Difference between global vs private forward declarations in pimpl for the end user of the pimpl

Question

Consider the following two ways of implementing the pimpl idiom:

// file  g_visible.h 

//global forward declarations
class HiddenStuff_A;
class HiddenStuff_B;

class g_visible
{
   public:

      // API goodies for the end-user

   private:
      std::unique_ptr< HiddenStuff_A >  hs_a;
      std::unique_ptr< HiddenStuff_B >  hs_b;
}

and

// file  p_visible.h 

class p_visible
{
   public:

      // API goodies for the end-user

   private:
      // private forward declarations
      class HiddenStuff_A;
      class HiddenStuff_B;
      std::unique_ptr< HiddenStuff_A >  hs_a;
      std::unique_ptr< HiddenStuff_B >  hs_b;
}

Question: For the end-user -- i.e., for the developer who would include one of the [gp]_visible.h files -- what would be difference between including g_visible.h or p_visible.h?

Answer 1

The difference is that they have forward declarations of the two "Hidden" classes in global scope (or if your code put g_visible in a namespace, then whatever namespace that happened to be).

Unnecessary namespace pollution. The practical consequences:

• they can't use the same identifier again in that namespace, whereas I tend to use Impl for all pImpl classes
• their IDE's completion list wastes space on identifiers they'll never want to pick.
• client code could do things like define variables or function arguments of HiddenStuff_A* or ``HiddenStuff_B&type, but unlessg_visibleprovides a factory orHiddenStuff_X`'s definition's included in the translation unit it can't actually get variables to point at or call with

In short, not having the Impl declarations private is a poor choice.