Is C still being widely used in game engines?
First and foremost I must admit that I'm not a games developer, even though I have developed a fully functional 3D game engine in the past.
That aside, I do have a few words about optimizations, "spoiling" languages and so on.
When developing an application — any application — the golden rule of optimizations is "don't optimize before you make it work." You need to fully support all the functionality you want in your application, and only then you should optimize it. The reasons vary; the most important one is that while you may believe something is "slow," it may not be your bottleneck. You may be spending a lot of time optimizing something that doesn't require optimization. Furthermore, optimizations often blur your code simplicity and maintainability, which may lead to bugs in the near or far future. If this piece of code didn't require optimizations, you've complicated the code for nothing.
While that is the golden rule of optimizations, there is one big exception. When you know in advance that your application will be stressed out to the max, you need to make some different choices in the beginning (in the architecture or in the design phases) and also along the way. Also, the general rule that the platforms are getting better and faster doesn't apply for games, where the competition between the developers is on the very edge of technology. Here are several points to consider:
- Some lingual features may be costly. Exceptions in C++ are a good example.
- Some lingual features may actually save you code and will cost little or nothing during runtime. C++ templates are a good example, though you still may wind up creating lots of code during compilation, leading to a bigger binary, and therefore lower performance, potentially.
- Infrastructures (STL for example) are generic. By making a solution specific to your domain, you MAY be improving your performance. A simple example — if you have a vector that will always be 3 items long, you will definitely be better than the stl::vector implementation. However, this is not always the case. For further reading, see chapter 11 of "Efficient C++" by Dov Bulka. This is an excellent book in general for your question.
In general, using C with classes is a good start for writing fast code while retaining a structured and well-designed project, but one should not neglect the whole of C++ excellent features. More often than not, trying to roll out your own solution to a problem covered by C++ (e.g., polymorphism) will be slower than the out-of-the-box solution, unless you can make the solution VERY specific to the problem at hand.
C++ without STL
As I have seen it in my professional history, what is mostly used in game development is not C with classes, but rather C++ without STL. Some parts of STL are considered too slow for a reasonable game engine performance, most notably the streams. Other parts provide reasonable general performance, but the way memory allocation is handled is mostly unacceptable for games - this applies to string and vector libraries in most scenarios. Excellent extensive discussion of this can be found in EASTL white paper. Game developers frequently use boost or even implement their own alternative to part of whole of STL (see Game STL or EASTL).
No exceptions
There is one particular language feature which is never used in any performance critical engine parts - the exception handling. This is because on the most important game development platform (Visual Studio x86/x64) the exceptions are quite costly, and you pay some cost even when no exceptions are hit. The exceptions are avoided into the extent some development console platforms even do not provide them, or the support is known to be incomplete, unreliable and basically "broken".
Used to C
Other than that, it often happens that game programmers use C instead of C++ simply because they are used to it.