The first three generations of the Unreal Engine included a sandboxed scripting language, UnrealScript, which provided a simple interface for gameplay programming that was shielded from the complexity of the C++ engine.
The scripting approach is very welcoming to new programmers, but eventually it breaks down and becomes an obstacle to innovation and shipping. We experienced this over time as the Unreal Engine grew until finally, in 2011, we moved to a pure C++ architecture. The causative factors were both pervasive and general:
As an engine and its community grows, there is increasing pressure to expose more of the its native C++ features to the scripting environment. What starts out as a sandbox full of toys eventually grows into a desert of complexity and duplication.
As the script interface expands, there is a seemingly exponential increase in the cost and complexity of its interoperability or “interop” layer where C++ and script code communicate through a multi-language interface for calling functions and marshaling data. Interop becomes very tricky for advanced data types such as containers where standard scripting-language idioms differ greatly in representation and semantics from their templated C++ counterparts.
Developers seeking to take advantage of the engine’s native C++ features end up dividing their code unnaturally between the script world and the C++ world, with significant development time lost in this Interop Hell.
Developers need to look at program behavior holistically, but quickly find that script debugging tools and C++ debugging tools are separate and incompatible. Seeing where script code had gone wrong is of little value if you can’t trace the C++ that code led to it, and vice-versa.
It is these reasons, ultimately, that led to Epic’s move to pure C++. And the benefits are numerous: UE4 is a unified and fully-debuggable code base, freed from Interop Hell and totally open to programmers to study, modify, and extend. There are side-benefits, too, such as increased performance in gameplay code, and ease of integrating other middleware written in C++.
Building Unreal Engine 4 as a unified C++ codebase has been very freeing, giving engine and gameplay programmers enormous flexibility to write code without unnecessary interop barriers.
By making peace with complexity and writing code in C++, there is absolutely no limit to what you can accomplish, whether it involves debugging your entire codebase in-context, interfacing to low-level engine systems, modifying them, or talking to the operating system or advanced third-party libraries.