Detecting whether the -opt flag was passed to cppwinrt.exe: Using __has_include

I was upgrading the Window UWP Samples repo to take advantage of the new -opt flag introduced in C++/WinRT 2.0. This provides performance improvements for accessing static class members, and avoids having to register the type in your manifest for strictly in-module consumption.

The new -opt flag enables these optimizations, but it also adds a new requirement: Your implementation file needs to #include <ClassName.g.cpp>. The problem is that I wanted to upgrade the samples one at a time, but that meant that the shared files needed to support both optimized and unoptimized builds, at least until I get them all converted.

I was at a bit of a loss, because there was no obvious #define in winrt/base.h that tells me whether the -opt flag was passed.

And then I realized: I could use __has_include.

C++17 introduced the __has_include preprocessor keyword which snoops around to determine whether a header file exists. The idea is that you could conditionalize based on whether an optional header file is present. For example, you might check for the presence of xmmintrin.h and conditionally enable SSE operations.

In my case, I wouldn’t be probing for a system header file, but rather for a generated .g.cpp file produce by cppwinrt.exe in -opt mode.

#if __has_include(<MainPage.g.cpp>)
#include <MainPage.g.cpp>
#endif

If cppwinrt.exe were run with the -opt flag, then the MainPage.g.cpp file will exist in the Generated Files directory, and I can include it. If it were run without the -opt flag, then the MainPage.g.cpp file will not exist, and I skip over it.