Default template parameters with forward declaration

Question

Is it possible to forward declare a class that uses default arguments without specifying or knowing those arguments?

For example, I would like to declare a boo

Answer 1

I don't think you can forward declare a template with default arguments unless the library in question provided its own forward declaration header. This is because you can't respecify the default arguments (even if they match... gcc will still report “error: redefinition of default argument”).

So to the best of my knowledge the solution is for the library to supply a forward declaration header Foo_fwd.h:

#ifndef INCLUDED_Foo_fwd_h_
#define INCLUDED_Foo_fwd_h_
template class Foo; // default U=char up here
#endif

and then the full implementation in Foo.h would be:

#ifndef INCLUDED_Foo_h_
#define INCLUDED_Foo_h_
#include "Foo_fwd.h"
template class Foo { /*...*/ }; // note no U=char here
#endif

So now your code could use Foo_fwd.h as well... but unfortunately, since this approach requires modifying the original Foo.h to remove the default arguments this doesn't scale to 3rd party libraries. Maybe we should lobby the C++0x crew to allow equivalent respecification of default template arguments, à la typedefs...?