[Boost-users] Re: passing shared_ptr<> by_value or by-reference?

From: Jonathan Biggar (jon_at_[hidden])
Date: 2005-01-21 14:56:10

Peter Dimov wrote:
>>>> Is there a common sense about how a shared_ptr<> should be passed to
>>>> functions/methods: by-value or by-(const-)reference?
>>>>
>>>>    1) void test(boost::shared_ptr<A> a);
>>>>    2) void test(boost::shared_ptr<A>& const a);
>>>
>>>
>>>
>>> You should pass by value. Passing a shared_ptr means giving the
>>> opportunity to the callee to use the data without worrying about its
>>> lifetime. Would you pass it by ref, the callee won't own the the 
>>> pointee and
>>> hence cannot use it reliabily (it may get deleted behind its back).
>>> By passing it by value, the callee makes a copy of the shared_ptr,
>>> hence incrementing the ref count. The pointer won't get deleted
>>> while the callee use the pointer.
>>> Hope it makes sense.
>>
>>
>> I'm not 100% sure that such a scenario (the pointer gets deleted
>> behind its back if the shared_ptr is passed by ref) is technically
>> possible, but I agree that logically it's better to pass by value.
> 
> 
> It is possible. In a multithreaded program, another thread may reset() 
> a. In a single-threaded program, a function called by test() may reset 
> a. I have encountered it myself.

But doesn't that violate the thread-safety guarantee of the shared_ptr
interface anyway?  Using two different shared pointers to the same
object is guaranteed to be thread safe, but you aren't supposed to let a
thread modify a particular shared pointer instance while another thread
is accessing it.

The fact that it is passed by const reference in a function call is a
red herring, since the multiple access violation can occur before or
after the call too.

-- 
Jon Biggar
Levanta
jon_at_[hidden]