dynamic_library.md

Use metapp in dynamic library

Overview

We can use metapp in dynamic library. That's to say, we can expose meta data reflected in one dynamic library, then use the meta data in the main program, or in another dynamic library.

metapp is ready for use in dynamic library. The only thing we need to take care of is, we must give each meta type a unique typeKind. When comparing meta types in different modules (different dynamic libraries or program), metapp use typeKind to identify the meta type, so the typeKind must be unique.
When we use metapp within a single module (a single program or a single dynamic library, no crossing dynamic libraries), metapp compares meta types using a unique pointer, so whether typeKind is unique doesn't matter.

To assign each meta type a unique typeKind, we need to DeclareMetaType

Note on performance

When checking MetaType::equal across modules, such as the main program (module A) call Variant::get/cast or invoke callable that is from a dynamic library (module B), the performance can be much slower than doing the same action within a single module, especially when the related MetaType has a lot of up types or deep level nested up types.
The reason is, when checking MetaType::equal within a single module, the checking only requires up to two pairs of pointers comparison. But when doing it across modules, the checking needs to traverse through all up types recursively and compare the type kind.