Add a section to godot-cpp explaining core types and where to find their documentation.

Author: Ivorforce

engine_details/architecture/index.rst

@@ -1,5 +1,7 @@
 :allow_comments: False
 
+.. _doc_engine_architecture:
+
 Engine architecture
 ===================
 

tutorials/scripting/cpp/core_types.rst

@@ -0,0 +1,63 @@
+.. _doc_godot_cpp_core_types:
+
+Core functions and types
+========================
+
+godot-cpp's API is designed to be as similar as possible to Godot's internal API.
+
+This means that, in general, you can use the :ref:`Engine details <doc_engine_architecture>` section to learn how to
+work with godot-cpp. In addition, it can often be useful to browse the `engine's code <https://github.com/godotengine/godot>`__
+for examples for how to work with Godot's API.
+
+That being said, there are some differences to be aware of, which are documented here.
+
+Common functions and macros
+---------------------------
+
+Please refer to :ref:`doc_common_engine_methods_and_macros` for information on this. The functions and macros documented
+there are also available in godot-cpp.
+
+Core types
+----------
+
+Godot's :ref:`Core types <doc_core_types>` are also available in godot-cpp, and the same recommendations apply
+as described in that article. The types are regularly synchronized with the Godot codebase.
+
+In your own code, you can also use `C++ STL types <https://en.cppreference.com/w/cpp/container.html>`__, or types from
+any library you choose, but they won't be compatible with Godot's APIs.
+
+Packed arrays
+~~~~~~~~~~~~~
+
+While in Godot, the ``Packed*Array`` types are aliases of ``Vector``, in godot-cpp, they're their own types, using the
+Godot bindings. This is because these types, along with their methods, are exposed through the GDExtension interface,
+which would not be compatible with the templated nature of ``Vector``.
+
+In general though, the ``Packed*Array`` types work the same way as their ``Vector`` aliases. However, you should be
+aware that the ``Variant`` wrappers for ``Packed*Array`` treat them as pass-by-reference, while the ``Packed*Array``
+types themselves are pass-by-value (implemented as copy-on-write).
+
+In addition, it may be of interest that GDScript calls use the ``Variant`` call interface: Any ``Packed*Array``
+arguments to your functions will be passed in a ``Variant``, and unpacked from there. This can create copies of the
+types, so the argument you receive may be a copy of the argument that the function was called with. In practice, this
+means you cannot rely on that the argument passed to you can be modified at the caller's site.
+
+Variant class
+-------------
+
+Please refer to :ref:`doc_variant_class` to learn about how to work with ``Variant``.
+
+Most importantly, you should be aware that all functions exposed through the GDExtension API must be compatible with
+``Variant``.
+
+Object class
+------------
+
+Please refer to :ref:`doc_object_class` to learn how to register and work with your own ``Object`` types.
+
+We are not aware of any major differences between the godot-cpp ``Object`` API and Godot's internal ``Object`` API,
+except that some methods are available in Godot's internal API that are not available in godot-cpp.
+
+You should be aware that the pointer to your godot-cpp ``Object`` is different from the pointer that Godot uses
+internally. This is because the godot-cpp version is an extension instance, allocated separately from the original
+``Object``. However, in practice, this difference is usually not noticeable.

tutorials/scripting/cpp/index.rst

@@ -15,4 +15,5 @@ the official C++ GDExtension bindings maintained as part of the Godot project.
    about_godot_cpp
    gdextension_cpp_example
    build_system/index
+   core_types
    gdextension_docs_system