a(b1, b2);

A little more verbose than I had hoped, but ok as long as it is doing
what it's supposed to.

> HTH,

Yes it did, thank you.

Best regards,
Christian