Problems using STL vectors in trees with TTree::Draw()

From: Nathaniel Tagg <tagg_at_minos.phy.tufts.edu>
Date: Fri, 23 Feb 2007 08:32:33 -0500

Hi ROOTers,

I'm trying to use STL vectors in trees. Vectors are by FAR easier to work with than TClonesArrays, so I want to use them in our code. However, the resulting TTrees do not work with TTree::Draw commands. I'm using ROOT 5.13 and 5.14 for my tests.

Problem (1)

---

If I branch a tree like the following:

template<class T>
class Vector : public std::vector<T>, public TObject
{

  ....
}

Vector<Thing>* a;
tree->Branch("branch","Vector<Thing>",&a);

I make sure that Thing and Vector<Thing> are correctly built with rootcint. The resulting root tree, however, is pretty ugly:

tree->Print() throws errors of Error in <TStreamerInfo::BuildOld>: Could not find STL base class: vector<Thing> for Vector<Thing>

tree->Draw commands are not simple. If I have two vectors with overlapping leaf names, then there is no way to specify which tree I want to draw. That is,
tree->Draw("branch.mElement") doesn't work.

What does work is
tree->Draw("vector<Thing>.mElement")
but it gives inconsistent results! Subsequent draw commands, like tree->Draw("vector<Thing>.mElement - 10") will give different plots!

Problem (2)

---

I read somewhere on roottalk that top-level vectors aren't very good, so I tried this instead:

template <class T>
class Vector : public TObject
{

...
private:

        std::vector<T> mV
};

However, this gives a similar problem... you cannot address specific branches on the tree.

tree->Draw("branch.mV.mElement");
Error in <TTreeFormula::Compile>: Bad numerical expression : "branch.mV.mElement"
tree->Draw("branch.mV[0].mElement");
Error in <TTreeFormula::Compile>: Bad numerical expression : "branch.mV[0].mElement"

This is useless if you have two Vector objects with similarly-named mElement leaves.

Problem(3)

---

I attempted to get around this by writing a custom streamer for my object. However, the custom streamer was never called when I did TTree::Fill(). Instead, ROOT defaulted to it's standard STL steamers. This is an undocumented behaviour.

In all of these problems, I THINK it's because I have my splitlevel turned up quite high. I would like to keep this since it allows much faster data retrieval.

Any advice what I should try?

--Nathaniel Tagg Received on Fri Feb 23 2007 - 14:32:17 CET