Is there a way to Make NSInvocation surport variable parmas function line [NSstring stringWithFormat:..]

Apple doc says "NSInvocation does not support invocations of methods with either variable numbers of arguments or union arguments. "

i searched for hours ,some people says var_list works, but i tryed ,it does Not

but I think there may be a way to do the same thing (reflection) on variable params function,as i metioned ,[stringWithFormat:],

so , I found a way ,please readt the code bellow .

    @interface INTObj : NSObject
    @property (nonatomic, assign)   int  realvalue
    @end

    @interface FloatObj : NSObject
    @property (nonatomic, assign)   float  realvalue
    @end

    typedef id (*Obj_Imp)(id,SEL,...);
    Method md =  class_getClassMethod(obj, selector);  // here, obj means NSString
    Obj_Imp fun =  (Obj_Imp )method_getImplementation(md); // stringWithFormat:...

    NSString *arg1 = @"hello  %f  %d";

    FloatObj *fo = [[FloatObj alloc] init];
    fo.realvalue = 11.3;

    INTObj *io = [[INTObj alloc] init];
    io.realvalue = 5;


    NSObject *arr[3] = {arg1, fo,io};


   // it cracks  . exc_bad_Access code = 1
   NSString *result = fun(obj , selector, arr[0], [arr[1] realvalue],[arr[2] realvalue]) ;



   // it works but i cant do this ,because i don't know the type of argument 1 at Runtime
   NSString *result = fun(obj , selector, arr[0],[(FloatObj *)arr[1] realvalue],[arr[2] realvalue])

why does the second calling of the function "fun" works while the first one cracks?

is there a better way to to do this?