Hiding use of class inside a library - c++

Question

I am creating a simple library for Arduino, the intent of which is to wrap and hide another class inside of it. I would like to be able to hide the underlying class entirely from the end user.
I have somewhat simplified this example to keep things clear, but the same problem exists.
The class I am trying to wrap is the Wire library which is called TwoWire.
The header file of the wrapper library:
#ifndef __DERIVEDONEWIRE2_H__
#define __DERIVEDONEWIRE2_H__
#include <Wire.h>
class DerivedWire
{
private:
TwoWire wire;
public:
DerivedWire();
};
#endif
The CPP file:
#include "DerivedWire.h"
DerivedWire::DerivedWire()
{
}
And the Arduino sketch that uses it:
#include <DerivedWire.h>
DerivedWire derivedWire;
void setup()
{
}
void loop()
{
}
This fails to compile:
/Users/andrew/Documents/Arduino/libraries/DerivedWire/DerivedWire.h:9:
error: 'TwoWire' does not name a type
Including Wire.h at the top of the main sketch allows this to compile:
#include <DerivedWire.h>
#include <Wire.h>
DerivedWire derivedWire;
void setup()
{
}
void loop()
{
}
However I would like to completely hide from the user of the DerivedWire library that the Wire library/TwoWire class is used at all.
I have tried changing the line "TwoWire wire;" to "TwoWire* wire;" - now I get the following error:
/Users/andrew/Documents/Arduino/libraries/DerivedWire/DerivedWire.h:9:
error: ISO C++ forbids declaration of 'TwoWire' with no type
/Users/andrew/Documents/Arduino/libraries/DerivedWire/DerivedWire.h:9:
error: expected ';' before '*' token
I've tried a few other things, but it just seems that I need to include Wire.h at the top level.
How do I fix this?

Answer 1

The Wire instance is created on the last line in the lib.
https://github.com/lstoll/arduino-libraries/blob/master/Wire/Wire.cpp
You are NOT supposed to have multiple instances of TwoWire, and there is no reason to hide it, but if you really want to hide it, you have to remove that line.
The arduino build method requires that the lib is included from the main sketch so you can not hide that part without signifigant changes.
If you really really need to hide this Wire instance, you could copy the source of the Wire lib into your own lib, and remove that last line.

Answer 2

Check out the "pimpl idiom", here's a link to get you started: http://en.wikipedia.org/wiki/Opaque_pointer
Pimpl idiom is a way in C++ to hide a class implementation, and all functions that you do not want to be visible.