Editing Virtual method table (section)

==Efficiency==
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%.<ref>Driesen, Karel and Hölzle, Urs, [https://dl.acm.org/doi/10.1145/236338.236369 "The Direct Cost of Virtual Function Calls in C++"], OOPSLA 1996</ref> The cost of virtual functions may not be so high on modern {{abbr|CPU|Central Processing Unit}} architectures due to much larger caches and better [[branch predictor|branch prediction]].

Furthermore, in environments where [[Just-in-time compilation|JIT compilation]] is not in use, virtual function calls usually cannot be [[inline expansion|inlined]]. In certain cases it may be possible for the compiler to perform a process known as '''devirtualization''' in which, for instance, the lookup and indirect call are replaced with a conditional execution of each inlined body, but such optimizations are not common.

To avoid this overhead, compilers usually avoid using virtual method tables whenever the call can be resolved at [[compile time]].

Thus, the call to <code>f1</code> above may not require a table lookup because the compiler may be able to tell that <code>d</code> can only hold a <code>D</code> at this point, and <code>D</code> does not override <code>f1</code>. Or the compiler (or optimizer) may be able to detect that there are no subclasses of <code>B1</code> anywhere in the program that override <code>f1</code>. The call to <code>B1::f1</code> or <code>B2::f2</code> will probably not require a table lookup because the implementation is specified explicitly (although it does still require the 'this'-pointer fixup).