Re: does virtualizing all methods slow C++ down ?

Lynn McGuire <lmc@winsim.com> wrote:


  The cost of calling a virtual function (compared to calling a normal
member function) is often greatly exaggerated. I once tested the speed
difference myself and the difference was not measurable (in other words,
both tests returned time values that were so close together that even
the variance between runs of the same test were larger).

  Of course my sample size is that of 1 (iow. I have tested it only in
one system with one compiler, using one specific test), so the result is
far from definitive. However, it does tell me that the difference is
quite minuscule in practice, and often overwhelmed by everything else
that happens when calling a function (pushing parameters onto the stack
and so on, which, depending on the amount and type of parameters, will
probably take a lot more clock cycles than the indirect virtual function
call.)

  However, this is comparing virtual functions with regular *non-inlined*
functions. Function inlining can have a much greater difference, and if
you virtualize an inline function, it might slow it down significantly
in comparison. (Either the compiler will not inline it at all, or else
it will have to add some kind of runtime conditional in the compiled code
to check if the function has been overridden.) Thus if you have for example
some really small getter/setter functions that are better inline and really
don't need to be virtual, don't make them.