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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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tim@franck.Princeton.EDU (Tim Hollebeek) wrote:

>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();
>}

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.

- Don


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