Re: Efficient insertion in a std::multimap
On Aug 24, 5:24 am, Pavel <dot_com_yahoo@paultolk_reverse.yourself>
wrote:
Pete Becker wrote:
Barry wrote:
Hope this doesn't get lost beneath all the spam.
I have the following container which I wish to create and store
objects of type MyObject in-
class Phase : public std::multimap<double, MyObject>
{
public:
Phase();
};
Phase::Phase()
{
MyObject myObject;
std::pair<double,Note> pair(0.0,myObject);
insert (pair);
}
A lot of copy constructors are being called for MyObject
which I'd like to avoid. But first, I'm not understanding
what is happening for the line: insert (pair);. Here, the
copy constructor is called twice and the default destructor
once which suggests to me that a temp object is create, but
why? Finally, is there a way to cut down on all the copies?
my_map.emplace(0.0, myObject), coming with C++0x. It takes
its arguments by reference, and constructs the stored object
in place. No copying.
Just a reminder that map<>::emplace is not a part of current
C++. It's part of the current draft for the next version of the
standard. It's highly unlikely that you can use it with your
compiler today.
Still, myObject will have to be copied... by constructing the
stored object in place, anyway.
If the final standard conforms to the current draft (and I see
no reason why it shouldn't in this regard), no copy will be
needed. You just pass whatever arguments you'd have passed to
the constructor to emplace, and it constructs the object in
place.
With map of pointers this can be avoided.. at the cost of
dynamic memory allocation for myObject, of course. It is a
usual trade-off -- sometimes mapping pointers instead of
objects pays off, sometimes it does not. If myObject is big
enough, it often does.
Most of the time, the motivation for mapping pointers is that
the objects have identity (and are often polymorphic). I've yet
to see a case where you'd map pointers with a value oriented
object, which can be copied.
(Note that if I understand correctly, objects created with
emplace in node based containers, like std::map, will not need
to be copiable.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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