Template class forward declaration [duplicate]

Question

This question already has answers here:

How to forward declare a template class in namespace std? (4 answers)

Closed 8 years ago.

I am forward declaring a template outer and inner class as follows

template<class T>
class outer;

class inner;

Just after the above declaration I have a boost::serialization declaration defined as

namespace boost
{
    namespace serialization
    {
         template<class Archive> 
         void serialize(Archive &ar, outer &var, ...) { }
    }

}

Outer is a template class and thus requires specification of template arguments. If I attempt to do so as follows

...
     void serialize(... outer<T> &var ... ) { }
...

this is an error as only one template declaration is allowed. What is the proper way to define such a forward declaration?

Answer 1

Try using two template parameters:

namespace boost
{
    namespace serialization
    {
         template<class Archive, class T> 
         void serialize(Archive &ar, outer<T>& var, ...) { }
    }

}

Answer 2

I might have misunderstood, but can you not do this:

template <class Archive, typename T>
void serialize( Archive archive, out<T> &var, ... );