Whenever possible, your code implementation should be in the cpp file, and
the declaration in the h file.
exceptions:
- template classes should go into the h file completely because there is no
other option.
- IDE generated classes like Window Forms classes. These are implemented in
h files because the wizard does not understand the concept of difference
between declaration and implementation.

The reasons for the split is that a change in the h file causes a
recompilation in all the cpp files that include it. you want to prevent as
much as possible.
It is also the worldwide standard to program this way,
And finally, If you build libraries you have to have a separate header file
with the declarations anyway.