Write/Fill Hook for (split) TObjects in a tree

ROOT
#1

Hi all,

I have a class storing variables that also have functions defining some derived value vased upon this variable, e.g.:

class MyClass : public TObject {
  public:
    double MyVar; // a variable
    double GetDerivedValue() const 
    {
        // perform some calculation based on MyVar
        ...
    }
};

The problem is, if this is stored in a tree and then one calls the draw command:

  tree->Draw("MyClass.GetDerivedClass()");

this can be very slow because the entire object must be pulled in (of course) to properly call the function.

My question is the following: Is it possible to somehow have a Hook called for this class when it’s Filled in a tree (and/or written to a file) that calculates cached variables in protected variables?

For example:

class MyClass : public TObject {
  public:
    double MyVar; // a variable
    double GetDerivedValue() const 
    {
        // perform some calculation based on MyVar
        ...
    }
    
    virtual void SomeHook() 
    {
       MyDerivedValue = GetDerivedValue();
    } 
  protected:
    double MyDerivedValue; // cached value
};
 MyClass* my = new MyClass();
 tree->Branch("MyBranch", my);
 my->MyVar = 1.;
 tree->Fill(); // Somehow calls MyClass::SomeHook() to fill the protected variables

I’ve had a look in the code to see if there are any hooks called before a Fill for a TObject, but I can’t find any. Does anyone have any ideas of how one might do this? The advantage would be of course to be able to have fast access to this value in a TTree::Draw command.

I of course understand this could be done manually, but ensuring this is done could be relatively error prone.

#2

Hi,

Why do you want/need the hook to be at the time of Fill? It would seems simpler to put it at the time MyVar is modified.:class MyClass : public TObject { protected: double MyVar; // a variable double MyDerivedValue; // cached value public: double GetDerivedValue() const { return MyDerivedValue; } double GetMyVar() const { return MyVar; } double SetMyVar(double val) { MyVar = val; MyDerivedValue = ....; } };

Cheers,
Philippe.