C++ namespaces and defining classes in separate files

Question

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.

Solution 1

Yes, you can split the namespace into multiple blocks (and hence files). Your classes will belong to the same namespace as long as they are declared in the namespace block with the same name.

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

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

Solution 2

Namespaces can be discontiguous. You can take advantage of this by keeping relevant classes in your 2DEngine.h which probably is going to be used by client code and will be shipped as part of your library.

Anything else, that is not to be revealed to the outside world can still be put in the same namespace but in a separate header file (which is not shipped).

Header H1.h (part of the library interface to the external world)

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

Header H2.h (not part of the library interface to the external world)

#include "H1.h"
namespace TwoEngine      // reopen the namespace and extend it
{
    class Primitive 
    { 
        // Define all of Primitive here 
    }; 
}

Solution 3

Yes just use the name space directive inside the implementation files also.

Comments

[Gusev Slava]

For functions the same? and for nested namespaces?