TITLE: What is explicit instantiation for?

(Newsgroups: comp.std.c++, 27 Jul 98)

DOUGLASS: scott douglass <sdouglass%_%junk@_.arm.com>

> I'm trying to figure out what explicit instantiation if for.


OLIVA: Alexandre Oliva <oliva@dcc.unicamp.br>

There are two situations in which explicit instantiation is useful:

1) for programmers to control in which object file they intend a
certain symbol to be defined.  By carefully selecting places in which
to instantiate templates, it may be possible to reduce the
dependencies between modules of a program, causing some modules not to 
be needlessly included in an executable.

2) in order to allow templates to be defined separately from their
declarations, because, if the definition of a template is not provided
in every translation unit in which it is used, it must be explicitly
instantiated in some translation unit.  This is not equivalent to
exporting a template definition; exporting has to do with *all*
possible specializations of a template, whereas by explicitly
instantiating a template for a particular set of types, then hiding
its implementation, you can restrict the set of specializations that
can be used.

DOUGLASS:

> Can you give me an example of a program that won't work without
> explicit instantiation?

OLIVA:

The second item above, in which a template is used in several
translation units, but is defined in only one of them, is one such
case.

DOUGLASS:

> Is it legal to explicitly instantiate a template in one compilation
> unit and implicitly instantiate the same template in another?

OLIVA:

Yes.

DOUGLASS:

> Similarly, is it legal to explicitly specialize a template in one
> compilation unit and implicitly instantiate the same template in
> another?

OLIVA:

If you really meant `specialize' in the sentence above, then it is
illegal, because it violates the ODR, but no diagnostic is required.