Re: calling a virtual funktion from a parent class

From:
Peter Cranz <peter.cranz@novis-software.de>
Newsgroups:
comp.lang.c++
Date:
Wed, 05 Jul 2006 14:56:29 +0200
Message-ID:
<e8gctv$pbk$01$1@news.t-online.com>
Murali Krishna schrieb:

Peter Cranz wrote:
[code snip>

at implementation I'd like to call A::X::f() inside A::B::X::f()

I have tried the following (but it doesn't work .. can't get access of
protected element f() inside a::X <- confusing, its the parent ^^) :


<code snip>

How can I get access to the parental f()-function from inside the
childfunction-f()? I must use these namespaces and also the identical
classnames. :(


Hi Peter,

I know very little. I will tell what I know.

if you create in either identical namespaces or in different
namespaces, you are using virtual. That is more important.

namespace A {
    class X {
    protected:
// public:
        virtual void f() { cout << "A::X::f()" << endl; }
        void g() { cout << "A::X::g()" << endl; }
    };
}

namespace A {
    namespace B {
        class X : public A::X {
        public:
            void f() {
                cout << "A::B::X::f()" << endl;
                (this->*A::X::f)(); // will again call child class's fn
            }
            void g() {
                cout << "A::B::X::g()" << endl;
                (this->*A::X::g)(); // will call parent's fn
                // X::g(); // this will work if names of Parent and child are diff.
            }
        };
    }
}

int main(int argc, char* argv[])
{
    A::B::X xObj;
// xObj.f();
    xObj.g();

    return 0;
}

if you run this, you will get fn g() in derived as well as in base
class.
if you remove comments on xObj.f(), you will find repetitive calls of
Child's f() fn. This is because base class virtual table function
always points to it's child class over-ridden functions.

/* point out of scope */
if you use static_cast on this, you will not be able to access parent's
function because they are protected members.
so if you make them public, again child class's functions are called.

So finally I am not able to call virtual function of a base class "or"
can I say it is not possible?

-- Murali Krishna.


thank you murali,

I think, it's possible to call a protected virtual function of the
base-class from inside the "same" virtual function inside the
child-class. there are no difficulties, if base-class and child-class
has different names. it's no matter what namespace is used. so here is
an example of what I said:

  namespace A {
   class X {
   protected:
   virtual void f()
            {
               cout << "A::X::f()" << endl;
            }
        public:
            void print() {f();}
   };
  }

  namespace A {
      namespace B {
   class Y : public A::X {
   protected:
   virtual void f()
            {
               cout << "A::B::Y::f()" << endl;
               using namespace A;
               X::f();
            }
   };
      }
  }

.....
Y* y = new Y(); y->print(); // will print out the following:

 >A::B::Y::f()
 >A::X::f()

X* x = y; x->print(); // will print out the following:

 >A::X::f()

please correct me if I'm wrong.

My only problem is, that I do not know ... how to realize this with same
  classnames in different namespaces. so, maybe, I have an understanding
problem of how to use namespaces in c++ or is there a little
inconsistency? In my opinion, namespaces are supposed to be a delimiter
that makes programmers possible, to use same names for different things
in different namespaces. does this not match for inheritance?

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