TTreeReaderValue

TTreeReaderValue is a class in the ROOT data analysis framework used for reading leaf values from TTrees and TNtuples. It is part of the TTreeReader facility, which provides a simplified, stream‑based approach to data access compared to the traditional GetEntry method. A TTreeReaderValue object is templated on the leaf data type, enabling compile‑time type safety for the values retrieved from a TTree.

When a TTreeReaderValue is constructed, it takes a reference to a TTreeReader and the name of the leaf to be accessed. Internally, it resolves the leaf address once upon construction and then reads the current entry's value whenever the object is dereferenced or used as a pointer. This mechanism eliminates the need for manual address binding for each branch and reduces boilerplate code in analytical scripts.

Typical usage involves creating a TTreeReader with the name of the tree and its associated TFile, then declaring one or more TTreeReaderValue instances for the desired leaves. Within a while loop that iterates over the reader, the leaf values can be accessed just like ordinary C++ variables, for example, (energy)->GetValue() for a double leaf named "energy". The reader automatically moves to the next entry on each loop iteration.

TTreeReaderValue is particularly convenient in interactive ROOT sessions, CINT/Cling macros, and compiled analysis code. It offers a modern, type‑safe alternative to the older TTree::SetBranchAddress pattern, while maintaining performance comparable to the legacy approach because the address resolution occurs only once.