How to structure explicit instantiation of a template for compilation speed?

Question

I am specifically asking about file structure and where to put what. Consider the following (this is an example of my current structure):

foo.h consists of:

template <typaname T>
class Foo {
  void bar();
}

#include "foo-inl.h"

foo-inl.h consists of:

template<typaneme T>
void Foo::bar() {
}

Some baz.cpp has:

#include "foo.h"

Foo<X> foo;

and some other nom_test.cpp has:

#include "foo.h"

Foo<TestY> foo;

How do I restructure this to take advantage of the explicit instantiation in both the prod code (baz.cpp) and test code (nom_test.cpp). Mind, I wouldn't like to expose the test types to the prod build.

[using-extern-template-c11](https://stackoverflow.com/questions/8130602/using-extern-template-c11) might interest you. — Jarod42, Jan 02 '20 at 16:24
Is `Foo` only occurring in `baz.cpp` or also in other files? — rustyx, Jan 02 '20 at 20:29
Related: https://stackoverflow.com/questions/2351148/explicit-template-instantiation-when-is-it-used — Ciro Santilli OurBigBook.com, Apr 30 '20 at 09:29

score 0 · Answer 1 · answered Jan 02 '20 at 20:45

foo.h:

template <typename T>
class Foo {
  void bar();
}
#include "foo-inl.h"

extern template class Foo<X>; // means "Foo<X> is instantiated elsewhere"

foo-impl.cpp:

#include "foo.h"

template class Foo<X>; // instantiates Foo<X> here

baz.cpp:

#include "foo.h"
// can simply use Foo<X>, the impl will be linked from foo-impl.o

Similarly for the test build, foo-impl-test.cpp would contain:

#include "foo.h"

template class Foo<TestY>;

Keep in mind though that the semantics of explicitly instantiated templates is different - they are no longer inline! For this reason the usefulness of explicitly instantiated templates is limited. C++20 modules solve this in an entirely different and much more flexible way, so it might be worth waiting for that instead.

Potentially, `#include "foo-inl.h"` might be moved into cpp foo-impl.cpp and foo-impl-test.cpp — Jarod42, Jan 02 '20 at 22:28
Yes, if you're sure you explicit-instantiate *all* possible `T`s. — rustyx, Jan 02 '20 at 22:54

How to structure explicit instantiation of a template for compilation speed?

1 Answers1