Hot Reloading C++

When writing game code in C++, the most annoying aspect are the iteration times. Due to how C++ works, it is nearly impossible to swap out code and replace it with a newer version. Some engines try to do this, but there are always limitations, and the effort to get this working and keep it from breaking is quite big.

Plasma Engine is no different here, reloading code at runtime is not possible. However, the editor is split into two processes: The actual editor process which displays the UI, and manages the scene state, and the engine process which does the scene rendering and executes the actual game code. This separation makes the editor more resilient to crashes.

To recover from the crash you can click the viewport. When you click the button, the editor launches a new engine process, synchronizes the latest scene state over and continues as if nothing happened.

That of course means, that when the new engine process launches, it also loads the latest state of all plugins. So if any of the plugins was modified, we would now see these modifications. So by simply nuking and restarting the entire engine process, we can get some form of C++ code hot reloading.

Unfortunately, you can't compile a plugin while a process is using it, because the process prevents other applications from writing the DLL. We can solve this problem, by making a copy of our game plugin DLL, and loading that instead. That means that the original DLL is not actually used, and our IDE can modify it further.

This trick works quite well if we only do it for plugins that are loaded fully dynamic, meaning that no other plugin tries to link against it. If that were the case, both the original plugin, as well as the copied plugin would get loaded, which is not good. Therefore only select game plugins may use this copy mechanic, which is why you need to set this up manually.