C++ namespaces and defining classes in separate files

I want to make a namespace that will contain several classes as part of a "package".

Do I have to declare all of the classes within the namespace?

For example, if I have a "2dEngine.h" which defines the 2dEngine namespace, do I have to declare all of the individual classes within that header file? Or can I still separate them into separate header (.h) files and have them be part of the namespace?

Pseudo example:

TwoEngine.h

namespace TwoEngine{    class Canvas    {        // Define all of Canvas here    };    class Primitive    {        // Define all of Primitive here    };}

Instead of doing that, I want to have Canvas and Primitive be their own .h files and just somehow state that they are part of that namespace.

Sorry, I'm still pretty new to this.