Re: [Boost-users] boost::thread() calls destructor!!

From: Michael B. Edwin Rickert (panda_at_[hidden])
Date: 2005-05-28 18:16:42

Patrik Jonsson wrote:

> Hi,
>
> I don't pretend to know how the boost thread functions work, but I've 
> used them and haven't had any problems. Are you sure it's not just 
> that the object is being copied 4 times and what you're seeing are the 
> destructors for the copies? In your test code, you wouldn't be able to 
> tell.  According to the docs, the thread constructor makes at least 
> one copy of its argument. Does your class have a working copy 
> constructor?

Since boost::thread takes a boost::function, which copies it's functor 
when it is copied to preven dangling references - so in the second case 
writerClass is probably indeed being copied.
4 times does seem slightly odd I'll admit... but I suppose the 
boost::function is simply getting passed around a bit by value 
internally. If you create a copy constructor, it should show writerClass 
getting copied a few times.

If you didn't want this behavior you'd use boost::ref IIRC:

writerClass writer;
boost::thread thrd2( boost::ref( writer ) );

>> and changed in the main()
>> --> boost::thread thrd2(&writer);
>> with
>> --> boost::thread thrd2((writerClass()));
>>
>> Everything works the same, but I have the destructor of wirterClass
>> called 4 times, when constructing thrd2!!!
>>
>> That's weird to me, but it's annoying because in my other program, the
>> dtor destroys some member data, and the program crashes because of it.
>> Why is the ~writerClass() called?
>> is there maybe some exception thrown??
>> how can I solve the problem?
>
Again, probably due to the fact that writerClass is getting copied a few 
times. Adding an appropriate copy constructor should do the trick (e.g. 
deep copy pointed to data or use a boost::shared_ptr so that all the 
classes share a single version of the data, but don't delete it until 
the last instance dies)

      -Mike