Re: How to convert Pointer between structs?

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sun, 22 Feb 2009 22:20:47 +0100
Message-ID:
<gnsffn$dbr$1@news.motzarella.org>
* Immortal Nephi:

I want to show you my example like polymorphism. Three
subclasses as Center share data member of base class as Top.


OK.

 I do not
have to use static data member. It may look like multiple diamond.


Yes, it is a diamond inheritance pattern.

Only Bottom sub-class is able to access three subclasses as Center. I
wonder why member functions can't access between Center1 subglass and
Center2 subclass so it has to go through Bottom subclass.


It's unclear what you mean.

class Top
{
public:
    Top() : integer(0) { cout << "Top()" << endl; }
    ~Top() { cout << "~Top()" << endl; }

    void Run() { cout << "Top::Run()" << endl; }

protected:
    int integer;
};

class Center1 : virtual public Top
{
public:
    Center1() { cout << "Center1()" << endl; }
    ~Center1() { cout << "~Center1()" << endl; }

    void Run() { integer += 0x1; cout << "Center1::Run() = " << hex <<
integer << endl; }
};

class Center2 : virtual public Top
{
public:
    Center2() { cout << "Center2()" << endl; }
    ~Center2() { cout << "~Center2()" << endl; }

    void Run() { integer += 0x20; cout << "Center2::Run() = " << hex <<
integer << endl; }
};

class Center3 : virtual public Top
{
public:
    Center3() { cout << "Center3()" << endl; }
    ~Center3() { cout << "~Center3()" << endl; }

    void Run() { integer += 0x400; cout << "Center2::Run() = " << hex <<
integer << endl; }
};

class Bottom : public Center1, public Center2, public Center3
{
public:
    Bottom() { cout << "Bottom()" << endl; }
    ~Bottom() { cout << "~Bottom()" << endl; }

// void Run() { cout << "Bottom::Run()" << endl; Center1::Run();
Center2::Run(); Center3::Run(); }
    void (Bottom::*pRun[3])();
};

int main(void)
{
    Bottom b;

    b.pRun[0] = &Center1::Run;
    b.pRun[1] = &Center2::Run;
    b.pRun[2] = &Center3::Run;

This is very dangerous because correct operation of a Bottom instance here
depends on the client code doing correct external initialization. Instead you
should do such initialization in the class' constructor. And if the ability to
change the assignments is desired, provide also that via safe methods.

     (b.*(b.pRun[0]))();
    (b.*(b.pRun[1]))();
    (b.*(b.pRun[2]))();

This is like having a good working car at hand and instead of driving it as it's
meant to (see below), delving into the engine and trying to make the car go
forward by hand-cranking the starter motor, trying all kinds of ingenious
reconnections of the starter motor wires in order to get rid of that
inexplicable resistance...

     b.~Bottom();

Don't invoke destructors explicitly. It's also done automatically. And at that
point (at the 'return' below) you have Undefined Behavior; you'd have to
terminate the program right here in order to avoid the UB.

     return 0;
}

Output to Screen
Top()
Center1()
Center2()
Center3()
Bottom()
Center1::Run() = 1
Center2::Run() = 21
Center3::Run() = 421
~Bottom()
~Center3()
~Center2()
~Center1()
~Top()


Try

     int main()
     {
      Bottom b;

         b.Center1::Run();
         b.Center2::Run();
         b.Center3::Run();
     }

By the way, which book are you using that doesn't discuss this?

It can be helpful to others to know about that book, to avoid it.


Yes, I am novice to OOP, but I have great knowledge of procedural
programming of C. I do self-study for personal use. I use C++
Practice book.


Uhm, author? I can't find it on Amazon, nor in ACCU reviews.

Cheers & hth.,

- Alf

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