Where to put the enum in a cpp program?

Question

If the enum is going to be used in more than one .cpp file, you should put it in a header file that will be included by each. If there's a common header file, you should use that, otherwise you may as well create a new header file for this enum

You should always attempt to limit the scope of types in C++, so the enum should probably be declaread at class scope. The enum will typically belong slightly more naturally in one class than the other - lets say class A, so you put it in the declaration of A in the a.h header:

// a.h

class A {
    public:
       enum Type { a, b };
    ...
};

Now you need to include a.h in the header that declares B:

// b.h
#include "a.h"

class B {
   public:
      void f( A::Type t );     // use the Type enum
   ...
};

I can see the point of Neil: it is a pet peeve for many programmers to see stuff on the global scope. otoh, imho, introducing a class just for an enum is not a good style: It is supposed to be enum not a class. However, putting the same enum list in both classes (is what you were asking) would be the worst idea: we don't want to be repeating stuff.

Moreover, in most non-trivial codes, one might end up using more of such shared entities (more enums, const parameters, etc...) for implementation. So, I'd begin lumping all this into an implementation namespace (say "detail") which is a child namespace of your classes, and resides in a separate header file (say "detail.hpp"), included by all. For example:

// file A.hpp 
#include "foo/detail.hpp"
namespace foo {
   class A 
   { 
   // accessing enum as detail::a 
   };
}

// file B.hpp
#include "foo/detail.hpp"
namespace foo { class B { ... }; }  

// file foo/detail.hpp
namespace foo { namespace detail {
   enum { a,b, ... }
   const int three = 3;
   // etc...
   // other implementation classes etc...
}}

And "detail" is nice and clean way of warning your class users to back off from whatever's declared in there. As your code gets bigger and these implementation details start growing in number you can break the dependencies into separate header files (detail1 detail2 etc...) and still keep one "detail" namespace (something which you can not do with a "class detail" for example).

The question is rather vague, but as a rule of thumb, you should try to minimize the redundancy in your code. Therefore, you should put the declaration of the enum to a header file.