How to read / write DICOMDIR using DCMTK

Quoted from the Reading DICOMDIR using dcmtk - Unix Linux Forum - Fixunix.com

class DcmDicomDir is what you are looking for (see
dcmdata/include/dcdicdir.h). This class allows you to read a DICOMDIR
file and to browse through the logical structure (patient - study -
series - instance) of the directory records.

Use DcmDicomDir::getRootRecord() to get access to the root directory
record, then use methods of class DcmDirectoryRecord to walk through
the tree.

DicomDirInterface is, as the documentation suggests, only a helper
class (a wrapper around class DcmDicomDir) that simplifies the task
of creating or extending a DICOMDIR from a number of DICOM files.

We can read and write the DICOMDIR file by using DcmDicomDir class.
This class strictly compliant to DICOM specification, so we need to care especially the "Transfer syntax" and "File name".