Re: efficiency of map::operator[] and map-insert

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++.moderated

Date:

3 Jun 2006 10:13:16 -0400

Message-ID:

<4eccu7F1eel6qU1@individual.net>

<gast128@hotmail.com> skrev i meddelandet
news:1149176899.591167.293850@u72g2000cwu.googlegroups.com...

Dear all,

item 24 of Scott Meyers (excellent) effective STL book is about the
performance difference of operator[] compared to map insert. In
short
he warns about the possible performance loss of operator[] due to an
extra copy.

Our code uses a lot of this constructs and therefore I profiled his
efficient update algorithm with a std::map<int, std::sting>.

I come to the following profile results (Visual Studio 2003 release
builds):

only adds:
operator[] 15.1
Scott 12.5
(+/- 20%)

dominating inserts:
operator[] 26.4
Scott 25.0
(+/- 4%)

Thus my guess will be that indeed only a very heavy object with
dominating adds in a container will make a difference.

I belive the reason is that the C++ committee has decided to change
the specification of operator[]

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#334

and this change is already implemented in the VS 2003 library.

Bo Persson

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