[usefulcat]

Even without that restriction it's not that easy. It generally comes down to a choice between:

a) #include <vector> -- Lots of dependencies, but I know it works and I don't want to reinvent the wheel

b) #include <my_vector.h> -- Ok, I'm going to reinvent the wheel, but at least my wheel might have slightly fewer dependencies.

c) //#include <vector> // I'll just do without the wheel entirely

[maccard]

With ue4 that's not really the problem, it mostly falls under category b). The problem is that a bunch of stuff has unneeded module dependencies that hide the real dependency tree. Say a.h includes b.h but doesn't use anything from module B, but b.h includes SomeFundamentalHeader.h which a does need. The choice here is leave it alone or fix the dependency but break any user code that relies on the same behaviour. I did just that in a few modules that were new and off by default but good luck making a change like that in any commonly used modules.