Can I start with C++ modules instead of pre-compiled header?

[rnikander]

I'm just starting to learn Windows C++ development. When I create a project in Visual Studio, I like to set the C++ version to /std:c++latest, and I want to use modules whenever I can. I created a new project using the template: "Blank App, Packaged (WinUI 3 in Desktop)". I see it uses a pch.h precompiled header.

This page in MS docs claims that modules are an improvement over pre-compiled headers. Is there a way to replace the pch.h here and use modules instead? If I just turn off the setting, I think I see the compilation take a lot longer. I'm not sure how one would arrange files and imports to replace (if possible) the pch.h here.

[DarranRowe]

Unfortunately, no.
The closest you could currently get is header units, and the Windows headers tend to not be "well behaved". To get to the point of you being able to use modules in a WinUI 3 project, there would have to be modules for Windows and C++/WinRT.

[rnikander]

Okay, thanks. Hopefully I can use modules for my own code without having to wait forever for the entire Windows API to be using modules. I'm going to try and see how it goes.

I also thought I might be able to get a compilation speedup like a pre-compiled header by including things in a global fragment and re-exporting them, like...

module;
#include [windows api stuff]
export module windows;
export ... (re-export stuff from the headers that I want to use...)

Haven't tried it yet.

I got a different answer here: https://learn.microsoft.com/en-us/answers/questions/1435360/should-i-turn-off-pre-compiled-headers-if-im-using

But I don't have a lot of confidence that they are speaking from experience there...

[jaigak]

Also, note that officially there is no statement denying you cannot use both precompiled headers and modules. However, from my experience, it does not work properly if you refer to a type defined in a header that's included in PCH from a module.