Re: Fastest way to clone a hash_map

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 30 Apr 2008 04:13:32 CST

Message-ID:

<290420081945573667%cbarron413@adelphia.net>

In article
<beb9809a-53ab-49e7-b1b2-5867b39de49b@e39g2000hsf.googlegroups.com>,
devdude <rottyguy70@yahoo.com> wrote:

Hi,

I have the need to take a snapshot of a hash_map during execution
(actually transform it to a vector). This map is a shared resource
and therefore must be locked prior to any read/write operations thus I
need to minimize the amount of time the map resource is locked.

The map is defined as type <string, boost::shared_ptr<myobject>>. My
algorithm is as such:

void SnapShotToVector( vector< pair< string,
boost::shared_ptr<myobject> >& vec )
{

    lockResource(this->map);
    vec.resize( map.size() );
    copy(this->map.begin(), this->map.end(),list.begin());
    unlockResource(this->map);
}

      I will not consider mult-threas but the code above may
not be the best to accomplish the results.
    vec.clear();
    vec.reserve(map.size());
    copy(map.begin(),map.end(),back_inserter(vec));
is likely to be faster,
    further has_map::value_type most likely is a pair<
const string,shared_ptr<myobject> > not pair<
string,shared_ptr<myobject> > so storing hash_map::value_type
in vec is likely to avoid converting const string to string.
    thats 3M default ctors of hash_map::value_type not called and
3M conversions of pair<U1,U2> to pair<T1,T2> not performed.

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