TTreeBranch

TTreeBranch is an object used in the ROOT data analysis framework to represent a branch of a TTree. It stores data for a single variable or a collection of variables that can be read or written during I/O operations. Each TTreeBranch is identified by a name and a title, and optionally by a branch identifier.

The branch can hold primitive data types such as int, float, double, or complex types like arrays, vectors, or class instances that are compiled as ROOT classes. To read data from a TTreeBranch one typically sets a pointer to a local variable using the Branch method, and then calls GetEntry to load the data for a particular event. Writing to a branch follows a similar procedure: the Branch method establishes the connection and PostData writes the local variable back into the TTree.

Branch metadata includes leaf lists, compression level, and split level. The split level controls how ROOT organizes nested objects within a branch, allowing efficient I/O for complex data structures. ROOT also provides methods such as SetAddress, SetBranchAddress, and SetBranchStatus to manage memory and control processing of the branch.

When constructing TTrees, developers create branches via the TTree::Branch or TTree::BranchC methods for C-style structs. The framework automatically synchronizes branch addresses during the I/O cycle, ensuring that the data recovered matches the original structure. This tight coupling between branch definitions and data storage makes TTreeBranch a central element in ROOT’s event data handling.