Hum, it’s a little difficult to explain, especially in the title…

Let’s say I have the following files: main.cpp, domain.cpp, domain.hpp, generated.hpp and generator.cpp. domain.cpp includes domain.hpp, which in turn includes generated.hpp. I want to build two executables:

• generator, consisting of generator.cpp and domain.cpp
• main, consisting of main.cpp and domain.cpp

Here’s the twist. Initially, generated.hpp is empty. Before building main, I want to build generator (if needed), and always run it before building main. generator calculates stuff related to domains.cpp, and rewrites generated.hpp – but only if its content would be different. Thus, running generator for the first time will rewrite the empty generated.hpp; running it again immediately afterwards will not touch generated.hpp; and running generator after making changes to the domain.?pp files may or may not rewrite generated.hpp.

Thus, there is a cycle because generator depends on its output, but it will be traversed only once.

Moreover, generator has to run before ninja/make/whatever starts examining the dependencies of main.

Can it be achieved in plain cmake, without playing tricks with scripts?

It would be nice if the generated generated.hpp goes in the build area.

The files are in C++17, thus solutions that involve __has_include are acceptable. But I doubt it can help.

PS In case you wonder about the context, it’s for my YOMM2 open multi-methods library. generator calculates offsets in v-tables, and writes them as constexpr offsets. If present, they are used to speed up method dispatch.