Question

I’m using MSVC 9.0 and have this function:

class RecipientList
{
public:
    template<class T>
    void fillMessageWithRecipients( typename boost::is_base_of<MsgContent, T>::type* msg );
};

template< class T >
void RecipientList::fillMessageWithRecipients( typename boost::is_base_of<MsgContent, T>::type* msg )
{
 // do stuff
}

I want template type deduction to work here, so I can use it like this:

class SomeMsg : public MsgContent {};

std::auto_ptr<SomeMsg> msg( new SomeMsg );

RecipientList recipients;
recipients.fillMessageWithRecipients( msg.get() );

However I get the compiler error:

error C2783: ‘void
RecipientList::fillMessageWithRecipients(boost::is_base_of::type
*)’ : could not deduce template argument for ‘T’

I have a feeling this has something to do with the fact that the type actually being passed in is a pointer type, and not the type by itself. Any idea how I can properly get type deduction working here?

Thanks in advance.

Answer 1

I have the feeling you are misusing boost::is_base_of. The nested type will be either true_type or false_type. Neither of those two make sense to take as an argument and your pointers will not be convertible to those.

What you really want:

#include <boost/type_traits/is_base_of.hpp>
#include <boost/utility/enable_if.hpp>

class MsgContent {};

class RecipientList
{
public:
    template<class T>
    typename boost::enable_if<
        typename boost::is_base_of<MsgContent, T>::type
      , void>::type
    fillMessageWithRecipients(T* t) { }
};

class SomeMsg : public MsgContent {};

int main()
{
  RecipientList recipients;
  SomeMsg m;
  recipients.fillMessageWithRecipients( &m );

  return 0;
}