NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/comp/lang/cplus/16814

Text File | 1992-11-24 | 1.7 KB | 49 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!The-Star.honeywell.com!umn.edu!atto.cs.umn.edu!hansen
From: [e-mail] (David M. Hansen)
Subject: Re: Calling pure virtual functions in base class constructor
Message-ID: <[e-mail]>
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Organization: University of Minnesota
References:  <[e-mail]>
Date: Tue, 24 Nov 1992 01:02:22 GMT
Lines: 36

In article <[e-mail]>, [e-mail] (Nikki Locke) writes:
|> Aha, a man who knows ! That's that one explained entirely to my
|> satisfaction.
|>
|> However, what I _really_ wanted to know is why, in a constructor or
|> destructor, you still have to specify the scope override operator to call
|> a pure virtual.
|>
|> Example :
|>
|> class A {
|> public:
|>     A() {
|>         pure_virtual();        // undefined behaviour - WHY ?
|>         A::pure_vurtual();    // O.K.
|>         }
|>     virtual void pure_virtual() = 0;
|>     };
|>
|> void A::pure_virtual()
|> {
|>     cout << "A::pure_virtual() called\n";
|> }
|>
|> What is the logical reason for this, then ?

At the bottom of p. 215 of the ARM it states, "The reason this makes technical
sense is that the explicit call A::f() avoids use of the virtual call
mechanism (S10.2) that cannot be used for a pure virtual function from within a
constructor.  If A::f() calls a pure virtual function for its object then it
in turn will encounter a disaster just as the constructor would have."

Note that this is valid only if the pure virtual function is defined.

                    -=Dave