Re: How to hide implementation details?

From:
Michael Doubez <michael.doubez@free.fr>
Newsgroups:
comp.lang.c++
Date:
Thu, 27 May 2010 06:38:33 -0700 (PDT)
Message-ID:
<feb1947c-15fa-4837-a11a-914195ba8371@y12g2000vbg.googlegroups.com>
On 26 mai, 04:33, Immortal Nephi <Immortal_Ne...@hotmail.com> wrote:

        I want to create an interface of a door. The client is=

 able to see

the implementation behind the interface because inheritance is
present. The code is stored in the header. How do I hide the
implementation?


I assume you mean that a specific implementation of the interface is
exposed.

You could look into creational patterns. For C++ there is the
compilation firewall idiom that allows you to expose an implementation
but hide the nitty gritty.

        A door is an abstraction of a particular arrangement of a=

 rectangular

piece of material with hinges and a doorknob. It has four routines of
Open(), Close(), isOpened(), and isClosed().


It could also be ajar :)

        And the doorknob is in turn an abstraction of a particula=

r formation

of brass, nickel, iron, or steel. It has four routine of Lock(),
Unlock(), IsLocked(), and IsUnlocked().
        And also, Color_Knob is in turn an abstraction of some co=

lors.

        How do I put three objects of Door, Knob, and Color_Knob =

together

into a function or another class. Is it the way how design looks
good?


[snip]

Redecomposing your design you have two abstraction:
  - a Door (which could be a specific Gate abstraction) which handle a
state (opened/closed) and cab be operated (open/close). But
eventually, the closing or opening could fail. Keeping as close as
possible to your design, I could write it (with robust interface not
failing if by example I try to open door already opened):

class Door
{
  public:
    Door(bool is_open = false):m_is_open{}
    virtual ~Door() {}

    // return true upon success
    bool Open()
    {if(!m_is_open)m_is_open = tryOpenDoor();
     return m_is_open; }
    // return true upon success
    bool Close()
    {if(m_is_open)m_is_open = !tryCloseDoor();
     return !m_is_open; }
    bool IsOpened() const{ return m_is_open; }

  protected: // real door implement opening/closing condition
    // return true upon success
    // pre Door is closed
    virtual bool tryOpenDoor()=0;
    // return true upon success
    // pre Door is opened
    virtual bool tryCloseDoor()=0;

private:
        bool m_is_open;
};

Now, as others pointed out, for normal people a knob is not a door.
And it doesn't necessarily have a state. The only thing you know is
that is you want to open with it, it can grant you or not to open the
door. Lets keep it simple:
struct Knob
{
    virtual ~Knob(){}
    // return true upon success
    virtual bool tryOpenWithKnob()=0;
    // return true upon success
    virtual bool tryCloseWithKnob()=0;
};

Now You can have a Knob with a Lock:
class KnobWithLock: public Knob
{
  public:
        KnobWithLock(bool is_locked)
          : Knob()
          , m_is_locked(is_locked){}

        bool Lock()
        { m_is_locked = true; // easy lock
          return m_is_locked;}
        bool Unlock()
        { m_is_locked = false; // easy unlock
          return m_is_locked;}
        bool IsLocked () { return m_is_locked; }

        virtual bool tryOpenWithKnob()
        { /* can open only if unlocked */
          return !m_is_locked; }
        virtual bool tryCloseWithKnob()
        { /* can always close */
          return true; }
private:
        bool m_is_locked;
};

Now if you want to make a door with a knob:

class DoorWithKnob: public Door
{
  public:
   DoorWithKnob(Knob* knob = NULL):m_knob(knob){}

  Knob* knob()const{return m_knob;}

  protected:
    virtual bool tryOpenDoor()
    { // can open if no knob
      return m_knob ? m_knob->tryOpenWithKnob() : true;
    }
    virtual bool tryCloseDoor()=0;
    { // can close if no knob
      return m_knob ? m_knob->tryCloseWithKnob() : true;
    }

  private:
    Knob* m_knob;
};

Now, concerning the colored knob. The color is in fact an attribute of
a Knob that doesn't modify it. You could use a kind of decoration:

template<class TKnobType>
class ColoredKnob: TKnobType
{
  // ... code to handle color of knob
};

And you main program is:
int main()
{
  ColoredKnob<KnobWithLock> knob;
  DoorWithKnob door(&knob);

  knob.Select_Brass();

  knob.Unlock();
  door.Open();
  knob.Lock();
  door.Close();

  return 0;
}

--
Michael

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