The Array Mode Data Area

For access to the columns there is a data area:

void** data = ...;

This gets passed into the tool and into every function that needs to access any column data directly. To avoid passing it in explicitly everywhere it is also included inside every ObjectId and ObjectRange, meaning it is readily available everywhere an xAOD decoration (and with that a column) might be accessed.

It is ambiguous whether this ought to be a const qualified pointer or not. It contains input and output columns, so either way at some point somewhere a const_cast has to happen. However, this also generally happens in a safe manner inside the columnar infrastructure, and users will only ever experience non-const pointers if the input pointer was non-const.

The meaning of each entry is completely determined by the index, which is given a meaning during setup time. All type and access checks are also based on checking that index.

For convenience entry 0 is always guaranteed to be a nullptr, thereby guaranteeing that an uninitialized index will lead to a nullptr dereference and a segmentation fault (as opposed to undefined behavior). In practical use this should only occur if a user marks a column as optional, but doesn't check first whether the column exists; or if the user uses a default initialized (i.e. empty) column accessor.