C++ Single class definition

[Bunny]

In the project I'm currently working on I define a class that only inherits from a parent class and takes one argument. Does this class need to be defined in the header or source file? I read different answers around the internet.
Or is it better to always split definition and logica, even for something like an operator?

[tcs]

A class specification describes the interface to objects of this class. If you'll need your classes specification anywhere else than in its implementation file, then you'll better put it into a header file. Otherwise there's no way to describe other classes your interface.

Its common to use one header and one implementation file for each class.