shared_ptr allocation optimization

Question

Somewhere I saw a post about an optimized way of creating a boost shared_ptr so that it allocated the ptr plumbing and the pointee at the same time. I did a SO search but there are a lot of posts on shared_ptr and I could not find it. Can somebody smart please repost it

edit: thanks for answer. extra credit question. Whats the correct (preferred?) idiom for returning a null shared_ptr? ie

FooPtr Func()
{
   if(some_bad_thing)
      return xxx; // null
}

to me

return FooPtr((Foo*)0);

looks kinda klunky

Answer 1

See boost::make_shared():

Besides convenience and style, such a function is also exception safe and considerably faster because it can use a single allocation for both the object and its corresponding control block, eliminating a significant portion of shared_ptr's construction overhead. This eliminates one of the major efficiency complaints about shared_ptr.