boost::openmethod::registry_state

The single shared instance of a registry's state.

Synopsis

Declared in <boost/openmethod/preamble.hpp>

template<class Registry>
struct registry_state;

Description

registry_state is a thin, function‐free class whose only member, st, holds all of a registry's mutable state (of type detail::registry_state_type). Reach it through Registry::state().

It is deliberately a separate one‐member class, rather than registry_state_type itself, because sharing the state across a DLL boundary on Windows requires exporting and importing a whole class:

  • MSVC honors __declspec(dllexport/dllimport) on a class explicit instantiation, but NOT on a variable template (clients silently get a private copy) nor on a static‐data‐member instantiation (error C2720). So the exported symbol must be a class member reached via whole‐class instantiation.

  • dllexporting registry_state_type directly would also decorate its member functions and, transitively, the policies' nested state types, which MSVC rejects (error C2513).

A one‐member, function‐free class is the only shape MSVC will export as a whole and import via extern template.

To share the state across modules, use BOOST_OPENMETHOD_IMPORT_REGISTRY, BOOST_OPENMETHOD_EXPORT_REGISTRY and BOOST_OPENMETHOD_INSTANTIATE_REGISTRY. They hide a platform incompatibility: the export goes on the declaration on ELF and Mach‐O, but on the instantiation on declspec platforms, where extern and __declspec(dllexport) cannot be combined.

// header, every translation unit of a client module:
BOOST_OPENMETHOD_IMPORT_REGISTRY(boost::openmethod::default_registry);
// header, every translation unit of the owning module:
BOOST_OPENMETHOD_EXPORT_REGISTRY(boost::openmethod::default_registry);
// exactly one .cpp of the owning module:
BOOST_OPENMETHOD_INSTANTIATE_REGISTRY(boost::openmethod::default_registry);

See Also

Shared Libraries

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