Re: base classes and inheritance problem

From:
Greg Herlihy <greghe@mac.com>
Newsgroups:
comp.lang.c++
Date:
Sun, 16 Mar 2008 02:25:16 -0700 (PDT)
Message-ID:
<5eb14851-01c5-471f-9777-3c0c3de98a9f@e6g2000prf.googlegroups.com>
On Mar 15, 11:32 pm, "Alf P. Steinbach" <al...@start.no> wrote:

* zion...@gmail.com:

Hello, considering this code:

class A {
   protected:
   int n;
};

class B : public A {
   public:
   void setn(int k){ n = k;}
};

int main()
{
   A *c;
   c = new B();
   c->setn(10);

   return 0;
}

This of course will not compile, i'm trying to avoid using virtual
functions in class A and i was wondering if there exist a way of
achieving something like this code.


Since you're wondering about that, and also since you're considering leavi=

ng a

data member uninitialized for some time, and also since you're considering=

 a

setter function for that data member, chances are near 100% that you're
relatively (understatement) new to the language and have embarked on a jou=

rney

of Evil Premature Optimization, which, if you continue, will lead to much =

grief.

What do your measurements say about the impact of virtual functions?

Aha, no measurements!

Well, there you are, it's a case of severe, evil premature optimization.

Here's one way to do things more properly:

   class A
   {
   private:
       int myN;
   public:
       A( int n ): myN( n ) {}
       int n() const { return myN; }
   };

   class B: public A
   {
   public:
       B( int n ): A( n ) {}
   };

   int main()
   {
       B c; // And yes, that's it, all you have to do.
   }


In the above program, "B" objects cannot be default-constructed, so
"c"'s declaration will not compile. Possible solutions include:
initializing "c" with an int value:

     B c(-3);

or, providing a default argument to B's constructor:

     B( int n = 5): A( n ) {}

Greg

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