Re: Smart Pointer Issue

James Kanze <>
Fri, 13 Feb 2009 00:54:28 -0800 (PST)
On Feb 12, 1:37 pm, wrote:

Hello, I have the following classes:
class ServiceBase
                ServiceBase() {}
                ~ServiceBase() {}
                void Stop() {}

class ServiceDerivate:public ServiceBase
                ServiceDerivate() {}
                ~ServiceDerivate() {}

And I have linked smart pointer class which, does not destroy,
but calls Stop method on its destruction:

template <class X> class ServicePtr



Since ServiceBase and ServiceDerivated have to be dynamic
castable i wish I could use a pool of ServiceBase classes, and
upon request to spawn new ServicePtr holding dynamic_cast to

I'm not sure I understand. In your example, there are no
virtual functions, so the classes aren't polymorphic. I
presume, however, that this is just an oversight in the example.
And of course, you don't need dynamic_cast to convert from
ServiceDerivate to ServiceBase; that's an implicit conversion.

Sipmle example:

int main()
ServicePtr<ServiceBase> sbase;
ServicePtr<ServiceDerivate> sderivate;


return 0;

which, produces:
"no matching function for call to
note: candidates are: ServicePtr<X>& ServicePtr<X>::operator=(const
ServicePtr<X>&) [with X = ServiceDerivate]

It also doesn't make sense unless sbase actually points to a
ServiceDerivate. If you want to support this, with dynamic
checking, you can add something like the following to your

    template< typename T >
    ServicePtr& operator=( ServicePtr< T > const& other )
        X* tmp = dynamic_cast< X* >( other.get() ) ;
        // rest of processing, however...

In this case, I'd probably derive ServicePtr from a non-template
ServicePtrBase which contained and handled the links.
Otherwise, you'll have to make all instances of the template
friends of the other instances. (As it stands,
ServicePtr< ServiceBase > cannot access any of the members of a
ServicePtr< ServiceDerivate >.)

Can anybody help with this. Probably I am missing something.
I tryied to overload:
 ServicePtr& operator=(const ServicePtr<ServiceBase>& r)
but I did not compiled either.

What was the error message? And what did the code look like?

As mentionned above, you're likely to have problems with
accessing private data members; each instantiation of the
template is a separate, unrelated class. Moving the list
handling down to a common, non-template base class should solve
that. (It also has the advantage that you can get it out of the
header files, reducing their size.)

James Kanze (GABI Software)
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