Converting a large mathematical software package written in C++ to C++20 modules

To be fair, C++’s modules make no sense, just like their namespaces that span multiple translation units.

It’s just more heavy clunky abstractions for the sake of abstractions.

Modules are an attempt to make part of the language what currently requires a convention:

- A component is a collection of related code.

- The component has an interface and an implementation.

- The interface is a header file (e.g. *.h) that is included (but at most once!) using a preprocessor directive in each dependent component.

- The header file contains only declarations, templates, and explicitly inline definitions.

- The implementation is one or more source files (e.g. *.cpp) that provide the definitions for what is declared in the header, and other unexposed implementation details.

- Component implementations are compiled separately (usually).

- The linker finds compiled definitions for everything a component depends upon, transitively, to produce the resulting program/dll.

So much can go wrong! If only there were a notion of components in the language itself. This way we could just write what we mean ("this is a component, here is what it exports, here are the definitions, here is what it imports"). Then compiler toolchains could implement it however they like, and hopefully optimize it.

It makes lots of sense to anyone used to large scale software development.

It is no accident that Ada, Java, .NET, and oldies like Delphi, Eiffel, Modula-2 and Modula-3 have similar approaches.

Even the way D and Python modules and packages work, or the whole crates and modules approach in Rust.

Naturally folks not used to Web scale don't get these kind of features.