[ fromfile: qtcore.xml id: qtcore ]

Qt is a large library consisting of several smaller libraries, or modules. The most popular ones are

Except for core and gui, these modules need to be "enabled" in qmake project files. For example:

QT += xml  # to use the xml module
QT -= gui  # to not use QWidgets
QT += sql  # to use SQL module

Section 3.2.1 will introduce some of the core library classes.