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example: register the shared classes in both modules, as Windows requires #114

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jll63 merged 1 commit into from
Sep 16, 2026
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example: register the shared classes in both modules, as Windows requires #114

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1 change: 1 addition & 0 deletions doc/modules/ROOT/examples/shared_libs/implicit_linking/main.cpp
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Expand Up @@ -16,6 +16,7 @@ using namespace boost::openmethod::aliases;
struct Tiger : Carnivore {};

BOOST_OPENMETHOD_CLASSES(Tiger, Carnivore);
BOOST_OPENMETHOD_CLASSES(Animal, Herbivore, Cow, Carnivore, Wolf);

BOOST_OPENMETHOD_OVERRIDE(
meet, (virtual_ptr<Herbivore> a, virtual_ptr<Carnivore> b), std::string) {
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15 changes: 14 additions & 1 deletion doc/modules/ROOT/pages/shared_libraries.adoc
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Expand Up @@ -119,7 +119,8 @@ include::{shared}/implicit_linking/extensions.cpp[tag=content]
----

The program extends it. It adds a class the library has never heard of, two
overriders that specialise `meet`, and imports the state through the header:
overriders that specialise `meet`, and imports the state through the header. It
also registers the library's classes a second time - see below:

[source,c++]
----
Expand All @@ -131,6 +132,18 @@ the library's `Animal, Animal` overrider, so control crosses the module boundary
in the middle of a single dispatch, and `cow meets wolf` prints
`do not greet, run`.

The program registers `Animal`, `Herbivore`, `Cow`, `Carnivore` and `Wolf`,
although the library has registered them already. That is the rule
cpp:use_classes[] states: a class must be registered in as many translation
units as it has type ids. On ELF and Mach-O it has one - the dynamic linker
merges the `type_info` objects, and the second registration is redundant but
harmless. Windows keeps one `type_info` per module, so the program's `Wolf` and
the library's `Wolf` are two different type ids, and the program creates its
`Wolf` with `new`. Without the second registration the program's id is unknown
to the registry, and the first dispatch on it fails with `unknown class Wolf`.
Registering in both modules is what makes the example portable, and costs
nothing where it is not needed.

## Dynamic Linking

By "dynamic linking", we mean a program loading a shared library after it has
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