objective c - Method Swizzling isEqualToString -

i'm running odd behavior when trying method swizzle isequaltostring: on nsstring class. here code in question:

#import <foundation/foundation.h> #import <objc/objc-runtime.h>  @interface nsstring (swizzlestring) - (bool) custom_isequaltostring:(nsstring *)astring; - (nsrange)custom_rangeofstring:(nsstring *)astring; @end  @implementation nsstring (swizzlestring)  - (bool) custom_isequaltostring:(nsstring *)astring; {     nslog(@"inside custom_isequaltostring method definition");     return [self custom_isequaltostring:astring]; }  - (nsrange)custom_rangeofstring:(nsstring *)astring; {     nslog(@"inside custom_rangeofstring method definition");     return [self custom_rangeofstring:astring]; }  @end  int main(int argc, const char * argv[]) {      method m1, m2;      m1 = class_getinstancemethod([nsstring class], @selector(isequaltostring:));     m2 = class_getinstancemethod([nsstring class], @selector(custom_isequaltostring:));     method_exchangeimplementations(m1, m2);      m1 = class_getinstancemethod([nsstring class], @selector(rangeofstring:));     m2 = class_getinstancemethod([nsstring class], @selector(custom_rangeofstring:));     method_exchangeimplementations(m1, m2);      nsstring *foo = @"foo";      // not log anything, still using isequaltostring: implementation     [foo isequaltostring:@"foo"];     // not log anything, since using method implementation isequaltostring:     [foo custom_isequaltostring:@"foo"];      // log because rangeofstring uses custom_rangeofstring imp     [foo rangeofstring:@"foo"];     // not log because uses method implementation rangeofstring:     [foo custom_rangeofstring:@"foo"]; } 

isequaltostring: , rangeofstring: both defined in category on nsstring called (nsstringextensionmethods), included rangeofstring swizzle show i'm swizzling methods correctly , nsstring object eliminate questions class cluster problems.

when produce assembly code above, instead of seeing normal objc_msgsend calls instead see stuff l_objc_msgsend_fixup_isequaltostring_. led me finding out more objective-c vtable, in seems isequaltostring: can found:

static const char * const defaultvtable[] = {     "allocwithzone:",      "alloc",      "class",      "self",      "iskindofclass:",      "respondstoselector:",      "isflipped",      "length",      "objectforkey:",      "count",      "objectatindex:",      "isequaltostring:",      "isequal:",      "retain",      "release",      "autorelease",  }; 

i've been digging through objective-c source , internet day on how possible somehow still able swizzle isequaltostring:.

as you're aware, class cluster. need actual class, not public class, ask string object have:

nsstring *foo = @"foo";  m1 = class_getinstancemethod([foo class], @selector(isequaltostring:)); m2 = class_getinstancemethod([foo class], @selector(custom_isequaltostring:)); method_exchangeimplementations(m1, m2); 

there's fragile alternative of using class name itself:

nsclassfromstring(@"__nscfconstantstring") nsclassfromstring(@"__nscfstring")