Avoid Virtual Functions

Avoid Virtual Functions

Object-oriented programming purists will hate me for this section. C++ virtual functions are a wonderful incarnation of OOP and they can be beautiful and elegant. But you need to avoid them sometimes if speed is your goal.

They're also very fast function calls, even though done dynamically. Although virtual function calls seem like they're complicated and possibly slow, they're actually very carefully designed to be very fast to call in C++ class hierarchies. There's lots of painstaking work for compiler designers to get them to compile correctly, but their runtime efficiency is great for programmers. The implementation is effectively a small lookup table with function pointers. It's a couple more assembler statements before the function call, and the overhead of calling a function will dwarf that cost.

So, why do I say to review your use of virtual functions? Because they're an optimizer blocker. Since they're a dynamic runtime function call, there's much less opportunity for the C++ compile-time optimizations to remove these calls. Indeed, the compiler cannot always determine what function is being called and you can lose these speedups:

• inline functions
• constexpr function evaluation

Hence, I say you have to choose carefully in the use of virtual functions. Avoid them for speed-critical functions, and don't use them only for good OOP style if you don't really need them. But also, don't be afraid of using them in other instances because they're only marginally slower than a non-inlined function call. Kudos to the C++ language designers for that!

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