Earlier today I livestreamed what it would take to build a small part of the recently open sourced O3DE game engine. The attempt did not get very far, so here is a followup. It should not be considered exhaustive in any way, it is literally just me poking the code for a few hours and writing down what was discovered.
The code can be obtained from the upstream repo. There's not much more you can do with it since it does not actually build on Linux (tested on latest Fedora) but instead errors out with a non-helpful CMake error message.
When I finally managed to compile something, the console practically drowned in compiler warnings. These ranged from the common (missing virtual destructors in interface classes) to the bizarre (superfluous pragma pops that come from "somewhere" due to macros).
The code explicitly only supports Visual Studio and Clang. It errors out when trying to build with GCC. Looking through the code it seems like it is mostly a case of adding some defines. I tried that but pretty quickly ran into page-long error messages. A person with more knowledge of the inner workings of GCC could probably make it work with moderate effort.
O3DE is based on CryEngine, which predates C++ 11. One place where this shows up is that rather than using threading functionality in the standard library they have their own thread, mutex, etc classes that are implemented with either pthread or Windows threads. There may be some specific use cases (thread affinity?) why you'd need to scoop to plain primitives but otherwise this seem like legacy stuff nobody has gotten around to cleaning.
Yes, there is a string class. Several, in fact. But you already knew that.
This is where things get weird. The code uses, for example, RapidXML and RapidJSON. For some reason I could not get them to work even though I used the exact same versions that are listed in the CMake definition. After a fair bit of head scratching things eventually became clear. For example the system has its own header for RapidXML called rapidxml.h whose contents are roughly the following:
Upstream does not provide its header in a rapidxml subdirectory, it is under include. The same happens when the header is installed on the system. Thus the include as given can not work. Even more importantly, the upstream header is not named rapidxml.h but instead rapidxml.hpp.
It turns out that O3DE has its own dependency system which takes upstream source, makes arbitrary changes to it, builds it and then is provided as a "package" which is basically a zip file with headers and prebuilt static libs. These are downloaded from not-at-all-suspicous-looking URLs like this one. What changes are done to these packages is not readily apparent. There are two different repos with info but they don't seem to have everything.
When using external libraries like this there are two typically two choices. Either you patch the original to "fit in" with the rest of your code or you can write a very small adapter wrapper. This project does both and with preprocessor macros no less.
The whole dependency system seems to basically be a reimplementation of Conan using pure CMake. If that sentence on its own does not make cold sweat run down your back then let it be noted that one of the dependencies obtained in this way is OpenSSL.
The way the system is set up prevents you from building the project using system dependencies. This includes Qt as the editor GUI is written with it. Neither can you build the entire project from source yourself because the existing source only works with its own special prebuilt libraries and the changes applied do not seem to be readily available as patches.
Most of this is probably because CryEngine was originally written for internal use on Windows only. This sort of an approach works for that use case but not all that well for a multiplatform open source project.