Re: [Boost-users] shared_ptr as reference argument

Subject: Re: [Boost-users] shared_ptr as reference argument
From: Chris Uzdavinis (cuzdav_at_[hidden])
Date: 2009-03-20 11:30:01

On Thu, Mar 19, 2009 at 12:17 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
> anony wrote:
>> Is there some utility in passing shared_ptr's as references as here:
>>   void bla(boost::shared_ptr<T>& bla_ptr);
>> This avoids creating of a new object and copying overhead, but is it
>> safe? When would you recommend it and when not?

> Why wouldn't it be safe?  Unless the function stores a pointer
> or reference to the shared_ptr after it returns, there is no danger.

That's not really sufficient to be totally safe.  Code like this could
fail too:

void do_work(some_shptr const & myobj)
{
  std::cout << *myobj << '\n';
  do_something();
  std::cout << *myobj << '\n'; // oops (?)
}

Suppose that the shptr reference is invalidated in the call to
do_something.  This is entirely possible if the underlying object is
owned by a global lookup table, and the caller of do_work() does not
retain a reference to the object being passed in. When this happens,
the global lookup table holds the only reference, and do_something
might remove the element and invalidate the shptr.

>From experience.  :)

Chris