Unite 2010GDC ChinaAsia Game Show 2010GDC 2011
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Part 2: Inlining, and the inline keywordWhat is inlining? I'll answer that through the description of the inline keyword. It tells the compiler to "expand the function inline", which is a lot like the way that macros (#define's) work in C and C++, but with a small difference. Inline functions are type-safe, and subject to further compiler optimization. This is a REALLY good thing, because you'll have the speed of a macro (that means you won't suffer from function-call overhead ), with the type-safety of a function, with a bunch of other benefits. What are those benefits? Well, most compilers can only optimize code in a single module at a time. That's usually a single .h/.cpp combination. Using inline functions means that those functions will actually end up within the same module as the calling module, enabling certain optimizations, such as eliminating return-value copying, eliminating superfluous temporary variables, and a host of other possibilities. If you want to learn more, have a look at my references, specifically the performance C++ book, at the end of this article. The dreaded inline keyword. I have to mention this, because there seem to be a lot of misconceptions about it. The inline keyword does not force the compiler to inline that particular function, but rather gently requests it. Quoting from MSDN: "The inline keyword tells the compiler that inline expansion is preferred. However, the compiler can create a separate instance of the function (instantiate) and create standard calling linkages instead of inserting the code inline." Things that usually make the compiler ignore your request are: using loops in the inline function, calling other inline functions from within the function, and recursion. The above quote also hints at something else: the linkage for a function declared inline is internal. That means that your linker will choke on an inline function implemented in another object file, making them far less useful than I'd like. In human language, using ye olde declaration in .h file and implementation in .cpp file doesn’t work with inline functions, generally. The ANSI standard has a way to do it, but unfortunately, Visual C++ does not implement it. So, you ask, what is the solution? Simple, implement the inline functions within the same module. This is easy enough to do - just write the entire function in the .h file, and include it wherever you need to use the function. Not as clean as you might want, but it works. I don't like it much, in the interest of implementation hiding (I'm an Object Orientation freak), but I do use it in a lot of my classes lately. The good part is, I don't have to write in the inline keyword - if you write a function entirely within a class declaration, the compiler will automatically attempt to inline it. This way, I've ended up with entire classes contained in only a header file, since all of the functions needed to be inline. I'd suggest you ONLY do this when you really need the speed, and when you're not about to share the code with too many people - because the visibility of the implementation may lead to very annoying assumptions from the rest of your team. Trust me, it happens.
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