Re: [Boost-users] [Assign] reservation and insert efficiency

Subject: Re: [Boost-users] [Assign] reservation and insert efficiency
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-07-06 16:23:07

> > know to get the total length first, or will it keep re-allocating
> storage with each underlying insertion?  That is, do I get a
> performance benifit from manually doing:
> >
> > std::vector<record_type> newvec;
> > newvec.reserve (... add up the sizes of my three ranges ...)
> > newvec +=
> range(RecVec.begin(),db).range(newdata).range(de,RecVec.end());
> >
> > Hmm, I'm sure the last line would be somewhat different.
> > Anyway, do I need to reserve first, or will it be reasonably smart
> about that?
> 
> list_of() is not particularly fast: it uses a deque to store the
> elements that are being inserted. If you are into speed, look at
> ref_list_of/cref_list_of (which is very fast), or wait till the new
> version arrives with new utilities.
> 

I see.  So, with cref_list_of I would need to give a maximum number to the template, which is compile time!  There really isn't a reasonable limit on the size of the resulting list.

Is new stuff helpful to me, that I should look at becoming an early adopter?  Or should I forget about it?  Three separate range push_back's would not be too bad.  std::vector doesn't have range-based push-back, but I'm supposing that this is supplied with the range library?

But I still need to count up the total size to reserve ahead of time, for best efficiency.  What I need is a push_back that takes a list of ranges that are easy to specify, or be overloaded to some reasonable number of range arguments.  Anything like that hanging around already?

--John





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