swizzler.c

// Here a cave in which live all runtime dragons and wizards
#include <objc/runtime.h>

// We can swap instance methods, they start with '-' minus sign, you know
static void instance_methods_swap(id object, SEL firstSel, SEL secondSel) {
    method_exchangeImplementations(
        class_getInstanceMethod(object, firstSel),
        class_getInstanceMethod(object, secondSel));
}

// Also we can swap class methods, with '+' at start of definition 
static void class_methods_swap(Class c, SEL firstSel, SEL secondSel) {
    method_exchangeImplementations(
        class_getClassMethod(c, firstSel),
        class_getClassMethod(c, secondSel));
}