Comparison With Alternatives
The vtable is generally a good performance trade-off to achieve dynamic dispatch, but there are alternatives, such as binary tree dispatch, with higher performance but different costs.
However, vtables only allow for single dispatch on the special "this" parameter, in contrast to multiple dispatch (as in CLOS or Dylan), where the types of all parameters can be taken into account in dispatching.
Vtables also only work if dispatching is constrained to a known set of methods, so they can be placed in a simple array built at compile time, in contrast to duck typing languages (such as Smalltalk, Python or JavaScript).
Languages that provide either or both of these features often dispatch by looking up a string in a hash table, or some other equivalent method. There are a variety of tricks to speed this up (e.g., interning/tokenizing method names, caching lookups, just-in-time compilation), and dispatch time often does not have a significant effect on total processing time, but nonetheless, vtable lookup is obviously faster. Vtables are also simpler to implement and debug, and closer to the "C style" than hash tables of strings.
Note that Objective-C 2.1 now supports vtable dispatch under 64-bit Mac OS X 10.6+.
Read more about this topic: Virtual Method Table
Famous quotes containing the words comparison with, comparison and/or alternatives:
“[Girls] study under the paralyzing idea that their acquirements cannot be brought into practical use. They may subserve the purposes of promoting individual domestic pleasure and social enjoyment in conversation, but what are they in comparison with the grand stimulation of independence and self- reliance, of the capability of contributing to the comfort and happiness of those whom they love as their own souls?”
—Sarah M. Grimke (1792–1873)
“What is man in nature? A nothing in comparison with the infinite, an all in comparison with the nothing—a mean between nothing and everything.”
—Blaise Pascal (1623–1662)
“Clearly, society has a tremendous stake in insisting on a woman’s natural fitness for the career of mother: the alternatives are all too expensive.”
—Ann Oakley (b. 1944)