Re: cannot instantiate abstract class
"PaulH" <paul.heil@gmail.com> wrote in message
news:1191002477.582837.117380@g4g2000hsf.googlegroups.com...
I have a c++ applicaiton (as below) and I'm getting a linker error
that has me stumped. error C2259: 'myspace::PolicyImpl<T>': cannot
instantiate abstrace class.
If it's obvious to anybody what I'm doing wrong, please let me know.
Thanks,
PaulH
// MyClass.h
namespace myspace {
template< typename T, template< typename > class ClassPolicy =
Policy >
class ClassA
{
public:
virtual void Update( const T& hint ) = 0;
};
template < typename T >
class Policy
{
public:
virtual void RemoveAll( ClassA< T, myspace::Policy > *p ) = 0;
virtual void Add( ClassB< T, myspace::Policy > *p ) = 0;
In the derived class you define void Add( ClassB< T, myspace::PolicyImpl >
*p )
Note the mismatch between parameters, you end up hiding the base class
version instead of overridding. So the derived class is missing a
definition of the base class function and is therefore abstract as well.
Covariance in overriding is only allowed for the return type, and you don't
have covariance here anyway.
However, ClassA doesn't appear to need a second template parameter. It is
perfectly legal for the derived type ClassAImpl to have more template
parameters than its base class.
virtual void Remove( ClassB< T, myspace::Policy > *p ) = 0;
};
}
// MyClassImpl.h
namespace myspace {
template< typename T, template< typename > class ClassPolicy =
PolicyImpl >
This should not work as PolicyImpl needs a template parameter.
class ClassAImpl : public ClassA< T, ClassPolicy >
{
public:
virtual ~ClassAImpl()
{
//...
}
void Update( const T& hint )
{
//...
}
private:
ClassPolicy< T > classpolicy_; // ***linker ERROR C2259
here***
};
template < typename T >
class PolicyImpl : public Policy< T >
{
public:
PolicyImpl(){};
void RemoveAll( ClassA< T, myspace::PolicyImpl > *p )
{
//...
}
void Add( ClassB< T, myspace::PolicyImpl > *p )
{
//...
}
void Remove( ClassB< T, myspace::PolicyImpl > *p )
{
//...
}
private:
//...
};
}
The full text of the error message is:
1>MyClassImpl.h(45) : error C2259: 'myspace::PolicyImpl<T>' : cannot
instantiate abstract class
1> with
1> [
1> T=ClientConnectMsg
1> ]
1> due to following members:
1> 'void
myspace::Policy<T>::RemoveAll(myspace::ClassA<T,ClassPolicy> *)' : is
abstract
1> with
1> [
1> T=ClientConnectMsg,
1> ClassPolicy=myspace::Policy
1> ]
1> ClassA.h(43) : see declaration of
'myspace::Policy<T>::RemoveAll'
1> with
1> [
1> T=ClientConnectMsg
1> ]
1> MyClassImpl.h(60) : see reference to class template
instantiation 'myspace::ClassAImpl<T,ClassPolicy>' being compiled
1> with
1> [
1> T=ClientConnectMsg,
1> ClassPolicy=myspace::PolicyImpl
1> ]
1> MyClassImpl.h(57) : while compiling class template member
function 'myspace::ClassBImpl<T>::~ClassBImpl(void)'
1> with
1> [
1> T=ClientConnectMsg
1> ]
1> UsingClass.h(61) : see reference to class template
instantiation 'myspace::ClassBImpl<T>' being compiled
1> with
1> [
1> T=ClientConnectMsg
1> ]