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%. The cost of virtual functions may not be so high on modern CPU architectures due to much larger caches and better branch prediction.
Furthermore, in environments where JIT compilation is not in use, virtual function calls usually cannot be inlined. While a compiler could replace the lookup and indirect call with, for instance, a conditional execution of each inlined body, such optimizations are not common.
To avoid this overhead, compilers usually avoid using vtables whenever the call can be resolved at compile time.
Thus, the call to f1 above may not require a vtable lookup because the compiler may be able to tell that d can only hold a D at this point, and D does not override f1. Or the compiler (or optimizer) may be able to detect that there are no subclasses of B1 anywhere in the program that override f1. The call to B1::f1 or B2::f2 will probably not require a vtable lookup because the implementation is specified explicitly (although it does still require the 'this'-pointer fixup).
Read more about this topic: Virtual Method Table
Famous quotes containing the word efficiency:
“Nothing comes to pass in nature, which can be set down to a flaw therein; for nature is always the same and everywhere one and the same in her efficiency and power of action; that is, nature’s laws and ordinances whereby all things come to pass and change from one form to another, are everywhere and always; so that there should be one and the same method of understanding the nature of all things whatsoever, namely, through nature’s universal laws and rules.”
—Baruch (Benedict)
“I’ll take fifty percent efficiency to get one hundred percent loyalty.”
—Samuel Goldwyn (1882–1974)
““Never hug and kiss your children! Mother love may make your children’s infancy unhappy and prevent them from pursuing a career or getting married!” That’s total hogwash, of course. But it shows on extreme example of what state-of-the-art “scientific” parenting was supposed to be in early twentieth-century America. After all, that was the heyday of efficiency experts, time-and-motion studies, and the like.”
—Lawrence Kutner (20th century)