Include in header or cpp - what is preferred in Qt

Question

When I write a C++ class I write includes in header only is specific cases. I prefer forward declaration and I believe that is best practice in C++. But Qt classes often have so many fields that are standard Qt classes that writing them all in forward declaration seems like not a very good idea. Is there some preferred way? Qt has huge documentation, so I think that it has answer to my question, but I just started reading it.

Answer 1

My rule is for a header file to include only those files that define functionality that is used by the header file in question.

Suppose you have pointers and references to some class Foo in a header but the header never pokes into or instantiates one those objects. In this case, you don't need the definition of the class. All you need is a forward declaration.

On the other hand, a forward declaration does not suffice if you poke into an object of type Foo, or if you have a data member or variable of type Foo. Now you do need the full definition, so now it is time to #include the header file that defines class Foo. (Alternatively, if the use is in an inline function definition, you may want to rethink that inlining and put the implementation in a separate source file.)

Answer 2

A header file just says "what we can do". The cpp file says "We can do it this way and that is our intention".