A Common Idiom
The common idiom in the examples presented above, is that the selection of the appropriate algorithm is based on the call's argument types at runtime. The call is therefore subject to all the usual additional performance costs that are associated with dynamic resolution of calls, usually more than in a language supporting only single method dispatch. In C++, for example, a dynamic function call is usually resolved by a single offset calculation - which is possible because the compiler knows the location of the function in the object's method table and so can statically calculate the offset. In a language supporting double dispatch, this is slightly more costly, because the compiler must generate code to calculate the method's offset in the method table at runtime, thereby increasing the overall instruction path length (by an amount that is likely to be no more than the total number of calls to the function, which may not be very significant).
Read more about this topic: Double Dispatch
Famous quotes containing the words common and/or idiom:
“Academic and aristocratic people live in such an uncommon atmosphere that common sense can rarely reach them.”
—Samuel Butler (1835–1902)
“The “sayings” of a community, its proverbs, are its characteristic comment upon life; they imply its history, suggest its attitude toward the world and its way of accepting life. Such an idiom makes the finest language any writer can have; and he can never get it with a notebook. He himself must be able to think and feel in that speech—it is a gift from heart to heart.”
—Willa Cather (1873–1947)