On Mar 17, 2011, at 3:24 AM, Christoph Heindl wrote:
On Thu, Mar 17, 2011 at 10:56 AM, Joshua Juran <jjuran@gmail.com> wrote:
Is there a reason to pass the shared_ptr by value instead of const reference?
I'm not sure. After reading several discussions on the web [1,2] I concluded that I'll stick with by-value.
[1] http://stackoverflow.com/questions/327573/c-passing-references-to-boostshare... [2] http://stackoverflow.com/questions/3310737/shared-ptr-by-reference-or-by-val...
My understanding is that the arguments for pass-by-value boil down to: (1) The called function might cause some side effect which includes destroying the referenced object before using the object. This doesn't apply in your case, since you're just constructing a copy. (2) Another thread might destroy the object before the called function uses it. This is a red herring: Either the calling function has its own local copy, which will persist until the called function returns, or it's using storage accessible to another thread, in which case it's vulnerable -- but passing the shared_ptr by value to the called function doesn't close that vulnerability. The same hypothetical thread could either release the shared resource (reset()) or destroy the storage for the shared_ptr itself before the function call occurs. If this is in fact a risk (which it may well not be) then the calling code needs to arrange serialized access and either store its own local copy of the shared_ptr before passing it to another function or maintain a lock for the duration of the function call. One of the C++ best practices (from the book of the same name) is avoiding premature pessimization. Josh