Hi Aaron,
I understand that you may gain time in making this storage assumption.
However, this is not guaranteed to work on all compilers or with the
next version of the same compiler.
If you want to gain time, use TClonesArray instead.
Rene Brun
Aaron Dominguez wrote:
>
> Hi Walter and Rene,
>
> Instead of the for() loop for reading/writing the std::vector, I've been using
> ReadFastArray/WriteFastArray, like so:
>
> if (R__b.IsReading()) {
> int n;
> R__b >> n;
> fCoeff.resize(n);
> R__b.ReadFastArray(&fCoeff[0],n);
> }
> else {
> R__b.WriteVersion(MyClass::IsA());
> R__b << fCoeff.size();
> R__b.WriteFastArray(&fCoeff[0],fCoeff.size());
> }
>
> I'm pretty sure that this is ok since I believe that std::vectors are supposed
> to be contiguous in memory from beginning to end. At least this is working
> for us at CDF with ROOT v3.01/06a, KCC v4.0f or gcc v3. And we've always had
> great success speeding up streamers with Read/WriteFastArray.
>
> Cheers,
> Aaron.
>
> Rene Brun wrote:
>
> > Hi Walter,
> >
> > I have implemented your suggestion in rootcint. Now in CVS.
> >
> >
> > Walter F.J. Mueller wrote:
> > >
> > > Hi ROOTers,
> > >
> > > Currently (ROOT 3.02/03) the read part of an automatically generated
> > > streamer contains for reading of a STL vector code like
> > >
> > > fCoeff.clear();
> > > int R__i, R__n;
> > > R__b >> R__n;
> > > for (R__i = 0; R__i < R__n; R__i++) {
> > > double R__t;
> > > R__b >> R__t;
> > > fCoeff.push_back(R__t);
> > > }
> > >
> > > This works, but can cause an unnecessary amount of reallocations of the
> > > vector. Since the number of elements to be inserted is known up front
> > > this can be avoided by requesting enough capacity before the elements
> > > are inserted by calling the reserve() method ahead of time, like in
> > >
> > > fCoeff.clear();
> > > int R__i, R__n;
> > > R__b >> R__n;
> > > fCoeff.reserve(R__n);
> > > for (R__i = 0; R__i < R__n; R__i++) {
> > > ... as before ...
> > >
> > > This ensures that at most one reallocation is done when a vector is read.
> > > Only vectors and strings provide capacity management, so in practice this
> > > is only relevant for STL vectors and can't be applied to deque's ect.
> > >
> > > It be nice if rootcint would generate code like this. I haven't checked
> > > what the StreamerInfo based I/O does, but also there this might help.
> > >
> > > Cheers, Walter
> > >
This archive was generated by hypermail 2b29 : Tue Jan 01 2002 - 17:51:10 MET