> IMP usage is not well documented.  I'm able to use that type to cache
> methods as long as the return from the method involved is an object,
> but if the method returns a float or other nonobject, I can't make it
> work.  If you know, point me to some information please.  If you need
> more detailed info in the form of an example, I can provide it.

This is from NEXTSTEP (3.3) Object class description:

<quote>
    IMP is defined (in the objc/objc.h header file) as a pointer to a  
function that returns an id and takes a variable number of arguments (in  
addition to the two "hidden" arguments-self and _cmd-that are passed to  
every method implementation):

    typedef id (*IMP)(id, SEL, ...);

    This definition serves as a prototype for the function pointer that  
methodFor: returns.  It's sufficient for methods that return an object and  
take object arguments.  However, if the aSelector method takes different  
argument types or returns anything but an id, its function counterpart  
will be inadequately prototyped.  Lacking a prototype, the compiler will  
promote floats to doubles and chars to ints, which the implementation  
won't expect.  It will therefore behave differently (and erroneously) when  
called as a function than when performed as a method.

    To remedy this situation, it's necessary to provide your own  
prototype.  In the example below, the declaration of the test variable  
serves to prototype the implementation of the isEqual: method.  test is  
defined as pointer to a function that returns a BOOL and takes an id  
argument (in addition to the two "hidden" arguments).  The value returned  
by methodFor: is then similarly cast to be a pointer to this same function  
type:

    BOOL (*test)(id, SEL, id);
    test = (BOOL (*)(id, SEL, id))[target methodFor:@selector(isEqual:)];

    while ( !test(target, @selector(isEqual:), someObject) ) {
        . . .
    }

    In some cases, it might be clearer to define a type (similar to IMP)  
that can be used both for declaring the variable and for casting the  
function pointer methodFor: returns.  The example below defines the  
EqualIMP type for just this purpose:

    typedef BOOL (*EqualIMP)(id, SEL, id);
    EqualIMP test;
    test = (EqualIMP)[target methodFor:@selector(isEqual:)];

    while ( !test(target, @selector(isEqual:), someObject) ) {
        . . .
    }
</quote>