Class member function vs function with respect to speed

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Virtual Functions and Performance C++

Is this correct, that class member function takes more time than a simple function? What if inheritance and virtual functions are used?

I have tried to collect my functions into a simple interface class (only member functions, no data members), and it appears that I lose time. Is there a way to fix it?

P.S. I'm checking with gcc and icc compilers and using -O3 option.

No. I use a simple linux timer. struct timeval tp; gettimeofday(&tp, 0); return tp.tv_sec + (tp.tv_usec)*1.e-6; — klm123, Dec 12 '11 at 13:10
If you want an answer to the question of why something is slower, post two pieces of code, one faster and one slower, and hopefully someone will know. Asking us to validate if one of your guesses as to the problem is correct absent any code is fruitless. The question, as stated, is interesting, but not necessarily relevant to your real question. — Omnifarious, Dec 12 '11 at 14:52

score 9 · Accepted Answer · edited Jun 20 '20 at 09:12

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Premature optimization is the root of all evil

A nonstatic member function takes an additional argument, which is the object (a pointer or reference to it) on which the function is called. This is one overhead. If the function is virtual, then there is also one small indirection in case of a polymorphic call, that is, addition of the function index to the virtual table base offset. Both of these "overheads" are sooo negligable, you shouldn't worry about it unless the profiler says this is your bottleneck. Which it most likely is not.

Premature optimization is the root of all evil

edited Jun 20 '20 at 09:12

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answered Dec 12 '11 at 13:02

Armen Tsirunyan

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Optimizer will take care of that. – Puppy Dec 12 '11 at 13:02
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A polymorphic function call yields an indirection, not the virtuality of the function per se. – Sebastian Mach Dec 12 '11 at 13:18
@phresnel: Note taken, edited – Armen Tsirunyan Dec 12 '11 at 14:58
Upvoted. As far as I know, the standard does not mandate that non-polymorphic calls are implemented non-polymorphically, but g++ 4.x at least shows no indirection in such case. – Sebastian Mach Dec 12 '11 at 15:06
@phresnel: the standard doesn't talk about performance at all... so we are naturally talking about existing implementations – Armen Tsirunyan Dec 12 '11 at 15:11

Nawaz · Answer 2 · 2011-12-12T14:05:08.127

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Member functions, if they're not virtual, are same as free functions. There is no overhead in its invocation.

However, in case of virtual member functions, there is overhead, as it involves indirection, and even then, it is slower when you call the virtual function through pointer or reference (that is called polymorphic call). Otherwise, there is no difference if the call is not polymorphic.

edited Dec 12 '11 at 14:05

answered Dec 12 '11 at 13:01

Nawaz

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In case of polymorphic calls that is. On a value type, g++ shows me no difference and no overhead. – Sebastian Mach Dec 12 '11 at 13:16
@phresnel: Did I differ from that? – Nawaz Dec 12 '11 at 14:02
Oops, brainfart on my side, sorry. – Sebastian Mach Dec 12 '11 at 14:03
@phresnel: I made my post more explicit. I hope it is very clear now. – Nawaz Dec 12 '11 at 14:05

score 3 · Answer 3 · answered Dec 12 '11 at 13:02

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There is no additional time penalty involved for member functions. Virtual functions are slightly slower but not by much. Unless you're running an incredibly tight loop, even virtual function overhead is negligible.

answered Dec 12 '11 at 13:02

Puppy

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A virtual function is not slower, a polymorphic function call is. – Sebastian Mach Dec 12 '11 at 13:16

score 2 · Answer 4 · answered Dec 12 '11 at 13:05

For normal functions it's enough with a "jump" to them, which is very fast. The same goes for normal member functions. Virtual functions on the other hand, the address to jump to has to be fetched from a table, which of course involves more machine code instructions and therefore will be slower. However, the difference is negligible and will hardly even be measurable.

In other words, don't worry about it. If you have slowdowns, then it's most likely (like 99,999%) something else. Use a profiler to find out where.

Class member function vs function with respect to speed

4 Answers4

Premature optimization is the root of all evil

Premature optimization is the root of all evil

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