Using Internal Partitions

Our Core module is in code/Core.

The file Core/_Module.ixx contains:

export module Core;

export import :Attach;
export import :Container;
export import :CopyRegistry;
export import :Diagram;
export import :Element;
...

This is the interface of the Core module. The exported partitions are all external partitions. See also the blog posting “How a Module Should Look”.

We have an internal partition :UndoerImp in file Core/UndoerImp/UndoerImp.cppm :

module Core:UndoerImp;
...

The file UndoerImp.cppm declares a couple of classes for internal use in the implementation of the Core module.

Note that the internal partition :UndoerImp is not imported in _Module.ixx. Internal partitions are not meant to be imported in interfaces. They also cannot export anything.

To compile this file with the MSVC compiler, the /internalPartition flag must be set.

We separated definitions of functions declared in the :UndoerImp partition into a few cpp files. These files are all in the directory Core/UndoerImp:

>dir /b
PosUndoerGroup.cpp
SequenceUndoer.cpp
TransactionUndoer.cpp
UndoerImp.cppm

The file UndoerImp/SequenceUndoer.cpp contains:

module Core;
import :UndoerImp;
...

That’s a module implementation unit for the Core module. A module is the collection of all module units with the same name (Core).

Because we want to implement functions of the :UndoerImp partition, we need to import that partition. Readers who are not accustomed to how module implementation units work, might think that’s all we get there. But that’s not what happens.

The line module Core; not just declares that the file is a module unit of the Core module, but it also implicitly imports the whole interface of the Core module. That is, all the external partitions listed in the _Module.ixx file are pulled in as well. But we don’t need that.