In article <4gbilr$38m@dub-news-svc-1.compuserve.com> 71247.3221@compuserve.com (Don Wallace) writes:

[example of derived class calling pure virtual function in base class]

> I suspect that this is perfectly legal C++ code. Every statement taken
> individually is correct. But even if it linked, the net effect of
> calling bar.foo() would be to attempt to activate a pure virtual
> function (In Borland C++ this generates a RTL program halt - it says
> 'pure virtual function called'.)
>
> The statement/function
>
> >    int foo() { return Foo::foo(); }
>
> refers to Foo::foo() which is inherently not defined. Hence the link
> error.

This is a common misconception: a pure virtual function may or may
not have a definition. It it doesn't have a definition and you try to
call it anyway, you will probably get a link-time- or run-time error or
your program could just go haywire; if it does have a definition, then
calling it is perfectly OK. The canonical example of a pure virtual
function that does have a definition is a pure virtual destructor:
without a body, such a class would not be very useful.

- Wil

Wil Evers, ITT Publitec Research and Development, <wil@ittpub.nl>
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Is the following legal C++ code?  g++ compiles it, but it doesn't link
(undefined function Foo::foo()).  I'm suprised it compiles at all:

class Foo {
public:
    virtual int foo() = 0;
};

class Bar : public Foo {
public:
    int foo() { return Foo::foo(); }
};

int main() {
    Bar bar;
    bar.foo();
}

---------------------------------------------------------------------------
Tim Hollebeek      | Disclaimer :=> Everything above is a true statement,
<space for rent>   |                for sufficiently false values of true.
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-------------------| http://wfn-shop.princeton.edu/~tim (NEW! IMPROVED!)
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tim@franck.Princeton.EDU (Tim Hollebeek) writes:

>Is the following legal C++ code?  g++ compiles it, but it doesn't link
>(undefined function Foo::foo()).

No, the following code is not legal, because you don't have a definition
for the pure virtual function Foo::foo().  But if you add one, e.g.

 int Foo::foo() { return 42; }

then the code is quite legal.

>I'm suprised it compiles at all:

It has always been legal in C++ to provide a definition for pure
virtual functions, although the only way to invoke such a function is
by using an explicit qualification.

Yes, this suprised me too ;-)  I think it would have been simpler to
disallow definitions of pure virtual functions.

--
Fergus Henderson              WWW: http://www.cs.mu.oz.au/~fjh
fjh@cs.mu.oz.au               PGP: finger fjh@128.250.37.3
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