Re: Undefined reference to...

From:

Leigh Johnston <leigh@i42.co.uk>

Newsgroups:

comp.lang.c++

Date:

Thu, 11 Nov 2010 17:22:39 +0000

Message-ID:

<TMCdnT9Gq5XAu0HRnZ2dnUVZ7tudnZ2d@giganews.com>

On 11/11/2010 16:58, Andrea Crotti wrote:

Leigh Johnston<leigh@i42.co.uk> writes:

On 11/11/2010 09:16, James Kanze wrote:

However, what you've just shown *is* the closest working
approximation of what he seems to be trying to do. Unless he
actually wants more than one instance---it's not really clear.
For more than one instance, he'd need a factory function, e.g.

      class Base
      {
      public:
          virtual void printOut() = 0;
          static std::auto_ptr<Base> getLower();
      };

      class Extended: public Base
      {
      public:
          void printOut() { cout<< "hello"; }
      };

      std::auto_ptr<Base> Base::getLower()
      {
          return std::auto_ptr<Base>( new Extended );
      }

This is UB as Base does not contain a virtual destructor.

/Leigh

Can't find anywhere what "UB" mean, but I guess something bad...
For understanding, why there should be a virtual destructor?

And in general, whenever inside the classes I'm creating I don't
allocate anything with "new", do I ever need a destructor?

Or you mean that the virtual must be present since otherwise one
subclass COULD have some memory leaks that could not be "closed" by the
auto_ptr??

"UB" means "undefined behaviour". If deleting via a base class pointer
(which is what std::auto_ptr will do in Mr Kanze's example) the base
class must have a virtual destructor.

Yes, a memory leak could occur if the derived class allocated an object
as its destructor would not be called is the base class destructor was
not virtual.

/Leigh