Name-lookup resolution is similar to the method resolution techniques employed by Objective-C and Smalltalk. It is currently about five times slower than offset resolution. It is more flexible, however. In particular, name-lookup resolution, unlike offset resolution, can be used when:
For example, a client program may use two classes that define two different methods of the same name, and it might not be known until run time which of the two methods should be invoked (because, for example, it will not be known until run time which class's instance the method will be applied to).
Name-lookup resolution is always performed by a class, so it requires a method call. (Offset resolution, by contrast, requires no method calls.) To perform name-lookup method resolution, the class of the intended receiver object obtains a method procedure pointer for the desired method that is appropriate for its instances. In general, this will require a name-based search through various data structures maintained by ancestor classes.
Offset and name-lookup resolution achieve the same net effect (that is, they select the same method procedure); they just achieve it differently (via different mechanisms for locating the method's method token). Offset resolution is faster, because it does not require searching for the method token, but name-lookup resolution is more flexible.
When defining (in SOM IDL) the interface to a class of objects, the class implementor can decide, for each method, whether the SOM Compiler will generate usage bindings that support name-lookup resolution for invoking the method. Regardless of whether this is done, however, application programs using the class can have SOM use either technique, on a per-method-call basis. Chapter 3, "Using SOM Classes in Client Programs," describes how client programs invoke methods.