Re: interface/implementation: export keyword?

[Nathan Myers <ncm at cygnus dot com>]

Jan Reimers wrote:

> > > > I'm trying to sensibly separate template definitions/declarations
> > > > for a project I'm writing in C++, as I believe (?) it's good 
> > > > practice to separate the interface from the implementation.
> > >
> > > The short answer is to put your template declarations in an .h file,
> > > and your template definitions in a .cxx (or .cc) file, and until
> > > egcs supports export, #include the .cxx file as well as the .h file
> > > when you use the templates.
> >
> > Thanks for the advice, but there's two reasons I didn't do this in the
> > first place: ...

It's better to conditionally include the template definitions in the 
headers directly, as:

  // foo.h
  #ifndef INCLUDED_FOO
  # define INCLUDED_FOO
  template <class T> void mangle(T*);
  # ifdef EXPORT_NOT_IMPLEMENTED
  #  define export
  #  include "bar.h" /* needed for mangle() definition */
  #  include "foo.tcc"
  # endif
  #endif

  // foo.tcc
  export template <class T>
    void mangle(T*) {}

This is the solution used in the libstdc++-v3 project.
When export is supported then you read the ".tcc" files
into your template repository.  In the meantime compilation
is slower because each compilation unit sees the whole 
definition.

> > Surely there must be an elegant way of doing this already, or does
> > nobody else use templates and require them to be space-efficient?
>
> I would guess that very few people don't use templates these days.  The
> standard is full of them, even istream and ostream are implemented with
> templates.

It's not a space-efficiency issue; it's a matter of convenience,
size of intermediate files, and compilation time.  The solution
to these is to implement export.

Nathan Myers
ncm@cantrip.org