Virtual Method Table - Comparison With Alternatives

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+.