What makes virtual functions so slow? C++

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Virtual functions and performance - C++

I'm trying to refactor my code, and everywhere people say that using virtual functions is a huuuuge nono performance-wise, why? and is there another way that I can inherit a class and redefine functions that are defined in the base class?

Do you actually have a performance issue? – David Rodríguez - dribeas Apr 19 '12 at 18:12 — David Rodríguez - dribeas, Apr 19 '12 at 18:12

score 3 · Accepted Answer · answered Apr 19 '12 at 18:00

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A good reference article for this topic can be found here: http://coldattic.info/shvedsky/pro/blogs/a-foo-walks-into-a-bar/posts/3

For the lazy, I guess the answer is "maybe slower slightly"

answered Apr 19 '12 at 18:00

Rob

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score 1 · Answer 2 · answered Apr 19 '12 at 17:58

Virtual functions are called through a vtable, which is basically an array of function pointers. So, every time one is called, there's an extra array lookup. I'm not sure if I'd call this a huuuuge nono though, in general they should be pretty fast.

From Wikipedia:

A virtual call requires at least an extra indexed dereference, and sometimes a "fixup" addition, compared to a non-virtual call, which is simply a jump to a compiled-in pointer. Therefore, calling virtual functions is inherently slower than calling non-virtual functions. An experiment done in 1996 indicates that approximately 6–13% of execution time is spent simply dispatching to the correct function, though the overhead can be as high as 50%.[4] The cost of virtual functions may not be so high on modern CPU architectures due to much larger caches and better branch prediction.

That's implementation defined, but most implementations do it that way. — Etienne de Martel, Apr 19 '12 at 17:59
I would also guess compilers have come a long way since 1996 :) — Mike Christensen, Apr 19 '12 at 18:04

What makes virtual functions so slow? C++

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