Missing virtual functions

Missing virtual functions

[Eric Britten]

Yesterday I found that if one declares virtual functions in a class
like:

class A
{
    A();
    virtual void B();
};

and the definitions for the functions(inline or external) are not found
by the linker then two types of linking errors result.

1.  Undefined reference to a vtable in the constructors of the class.
2.  Undefined reference to any other inline functions of the class that
are used by other code.

It took me a while to figure out that if all virtual functions are
defined, then these errors go away.  Is this GNU specific?  It would
help if the linker complained of undefined references to the virtual
function without a definition, rather than the missing vtable.  Of
course this would help those unfamiliar with this error, I already know
the cause.

Eric Britten

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Re: Missing virtual functions

[Fergus Henderson]

This is an off-topic answer to an off-topic question about GNU C++.

Eric Britten <ebritten@uci.edu> writes:

>Yesterday I found that if one declares virtual functions in a class
>like:
>
>class A
>{
>    A();
>    virtual void B();
>};
>
>and the definitions for the functions(inline or external) are not found
>by the linker then two types of linking errors result.
>
>1.  Undefined reference to a vtable in the constructors of the class.
>2.  Undefined reference to any other inline functions of the class that
>are used by other code.
>
>It took me a while to figure out that if all virtual functions are
>defined, then these errors go away.  Is this GNU specific?

The exact error messages you get are probably specific to GNU C++.
However, the fact that you do get errors if virtual functions
are not defined is not GNU specific.  The GNU behaviour conforms
to the draft ANSI/ISO C++ standard, which says that programs must
contain a definition of every virtual function.  (Implementations
are not _required_ to diagnose this error, however, so such code
_may_ work on other compilers...)

--
Fergus Henderson <fjh@cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: < http://www.cs.mu.oz.au/~fjh >   |  of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3         |     -- the last words of T. S. Garp.
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