Separating public interface from implementation details

Question

I have to design a Font class that will have multiple implementations spanning platforms or different libraries (Win32 GDI or FreeType for example). So essentially there will be single shared header/interface file and multiple .cpp implementations (selected during build time). I would prefer to keep the public interface (header file) clean from any implementation details, but this usually is hard to achieve. A font object has to drag some sort of a private state - like a handle in GDI, or FreeType face object internally.

In C++, what is the best way to track private implementation details? Should I use static data in implementation files?

Edit: Found this great article on the subject: Separating Interface and Implementation in C++.

p.s. I recall in Objective-C there are private categories that allow you to define a class extension in your private implementation file making quite elegant solution.

Answer 1

You can use the PIMPL design pattern.

This is basically where your object holds a pointer to the actual platform dependent part and passes all platform dependent calls through to this object.

Font.h

class FontPlatform;
class Font
{
   public:
       Font();
       void Stuff();
       void StuffPlatform();
   private:
       FontPlatform*  plat;
};

Font.cpp

#include "Font.h"
#if Win
#include "FontWindows.h"
#else
#error "Not implemented on non Win platforms"
#endif

Font::Font()
  : plat(new PLATFORM_SPCIFIC_FONT)  // DO this cleaner by using factory.
{}

void Font::Stuff()  { /* DoStuff */ }
void Font::StuffPlatform()
{
    plat->doStuffPlat();  // Call the platform specific code.
}