Doubt regarding 'Find Files' example - No forward declaration of class?

[norobro]

The Google C++ Style Guide (link) has some good information.

Here's what it says about forward declarations:

How can we use a class Foo in a header file without access to its definition?

We can declare data members of type Foo* or Foo&.
We can declare (but not define) functions with arguments, and/or return values, of type Foo. (One exception is if an argument Foo or const Foo& has a non-explicit, one-argument constructor, in which case we need the full definition to support automatic type conversion.)
We can declare static data members of type Foo. This is because static data members are defined outside the class definition.

On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data member of type Foo.

[tbscope]

One golden tip I can give is:
Don't rely on include files including other files.
Always try to explicitly include the header or forward declaration of any class you use.

In that respect, and my personal opinion, the example isn't 100% correct.

[Lykurg]

You surely find all header files on your disk, where you have installed Qt. That should be the fastest and easiest why to find the files.